Table Of Content

Be/X-ray Binaries Pablo Reig1,2 1 1 0 2 n a J 6 2 Abstract mation and dissipation of the equatorial disc, mass- ] The interest in X/γ-rayAstronomy has grownenor- ejection mechanisms, V/R variability, and rotation E mouslyinthelastdecadesthankstotheabilitytosend rates; those related to the neutron star, such as, mass H X-ray space missions above the Earth’s atmosphere. determination, accretion physics, and spin period evo- h. There are more than half a million X-ray sources de- lution; but also, those that result from the interaction p tectedandoverahundredmissions(pastandcurrently of the two constituents, such as, disc truncation and - operational) devoted to the study of cosmic X/γ rays. mass transfer. Until recently, it was thought that the o r With the improved sensibilities of the currently active Be stars’disc wasnot significantlyaffected by the neu- st missions new detections occur almost on a daily ba- tron star. In this review, I present the observational a sis. Among these, neutron-star X-ray binaries form an evidenceaccumulatedinrecentyearsontheinteraction [ important group because they are among the brightest between the circumstellar disc and the compact com- 1 extra-solar objects in the sky and are characterized by panion. The most obvious effect is the tidal truncation v dramatic variability in brightness on timescales rang- of the disc. As a result, the equatorial discs in Be/X- 6 ing frommilliseconds to monthsandyears. Their main 3 ray binaries are smaller and denser than those around source of power is the gravitational energy released by 0 isolated Be stars. 5 matteraccretedfromacompanionstarandfallingonto . the neutron star in a relatively close binary system. 1 Keywords X-rays: binaries – stars: neutron – stars: 0 Neutron-star X-ray binaries divide into high-mass binaries close –stars: emission line, Be 1 and low-mass systems according to whether the mass 1 of the donor star is above 8 or below 2 M⊙, respec- : ∼ ∼ v tively. MassiveX-raybinariesdividefurtherintosuper- 1 Definition and classification of X-ray binaries i giant X-ray binaries and Be/X-ray binaries depending X on the evolutionary status of the optical companion. r Inverygeneralterms,onecansimplydefineX-raybina- a Virtually all Be/X-ray binaries show X-ray pulsations. riesassystemsthatconsistofacompactobjectorbiting Therefore, these systems can be used as unique natu- anopticalcompanion. Theyare”close”binarysystems ral laboratories to investigate the properties of matter becausethere exists atransferofmassfromthe optical underextremeconditionsofgravityandmagneticfield. componenttothecompactobject. By”opticalcompan- The purpose of this work is to review the obser- ion”itisunderstoodthatnuclearburningisstilltaking vational properties of Be/X-ray binaries. The open place in its interior. Figure 1 shows a tree-diagramde- questions in Be/X-raybinaries include those related to picting all the different subsystems that comprise the the Be star companion, that is, the so-called ”Be phe- generic group of X-ray binaries. nomenon”, such as, timescales associated to the for- In referring to the two components in an X-ray bi- nary one should be careful and learn which is the sub- PabloReig ject of investigation as the same name can be used to 1IESL, Foundation for Reseach and Technology-Hellas, 71110, mean different components. In massive X-ray bina- Heraklion,Greece. ries,the most massivestar is normally termed primary 2Physics Department, University of Crete, 71003, Heraklion, whereasthelessmassiveoneiscalledsecondary. Inlow- Greece. mass systems, the term primary refers to the neutron 2 IV or V) and supergiant X-ray binaries (SGXBs), if XRB they contain a luminosity class I-II star. In SGXBs, the optical star emits a substantial stel- Black Holes Neutron stars White dwarf larwind, removingbetween10−6 10−8 M⊙ yr−1 with − a terminal velocity up to 2000 km s−1. A neutron star - (Classical) novae HMXB LMXB HMXB LMXB - Recurrent novae in a relatively close orbit will capture some fraction of - Dwarf novae this wind, sufficient to power a bright X-ray source. - Polars BeXB SGXB atoll Z - VY Sculptoris If mass transfer occurs via Roche lobe overflow, then - AM Canum theX-rayemissionishighlyenhancedandanaccretion - SW Sextantis - Transient - disc-fed disc is formed around the neutron star. At present, - wind-fed - Persistent - SFXT there is known only one disc-fed SGXB in the Galaxy (CenX-3)andthreeintotal(SMCX-1andLMCX-4), Fig. 1 Classification of X-ray binaries. while there are about a few tens of wind-fed SGXBs. Because of their brightness and persistent X-ray emis- star while the word secondary is reserved for the late- sion,SGXBswerethefirsttobediscovered. Theywere type companion. Other names also used are ”optical” initially thought to represent the dominating popula- or ”donor”for the largerstar and ”compact”,”gainer” tion of HMXBs, whereas BeXB were considered atyp- or ”accreting” for the denser companion. ical cases. Hence, the name classical or standard was given to SGXBs. They admit several classification schemes depend- In BeXB, the optical companion is a Be star. Be ing upon whether the emphasis is put on the type of stars are non-supergiant fast-rotating B-type and lu- the compact companion or the physical properties of minosity class III-V stars which at some point of their the optical star. X-ray binaries divide up into black- lives have shown spectral lines in emission, hence the holesystems,neutronstarX-raybinariesorcataclysmic qualifier ”e” in their spectral types (Porter & Rivinius variables(ifthecompactobjectisawhitedwarf). Nev- 2003; Balona 2000; Slettebak 1988). The best stud- ertheless, the term ”X-ray binaries” is normally re- ied lines are those of hydrogen (Balmer and Paschen served to designate binaries with neutron stars. series) but Be stars may also show He, Fe in emission (seee.gHanuschik1996). Theyalsoshowanamountof 1.1 High-mass X-ray binaries IR radiationthan is larger than that expected from an absorption-line B star of the same spectral type. This Neutron-star X-ray binaries are divided up into high- extralong-wavelengthemissionisknownasinfraredex- mass (HMXBs) and low-mass(LMXBs) X-raybinaries cess. depending on the spectral type of the mass donor, as The origin of the emission lines and infrared excess this feature determines the mode of transferring mass in BeXB is attributed to an equatorial disc, fed from to the compact object and the environment surround- materialexpelledfromtherapidlyrotatingBestarina ing the X-raysource. HMXBs containearly-type(Oor mannerthatitisnotyetunderstood(Porter & Rivinius B) companions, while the spectral type of the optical 2003). During periastron,the neutronstarpasses close starinLMXBsislaterthanA.HMXBsarestrongemit- to this disc, sometimes may even go through it caus- ters of X-ray radiation. Sometimes they appear as the ing major disruption. A large flow of matter is then brightestobjects of the X-ray sky. The high-energyra- accreted onto the compact object. The conversion of diation is produced as the result of accretionof matter thekineticenergyofthein-fallingmatterintoradiation fromtheopticalcompanionontotheneutronstar. The powerstheXrays. BeXBhavelargeorbitalperiodsand termaccretionreferstothegradualaccumulationorde- by definition they are non-supergiant systems. Hence, position of matter onto the surface of an object under the Be star is well within the Roche lobe. However, theinfluenceofgravity. Iftheaccretingobjectisaneu- transientRochelobeoverflowmayoccurduringperias- tron star (or black hole), then matter falls down onto tron passage in systems with large eccentric orbits or anenormouswellofgravitationalpotentialandisaccel- during giant X-ray outbursts when a large fraction of erated to extremely high velocities. When the matter the Be star’s disc is believed to be accreted. reachesthe surfaceofthe neutronstar,itis rapidly de- Most BeXB are transient systems and present mod- celeratedandthe free-fall kinetic energy radiatedaway erately eccentric orbits (e > 0.3), although persistent as heat which is available to power the X-ray source. sources also exist (Reig &∼Roche 1999). Persistent TheluminosityclassservestosubdivideHMXBsinto BeXB differ from transient BeXB in that they display Be/X-ray binaries (BeXB), when the optical star is a much less X-ray variability (no large outbursts are de- dwarf,subgiantorgiantOBestar(luminosity classIII, tected), lower luminosity (L <1035 erg s−1), contain x ∼ 3 inthisdiagram.Figure2displaysanupdatedversionanticorrelation,whileBeXBshowapositivecorrelationmasstransfer.SGXBsexhibitnocorrelationatallorangram(Corbet1986),whichreflectsthedifferenttypesofpositionsinthespinperiodversusorbitalperioddia-ThedifferenttypesofHMXBsoccupywell-definedtems(oftheorderof10000years).poweredphaseisrelativelyshortforOBsupergiantsys-SGXBsarelessnumerousthanBeXB.Theaccretion-tion,theevolutionarytimescalesinvolvedimplythattheirtriggeringmechanismbeingactivated.Inaddi-toobutthediscoveryofnewsystemsisalsorelatedtomissions.BeXBbenefitedfromthetechnicaladvancesthesensitivityoftheX-raydetectorsonboardspacesources,newdiscoveriescomefromtheimprovementoftransientemissioninBeXB.SinceSGXBsarepersistentenvelope),leadingtopersistentemissioninSGXBsandginoftheaccretedmass(stellarwindversusBestar’sBeXBforoneSGXB.Thereasonliesinthedifferentori-90’stherateofnewdiscoverieswasapproximatelyfourberofSGXBshadstabilised.Duringthe80’sandthenumberofBeXBwasgrowingfast,whilethenum-UntiltheadventoftheINTEGRALmissionin2002,belongindeedtoourGalaxy.wardstheGalacticplane,indicatingthatthemajorityconcentrationtowardstheGalacticcenterandalsoto-2010).Thedistributionofthesesourcesshowsaclearergyrange1–10keV(Liuetal.2006,2007;Birdetal.withfluxeswellabove10ergcmsintheen-1021−−−naries.Therearemorethan300brightX-raysourcesTable2givesthenumberofvarioustypesofX-raybi-beenshowntobetypicalofBeXB(seeSects.2and3).terpartsandwhoseX-rayandoptical/IRvariabilityhascludesonlythosesystemswithidentifiedopticalcoun-Table1.1liststheconfirmedBeXB.Thistablein-orbinwiderorbitsystems(P>200d).spinslowlyrotatingneutronstars(P>200s)andreside J2103.5+4545.to2S0114+65andOAO1657-41andtheopencircletoSAXPPdiagram.TheopentrianglescorrespondorbspinFig.2− orbP (d)11010010000.01 disc-fed SGXR 1 Pspin (100s)BeXB wind-fed SGXR 10000 Table 1 List of galactic BeXB. Only systems with known optical counterparts and well-established optical/X-ray behaviour are included X-rayname OpticalCtp. RA(J2000) Dec(J2000) Spec. type V J E(B-V) Pspin (s) Porb (d) e d (kpc) 4U0115+634 V635Cas 011831.90 +634424.0 B0.2Ve 14.8-15.5 10.8-12.3 1.55 3.6 24.3 0.34 8 IGRJ01363+6610 – 013618.00 +661036.0 B1IV-Ve 13.3 10 1.61 – – – 2 RXJ0146.9+6121 LSI+61235 014700.17 +612123.7 B1Ve 11.2 10 0.93 1412 330 – 2.3 IGRJ01583+6713 – 015818.20 +671326.0 B2IVe 14.4 11.5 1.46 19692 – – 4 RXJ0240.4+6112 LSI+61303 024031.67 +611345.6 B0.5Ve 10.7 8.8 0.75 – 26.45 0.54 3.1 V0331+530 BQCam 033459.89 +531023.6 O8-9Ve 15.1-15.8 11.4-12.2 1.9 4.4 34.3 0.3 7 4U0352+309(XPer) HD24534 035523.08 +310245.0 O9.5IIIe-B0Ve 6.1-6.8 5.7-6.5 0.4 837 250 0.11 1.3 RXJ0440.9+4431 LSV+4417 044059.32 +443149.3 B0.2Ve 10.8 9.2 0.65 203 – – 3.3 1A0535+262 V725Tau 053854.57 +261856.8 B0IIIe 8.9-9.6 7.7-8.5 0.75 105 111 0.47 2.4 IGRJ06074+2205 – 060724.00 +220500.0 B0.5V 12.3 10.5 1.88 – – – 5 XTEJ0658-073 – 065842.00 –071100.0 O9.7Ve 12.1 9.7 1.19 160.4 – – 3.9 4U0726-260 V441Pup 072853.60 –260629.0 O8-9Ve 11.6 10.4 0.73 103.2 34.5 – 6 RXJ0812.4-3114 LS992 081228.84 –311452.2 B0.2III-IVe 12.4 11.2-12.0 0.65 31.89 80 – 9 GS0834-430 – 083555.00 –431106.0 B0-2III-Ve 20.4 13.3 4.0 12.3 105.8 0.12 5 GROJ1008-57 – 100946.00 –581730.0 O9e-B1e 15.3 10.9 1.9 93.5 247.5 0.66 2 RXJ1037.5-5647 LS1698 103735.50 –564811.0 B0III-Ve 11.3 – 0.75 862 – – 5 1A1118-615 Hen3-640 112057.21 –615500.3 O9.5III-Ve 12.1 9.6 0.92 405 – – 5 4U1145-619 V801Cen 114800.02 –621224.9 B1Vne 9.3 8.7 0.29 292.4 187.5 >0.5 3.1 4U1258-613 GX304-1 130117.20 –613607.0 B2Vne 13.5 9.8 2.0 272 132.5 >0.5 2.4 2S1417-624 – 142112.80 –624154.0 B1Ve 17.2 13.3 2.0 17.6 42.12 0.45 10 GS1843+00 – 184536.90 +005148.2 B0-2IV-Ve 20.9 13.7 2.5 29.5 – – >10 XTEJ1946+274 – 194539.30 +272155.0 B0-1IV-Ve 16.8 12.5 2.0 15.8 169.2 0.33 8-10 KS1947+300 – 194930.50 +301224.0 B0Ve 14.2 11.7 1.09 18.76 40.4 0.03 9.5 EXO2030+375 V2246Cyg 203215.20 +373815.0 B0e 19.7 12.1 3.8 41.8 46.03 0.41 5 GROJ2058+42 – 205900.00 +414300.0 O9.5-B0IV-Ve 14.9 11.7 1.38 192 110 – 9 SAXJ2103.5+4545 – 210335.71 +454505.5 B0Ve 14.2 11.8 1.35 358.6 12.67 0.4 6.8 4U2135+57 CepX-4 213930.72 +565910.0 B1-B2Ve 14.2 11.8 1.3 66.3 – – 3.8 SAXJ2239.3+6116 – 223920.90 +611626.8 B0-2III-Ve 15.1 11.5 1.4 1247 262.6 – 4.4 4 Table 2 Statistics on HMXBsin theMilky Way Number of neutron-star X-ray binaries1 327 Number of suspected HMXB1 131 Number of suspected BeXB2 63 Number of confirmed BeXB3 28 1Sources intheLiuetal.(2007)andLiuetal.(2006)catalogs plusupdate fromthe4thIBIS/ISGRI softgamma-raysurveycatalog (Birdetal.2010) 2Sourcesintheupdatedon-lineversionoftheRaguzova &Popov(2005)catalog(http://xray.sai.msu.ru/∼raguzova/BeXcat/) 3Systemswithknownoptical counterpartandverifiedX-raybehaviour (fromTable1.1) of Corbet’s diagram. Disc-fed SGXBs (squares) show of a neutron star and a Be star. However, there seems short orbital periods and short spin periods and dis- to be no apparent mechanism that would prevent the playananticorrelationintheP P diagram. The formation of Be stars with black holes (BH) or white orb spin − small orbital separation and evolved companions make dwarfs(WD).Surprisingly,notasingleBeXBisknown Rochelobeoverflowthemostlikelymasstransfermech- to host a black-holecompanionin our Galaxy,whereas anism. Wind-fedSGXBs(triangles)showlongspinpe- the interpretation of γ Cas as a Be+WD system still riods and short orbital periods, occupying a more or remains very controversial. less flat region in the P P diagram. Two sys- Two ideas have been put forward to explain this orb spin − tems (open triangles) prevent the region from being a apparent lack of Be/BH binaries. Zhang et al. (2004) horizontal line: OAO1657-41, which might be making extended the application of the viscous decretion disc a transition to the disc-fed SGXB and 2S 0114+65,for model of Okazaki & Negueruela (2001) to compact which the association of the 104 s pulsations with the companionsofarbitrarymassandshowedthatthemost spin period of the neutron star remains controversial effective Be disc truncation would occur in relatively (Koenigsberger et al.2006). The spin and orbitalperi- narrowsystems. Usingthe populationsynthesisresults ods of BeXB (filled circles) exhibit a clear correlation. of Podsiadlowskiet al. (2003), which state that binary TheopencircleinFig.2representsSAXJ2103.5+4545, black holes are most likely to be born in systems with apeculiarsystemwhoseX-raypropertiesaresimilarto narroworbits(P <30days),thereasonforthelackof orb those of wind-fed systems but whose optical/IR emis- these systems can b∼e attributed to the difficulty to de- sion resemble that of BeXB (Reig et al. 2010a). tectingthem. Be/BHbinariesareexpectedtobeX-ray The observed correlation in Be/X systems is ex- transients with very long quiescent states. In contrast, plained in terms of the equilibrium period, defined as Belczynski & Ziolkowski(2009)showedthatBe/BHbi- the period at which the outer edge of the magneto- naries do not necessarily have narrow orbits. These sphererotateswiththeKeplerianvelocity(Davidson & Ostraiuktehrorsarguedthatthe predictedratioofBe/NSbina- 1973; Stella et al. 1986; Waters & van Kerkwijk 1989). ries to Be/BH binaries (FNS−to−BH 30 50) in our ∼ − If the the neutron star (and hence the magnetosphere) Galaxy, based on current population synthesis models rotates faster that the equilibrium period, then matter andevolutionaryscenarios,isconsistentwiththeobser- is spun away by the propeller mechanism; only when vations: there are60knownBe/NSbinaries,hence one thespinperiodislargerthantheequilibriumperiodcan would expect 0–2 Be/BH binaries, consistent with the matterbeaccretedontotheneutronstarsurface. This observedgalacticsample. BothZhang et al.(2004)and results in angular momentum transfer to the neutron Belczynski & Ziolkowski (2009) agree in that Be/BH star,increasingitsrotationvelocity(decreasingthespin binaries should exist in the Galaxy. period). Theequilibriumperioddependsmainlyonthe mass flux (or accretion rate) because it determines the 1.2 Other types of high-mass X-ray binaries sizeofthemagnetospherewhichisassumedtocorotate with the neutron star. In turn, the accretion rate de- The traditional picture of two classes of HMXBs, pends on the separation of the two components of the namely SGXB — subdivided into low-luminosity or binary systems, hence on the orbital period. wind-fed systems and high-luminosity or disc-fed sys- The compact object in all confirmed BeXB (Ta- tems — and BeXB — transient and persistent — is ble 1.1) is a neutron star. In fact, many times the neu- giving way to a more complex situation where newly tron star is taken as a defining property of BeXB. It is discoveredsystems may not fit in these categories. common to read in the literature that a BeXB consists 5 1.2.1 Low eccentricity BeXB There is a group of so far five BeXB (X Per, GS 0834– 430,KS1947+300,XTEJ1543–568,and2S1553-5421), characterised by P > 30 d and very low eccentricity orb (e<0.2). Their low e∼ccentricity requires that the neu- tro∼n star received a much lower kick velocity at birth than previously assumedby currentevolutionary mod- els (Pfahl et al. 2002). Most popular models for neutron star kicks involve Fig. 3 SpectralenergydistributionofLSI+61303. From a momentum impulse delivered around the time of the Chernyakovaet al. (2006) core collapse that produced the neutron star. These . modelsassumethatneutronstarsarebornwithspeeds in excess of 100-200 km s−1. Such velocities imply a INTEGRAL has unveiled a population of highly ob- probability close to zero that the post-supernova ec- scuredHMXBs withsupergiantcompanionsanda new centricity is less than 0.2. Hence, the discovery of type of source displaying outbursts which are signif- these low-eccentricity BeXB with wide orbits was un- icantly shorter than typical for BeXB and which are expected. Tidal circularizacion is ruled out since this characterized by bright flares with a duration of a mechanism requires that the star almost fills its Roche few hours and peak luminosities of 1036 1037 erg lobeandthattherebeaneffectivemechanismfordamp- s−1. These new systems have been termed−as Super- ing the tide. Tidal torques should have little effect on giant fast X-ray Transients (SFXTs, Smith et al. 2006; the orbit of a HMXB with P = 10 days, as long as orb Negueruela et al. 2006; Walter & Zurita-Heras 2007; the secondary is not too evolved and the eccentricity Negueruela et al. 2008). is not so large that the tidal interaction is enhanced Both obscured HMXBs and SFXTs display X-ray dramatically at periastron (as it is the case of typical and IR spectra typical of SGXBs. In some cases, BeXB). the X-ray sources are pulsed and orbital parameters Pfahl et al. (2002) developed a phenomenological typical of persistent SGXBs have been found (e.g., modelthatsimultaneouslyaccountsforthelong-period Bodaghee et al. 2006; Zurita Heras et al. 2006). The (>30 d), low-eccentricity (<0.2) HMXBs and which heavily-absorbedsourceshadnotbeendetectedbypre- d∼oes not violate any previo∼us notions regarding the vious missions due to high absorption, which renders numbers and kinematics of other neutron star popu- their spectra very hard. The current understanding is lations. They propose that a neutron star receives a that the entire binary system is surrounded by a dense relativelysmallkick(<50kms−1)iftheprogenitor,i.e., and absorbing circumstellar material envelope or co- thecoreoftheinitiall∼ymoremassivestarinthebinary, coon, made of cold gas and/or dust (Chaty 2008). rotates rapidly. This condition may be met when the SFXTs differ from SGXBs because they are only progenitor star experienced case B or C mass trans- e e detected sporadically, during very brief outbursts ferinabinarysystem2. Ifthehydrogen-exhaustedcore (Romano et al. 2010). A promising model to ex- of an initially rapidly rotating massive star is exposed plainSFXTsinvokeshighlystructured(clumpy)stellar following case Be or Ce mass transfer in a binary, then winds. The outburstoccurs as a resultof the accretion the core is also likely to be a rapid rotator. ofoneoftheclumpsofdensematterfromthewind. An 1.2.2 Obscured sources & Supergiant fast X-ray alternativemodel(”Be-type”model)assumesaveryel- transients lipticalorbitforthebinary. Inthismodeloutburstsare triggered when the compact object travels through its Since the launch of INTEGRAL in October 2002, the periastron. Other possibilities imply that SFXTs con- situation is also changing among the SGXB group. tain strongly magnetised neutron stars. The outbursts resultfromthe overcomingofcentrifugalandmagnetic 1The optical counterparts of XTE J1543–568 and 2S 1553-542 barriers (see Grebenev 2010, and references therein). arenotknown,hencetheydonotappearinTable1.1. 2In case B, mass transfer occurs during the shell hydrogen- 1.2.3 γ-ray binaries burning phase, but prior to central helium ignition, while case C evolution begins after helium has been depleted in the core. γ-ray binaries are HMXBs that emit most of their ra- Cases B and C are naturally divided into an early case (Be or diative output in the MeV-TeV range. Currently, only Ce), where the envelope of the primaryis mostly radiative, and fourHMXBsarewell-establishedmembersofthisgroup a late case (Bl or Cl), where the primaryhas adeep convective envelope. of high-energy sources: LS I +61 303, LS 5039, PSR 6 B1259-63, and Cygnus X-3, while other two are firm pulsar. In the pulsar wind scenario, the rotation of a candidates: Cygnus X-1, and HESS J0632+0573. The young pulsar provides stable energy to the nonthermal optical counterpart is either a luminous O-type star relativistic particles in the shocked pulsar wind mate- (Cygnus X-1, LS 5039), a Be star (LS I +61 303, PSR rial outflowing from the binary companion. As in the B1259-63, HESS J0632+057) or a likely Wolf-Rayet microquasar-jetmodels, the γ-rayemissioncanbe pro- star in Cygnus X-3. In addition to the variable γ-ray duced by inverse Compton scattering of the relativis- emission these systems share in common a resolved ra- tic particles from the pulsar wind on stellar photons dio counterpart with a jet or jet-like structure, multi- (Torres 2010, and references therein). In the pulsar wavelengthorbitalmodulationandspectralenergydis- wind scenario the resolved radio emission is not due tribution(Fig.3). Thewiderangeoforbitalparameters to a relativistic jet akin to those of microquasars, but (Paredes 2008) and the non-unique nature of the com- arises instead from shocked pulsar wind material out- pact companion (unknown in most systems but with a flowing from the binary (Dubus 2006). confirmedneutronstarinPSRB1259-63andafairlyse- cureblackholeinCygX-1)representachallengeforthe 1.2.4 γ-Cas like objects theoreticalmodellingofthesesystems. Twoalternative scenarios may explain the variable γ-ray emission: the A growing number of early Be stars discovered in X- microquasaroraccretion-poweredscenarioandthepul- ray surveys exhibit strong X/optical flux correlations sar wind scenario. and X-ray luminosities intermediate between those of All confirmed γ-ray binaries show a jet or jet-like normal stars and those of most BeXB in quiescence radio structure, which would indicate the presence of (Lopes de Oliveira et al. 2006). The optical properties relativistic particles. In black-hole binaries the radio are very similar to those of BeXB: i) the spectral type jet can account for their broad-band spectrum, from is always in the range B0-B1 III-Ve, and ii) show Hα radio to X-rays (Markoff et al. 2003) as well as for the equivalent widths stronger than 20 ˚A. However, they − origin of most of the timing variability (Kylafis et al. differ from the typical BeXB in their X-ray properties: 2008). Therefore, it is reasonable to think that the i) they show harder X-ray spectra that can be best jet may also be the origin of the very high-energy γ- fittedbyathinthermalplasmawithT 108 K,rather ∼ rays. Inthecontextofrelativisticjets,themostefficient thanapowerlawasseentypicallyinHMXBs, ii) there gamma-raymechanismwouldbeinverseComptonscat- is no evidence for coherent pulsations in any of these tering,bywhichrelativisticparticlescollidewithstellar systems but strong variability on time scales as short and/orsynchrotronphotonsandboosttheirenergiesto as100sisusuallyobserved,andiii)theydonotexhibit the VHE range. Two flavors of the microquasar model large X-ray outbursts. can be found in the literature depending on whether Theprototypeofthisgroupofsourcesisγ Cas. Two hadronic or leptonic jet matter dominates the emission models have been put forward to explain this type of at such an energy range. Among leptonic jet mod- system: accretion onto a compact object (most likely els, there are inverse Compton jet emission models in a white dwarf) and magnetically heated material be- which X-rays and γ-rays result from synchrotron self- tween the photosphere of the B star and the inner Compton processes (Atoyan & Aharonian 1999), or in part of its disc (Robinson et al. 2002). In the mag- which the seed photons come from external sources, netic corona model the hard X-rays result from high- i.e., companion star and/or accretion disk photons energyparticlesthatareemittedduetomagneticrecon- (Kaufman Bernad´o et al. 2002; Georganopoulos et al. nection, while the optical variability is due to changes 2002). In the hadronic scenario, the gamma-ray emis- in the density structure of the inner disc as a conse- sion arises from the decay of neutral pions created quence of turbulence generatedby changesin the mag- in the inelastic collisions between relativistic protons netic field. In support of this scenario there is the in the jet and either the ions in the stellar wind of fact that X-ray fluxes show random variations with or- the massive companion star (Romero et al. 2003) or bital phase, thereby contradicting the binary accretion nearby high-density regions (i.e. molecular clouds) model, which predicts a substantial modulation. In (Bosch-Ramo´n et al. 2005). favourofthebinarymodelisthesimilarityoftheX-ray Alternatively, relativistic particles can be injected spectra with those of cataclysmic variables, the rapid in the surrounding medium by the wind from a young variability andlarge numbers of Be + white dwarfsys- tems predicted by the theory of evolution of massive binaries. 3The Be star HD 215227, likely counterpart of the gamma-ray In the remaining part of this report I shall concen- sourceAGLJ2241+4454,hasbeensuggestedasanewcandidate (Wiliamsetal.2010). trate on the properties and variability of BeXBO˙nly 7 (Teff, log g) = (30000, 3.5) X Per 1.05 10-9 1 y -2-1m A)10-10 Intensit000..89.955 no disc X Per c -1g s 10-11 Flux (er10-12 ndois cdisc ntensity34000000 disc Hα He 10-13 I X Per 2000 1000 10000 6400 6500 6600 6700 6800 Wavelength (A) Wavelength (A) Fig. 4 IRexcess and Hαemission are thetwo main observational characteristics of Be stars. when for the sake of the discussion a comparison with on the geometry (size, shape) and dynamics (veloc- the behaviour of other type of X-ray binaries may be ity and density laws) of the envelope (Hanuschik 1986, illustrative, will these other type of binaries be men- 1995; Dachs 1992). tioned. Figure 4 illustrates graphically these two properties for the system X Persei. When an equatorial disc is present the near-IR emission exceeds that predicted 2 Optical/IR properties of BeXB by model atmosphere (shown is a Kuruzc model with T = 30000 K and logg = 3.5) and emission lines are eff The optical and infrared flux of a BeXB is completely prominently seen. dominatedby the Be starcompanion. While the X-ray emissionfromBeXBprovidesinformationonthephysi- 2.1 Hα line profiles calconditionsinthevicinityofthecompactobject,the optical and infrared emission reveals the physical state The Hα line is the prime indicator of the circumstel- ofthe massdonorcomponent. Since the fuelthatpow- lar disc state. Hα emission lines can be morphologi- ers the X-rayemission, namely matter from a powerful callydividedintwoclasses(Hummel & Vrancken1995; stellar wind orfroma equatoriallyconcentrateddenser Silaj et al. 2010): class 1 are symmetric and includes disc, comes from the massive companion, observations symmetric double-peak, wine-bottle and shell profiles of BeXB in the optical/IR are crucial to understand andclass2, whichareasymmetricandshowvariability the physicalconditions underwhichthe neutronstaris ontimescalesofyears. Eachclassdoesnotrefertodif- accreting. ferentgroupsofsourcesbecauseindividualBestarscan The two main observational characteristics of Be change from one to the other. These changes are nor- stars are the emission spectral lines, as opposed to the mally slow (of the order of years to decades in isolated normal absorption photospheric lines and an excess of Be stars and months to years in BeXB). Symmetric IR emission. Both properties, line emission and IR ex- profilesarebelievedto be generatedinquasi-Keplerian cess,originatefromextendedcircumstellarenvelopesof discs (see e.g. Hummel 1994), while asymmetric pro- ionisedgassurroundingtheequatoroftheBstar. They files are associated with distorted density distributions result from free-free and free-bound emission from the (Hanuschik 1995; Hummel & Hanuschik 1997). disc,i.e. recombinationoftheopticalandUVradiation Most BeXB show asymmetric split Hα profiles. The from the central star (Woolf et al. 1970; Gehrz et al. peaks adopt the names of the relative position of their 1974). The assumption of a common origin for the centralwavelengths. Inaspectrumwithmonotonically line emission and IR excess is strongly supported by increasing wavelength values, the left peak is known thecorrelationbetweentheintensitiesofcontinuousIR as the ”blue” (or violet) peak, while the right peak is emission,asmeasuredeitherbycolourindices(J M), named the ”red” peak. V/R variability refers then to − (J K) or fluxes at a certain wavelength and the in- the variation of the relative strength of the blue to the − tensity ofthe Hαline, asmeasuredeither asequivalent redpeak. Thereforethe V/Rratio,definedasthe ratio widthorfluxes(Dachs & Wamsteker1982;Dachs et al. of violet-side to red-side peak intensities above contin- 1988; Neto & de Freitas Pacheco 1982). By studying uum in units of continuum intensity represents a mea- thelinevariabilityonecanobtainimportantconstraints sure of the asymmetry of the line. A more convenient 8 quantity is the logarithmic of this ratio, log(V/R), be- cause in this case, positive values of log(V/R) corre- spond to a blue-dominated profile and negative values to a red-dominated line. The top panel of Fig. 5 dis- plays the V/R variability of the BeXB LS I +61 235 (Reig et al. 2000). V/R variability can be explained in terms of a non- axisymmetrical equatorial disc in which a one-armed perturbation (a zone in the disc with higher density) propagates(Okazaki1991,1996,1997;Papaloizouet al. 1992; Savonije & Heemskerk 1993). Double-peak sym- metricprofilesareexpectedwhenthehigh-densitypart is behind or in front of the star, while asymmetric pro- files are seen when the high-density perturbation is on 1 odnenesistiydepaorfttohfetdhiescd.iscMpoerretuprrbeactisioelny,iswlhoecnatetdheonhigthhe- log(V/R)-00-..0551 side of the disc where the rotational motion is directed 9.6 away from us, we see enhanced red emission (V < 9.8 J 10 R), while when the high-density part is moving toward 10.2 the observer, blue-dominated profiles V > R are ex- 0.4 0.3 pected (Telting et al. 1994). For systems seen at high- J-K0.2 0.1 inclinationangle,thetwosymmetriccasescanbe read- -20 ily distinguished since the central depression between αH )-15 the two symmetric peaks would be more pronounced W(-10 E -5 (reachingorgoingbeyondthe continuum, the so-called 48500 49000 49500 50000 50500 51000 51500 MJD shellprofile),whenthe perturbationis betweenthe ob- serverandthestar. Thereasonisthatshellprofilesare Fig. 5 Top: V/R variability observed in the Hα line of thoughttobeduetoaperspectiveeffect,namely,when LS I +61 235. From Reig et al. (2000). Bottom: Evolution the line of sight toward the star probes the equato- of the V/R ratio, J magnitude, J −K colour and the Hα equivalentwidth in LS I +61 235. From Reig et al. (2000). rialdiscandself-absorptionisproduced(Rivinius et al. 2006). If the density perturbation revolves around the star in the same direction as the material in the disc stars, which are found in the range 2-11 years with an (progradeprecession),thentheV/Rsequencewouldbe averageof 7 years (Okazaki 1997). accordingtoTelting et al.(1994): V =R(perturbation behind the star) V > R V =R (perturbation 2.2 EW(Hα) vs infrared colours −→ −→ in front of the star, shell profile) V <R. −→ One prediction of the model is that no changes in If the infrared excess observed in Be stars is due to the slope of the infrared continuum are expected be- the same processes as those responsible for Balmer cause the V/R variations are not the result of changes line emission, namely absorption and subsequent re- in the radial gradient of the circumstellar gas. This is emission of the optical and UV light from the underly- exactly the behaviour that it is found in LS I +61 235 ingstarinthecircumstellarenvelope,thenacorrelation (bottom panelofFig.5). While the individualinfrared between IR colours and the strength of the hydrogen photometric bands changed (∆J ∆H ∆K 0.3 lines shouldbe expected. Suchcorrelationhas been re- ≈ ≈ ∼ magnitudes)theinfraredcolours(e.g. J K)remained portedforisolatedBestars(Dachs & Wamsteker1982; − unchanged (Reig et al. 2000). Neto & de Freitas Pacheco 1982; Dachs et al. 1988) V/R variability is also seen in other lines, like HeI andBeXB(Coe et al.1994,2005,althoughmixedwith 6678˚A.Since helium lines are generatedatsmaller disc isolated Be stars). Figure 6 shows the first such dia- radii than the hydrogen lines (Hummel & Vrancken gram made from BeXB only. It displays the IR colour 1995; Jaschek & Jaschek 2004), the asymmetry of the index(J K) asafunctionoftheHαequivalentwidth, 0 HeI line profiles indicates that the internal changes of − EW(Hα),forthesourceslistedinTable3. Theinfrared the disc are global, affecting its entire structure. colours were corrected for interstellar extinction. Only V/R quasi-periods in BeXB range from 1-5 yr (Ta- contemporaneous data (when the time difference be- ble 5) and are shorter than those seen in isolated Be tween the IR and optical observations was less than 9 100 eitherdiffuseinterstellarbands,disc-lossepisodesorby disentanglingthe circumstellarandinterstellarredden- ing (see Fabregat & Torrej´on 1998). Under these conditions, namely, simultaneity of the 10 optical and infrared data and used of the extinction- α) W(H corrected (J − K) index, the EW(Hα) − (J − K)0 E becomes a useful tool to estimate the extra reddening 1 caused by the circumstellar disc. Once a value of the EW(Hα) is known, one can look for the corresponding intrinsic (J K) in Fig. 6. By comparing this value − with that expected according to the spectral type, an -0.4 -0.2 0 0.2 0.4 0.6 estimatedofthedisccontributionontheindex(J K) (J-K)0 − can be obtained. The relatively large scatter, however, Fig.6 CorrelationbetweentheequivalentwidthoftheHα may limit the usefulness of the diagram. line andtheinfrared colour J−K. Different colours repre- It should be noticed that although the Hα and in- sent different systems. This correlation implies a common fraredemissioncorrelateasexpectedforacommonori- origin for theinfrared excessandthelineemission, namely, gin in the disc, the spatial extension and precise lo- thecircumstellar disc round theBe star. calisation in the disc are different. From long-baseline interferometric observations in the K′ band Gies et al. one month) were used. The second and third columns (2007)foundthatthe angularsize ofthe infraredemis- in Table 3 represent the amplitude of change in the sion is consistently smaller than that determined for infrared colour (J K) and the EW(Hα) over the ob- the Hα emission. In other words, the near-IR emis- − served range, whereas Fig. 6 plots the actual values. sion forms closer to the star than does the Hα emis- In Fig. 6 we have included only those sources for sion. Furthermore there is good evidence in several whichweareconfidentthatthecolourexcessE(B V) cases (Clark et al. 2001; Grundstrom et al. 2007) that − is free of circumstellar effects. Due to the surround- anincreaseindiscbrightnessoccursfirstinthenear-IR ing envelope, the use of photometric magnitudes and fluxexcessandlaterinHα asexpectedforanoutwardly colours to derive astrophysicalparameters, like E(B progressing density enhancement. This difference can − V), may be misleading because they are contaminated be explained if the dependence with disc radius of the by disc emission. The effect of the disc is to make Hα optical depth is less steep than that of the infrared the photometric indices to appear redder than a non- optical depth (Gies et al. 2007). emitting B star of the same spectral type. The disc emission makes a maximum contribution to the op- 2.3 Identification of the optical counterparts tical (B V) colour of a few tenths of a magnitude − (Dachs et al. 1988; Howells et al. 2001). The values of With the improved sensitivities of the currently opera- E(B V) used to produce Fig. 6 were obtained from tionalspacemissionsmanynewX-raysourcesarebeing − discovered. AboutonethirdofthetheX/γ-raysources inthe 4thINTEGRAL catalogue(Bird et al.2010)are Table 3 List of BeXB used in Fig. 6 with colour excess thoughttobeX-raybinaries,ofwhichhalfarebelieved and amplitudeof change in (J−K)0 and EW(Hα). to contain early-type companions. An optical identi- X-raysource E(B−V) ∆(J−K)0 ∆EW(Hα) fication is necessary to facilitate a complete study of (mag) (mag) (˚A) these systems. Without a knowncounterpart, observa- 4U 0115+63 1.65 0.8 20 tions are limited to X-ray energies, and hence our un- RX J0146.9+6121 0.93 0.7 10 derstandingofthestructureanddynamicsofthosesys- V 0332+53 1.88 0.4 6 tems that remain optically unidentified is incomplete. 4U 0352+30 0.39 1.1 18 RX J0440.1+4431 0.65 0.3 5 If the only available information is that providedby 1A 0535+262 0.75 1.0 15 an isolated X-ray detection then potential HMXBs are 4U 0728-25 0.73 0.1 2 selected as those exhibiting properties of a magnetised RX J0812.4-3114 0.65 0.2 5 neutron star, namely, X-ray pulsations and/or an ab- GRO J1008-57 1.90 1.0 15 sorbed power-law continuum spectrum with an expo- 1A 1118-616 0.90 1.5 25 nential cutoff at 10-30 keV and cyclotron absorption 4U 1145-619 0.29 2.8 40 features. Extrainformationonthenatureofthesource EXO 2030+375 3.8 1.1 15 SAXJ2103.5+4545 1.35 0.1 1 can be obtained if the long-term X-ray variability is 10 -5 SAX J2103.5+4545 -5.2 -5.4 αH R--5.6 -5.8 -6 0.5 1 1.5 2 2.5 B-V -4 IGR J01363+6610 -4.5 αH R- -5 -5.5 -6 1 1.5 2 2.5 B-V Fig. 7 Colour-colour diagrams and V-band images of the field around SAX J2103.5+4545 (top) and IGR J01363+6610 (bottom). The position of the confirmed optical counterpart in the colour-colour diagram is marked with a filled circle. Goodcandidatesarethosethatoccupytheupperpartofthecolour-colourdiagramandlieinsideorveryclosetotheX-ray uncertainty region. For example, the stars that are close to SAX J2103.5+4545 in the colour-colour diagram lie far away from thesatellite error circles. known (information provided by all-sky monitors, like dates. Furthermore, uncertainty regions are given to ASM RXTE or BAT SWIFT). The presence of regular a certain confidence level, hence it is possible that the and periodic outbursts or unexpected giant outbursts true optical companion is located close but outside the reachingEddingtonluminosityattheirpeaksmayindi- X-ray error circle. catethe presenceofaBestar. Likewise,erraticflaring, Photometric detection of Be stars can be performed i.e., X-ray variability with changes in the X-ray inten- by selecting colours directly relatedto the excess emis- sity by a factor 3-5 over a few minutes might indicate sionboth in the Hα line (”red” colour)andthe contin- thepresenceofastrongstellarwindfromanearly-type, uum(”blue”colour). Observingthroughanarrowfilter probably evolved, companion. centred on the Hα line and a wider filter also contain- However,a systemwith X-raydata only willremain ing this line, e.g. Johnson R filter (Reig et al. 2005), in the category of suspected HMXBs until a confirmed Sloan-r (Guti´errez-Soto 2006) would account for the optical identification is performed. The most obvious emission line, while Johnson B V (Reig et al. 2005) − observational features to look for is Hα emission and or Stro¨mgren b y (Guti´errez-Soto 2006) can be used − near-infrared excess emission. While the detection of asreddeningindicators. TheuseofStro¨mgrenb,y,and these two observational features does not guarantee narrow-bandHα photometrythrough(b y, y)colour- − that the source is indeed an OBe type star, it defi- magnitude and (b y, y Hα) colour-colourdiagrams − − nitely narrows the number of candidates to a handful isparticularlysuitabletoidentifyBestarsinopenclus- of sources. ters (Grebel et al. 1992; McSwain & Gies 2005). The size of the uncertainty in the location of the Stars with a moderately or large Hα excess can be X-ray source, the so-called error circle, determines the distinguished from the rest because they deviate from typeofobservationaltechniquetouse. Iftheerrorcircle thegeneraltrendandoccupytheupperleftpartsofthe is small (a few arcseconds) then the number of visible diagram. Be star are expected to show bluer colours, starsinthe regionisexpectedtobesmallanditispos- i.e.,low(B V)or(b y),because theyareearly-type − − sible to perform narrow-slit spectroscopic observations stars(althoughthey normallyappearredderthannon- and look for early-type, Hα emitting line stars. If the emittingB starsdue tothe circumstellardisc)andalso errorradius is large (>1 arcminute), then it is likely to larger(i.e., less negative) R Hα colours because they − includealargenumbe∼rofsourcesandhencenarrow-slit showHαinemission. Oncethenumberofcandidatesis spectroscopy becomes impractical. In this case colour- reduced to a few systems, narrow-slitspectroscopy be- colour diagrams can be used to identify good candi- comes feasible. The final step is to obtain a spectrum

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Be/X-ray Binaries Pablo Reig1,2 1 1 0 2 n a J 6 2 Abstract mation and dissipation of the equatorial disc, mass- ] The interest in X/γ-rayAstronomy has grownenor- ejection mechanisms, V/R variability, and rotation E mouslyinthelastdecadesthankstotheabilitytosend rates; those related to the neutron star, such as, mass H X-ray space missions above the Earth’s atmosphere. determination, accretion physics, and spin period evo- h. There are more than half a million X-ray sources de- lution; but also, those that result from the interaction p tectedandoverahundredmissions(pastandcurrently of the two constituents, such as, disc truncation and - operational) devoted to the study of cosmic X/γ rays. mass transfer. Until recently, it was thought that the o r With the improved sensibilities of the currently active Be stars’disc wasnot significantlyaffected by the neu- st missions new detections occur almost on a daily ba- tron star. In this review, I present the observational a sis. Among these, neutron-star X-ray binaries form an evidenceaccumulatedinrecentyearsontheinteraction [ important group because they are among the brightest between the circumstellar disc and the compact com- 1 extra-solar objects in the sky and are characterized by panion. The most obvious effect is the tidal truncation v dramatic variability in brightness on timescales rang- of the disc. As a result, the equatorial discs in Be/X- 6 ing frommilliseconds to monthsandyears. Their main 3 ray binaries are smaller and denser than those around source of power is the gravitational energy released by 0 isolated Be stars. 5 matteraccretedfromacompanionstarandfallingonto . the neutron star in a relatively close binary system. 1 Keywords X-rays: binaries – stars: neutron – stars: 0 Neutron-star X-ray binaries divide into high-mass binaries close –stars: emission line, Be 1 and low-mass systems according to whether the mass 1 of the donor star is above 8 or below 2 M⊙, respec- : ∼ ∼ v tively. MassiveX-raybinariesdividefurtherintosuper- 1 Definition and classification of X-ray binaries i giant X-ray binaries and Be/X-ray binaries depending X on the evolutionary status of the optical companion. r Inverygeneralterms,onecansimplydefineX-raybina- a Virtually all Be/X-ray binaries show X-ray pulsations. riesassystemsthatconsistofacompactobjectorbiting Therefore, these systems can be used as unique natu- anopticalcompanion. Theyare”close”binarysystems ral laboratories to investigate the properties of matter becausethere exists atransferofmassfromthe optical underextremeconditionsofgravityandmagneticfield. componenttothecompactobject. By”opticalcompan- The purpose of this work is to review the obser- ion”itisunderstoodthatnuclearburningisstilltaking vational properties of Be/X-ray binaries. The open place in its interior. Figure 1 shows a tree-diagramde- questions in Be/X-raybinaries include those related to picting all the different subsystems that comprise the the Be star companion, that is, the so-called ”Be phe- generic group of X-ray binaries. nomenon”, such as, timescales associated to the for- In referring to the two components in an X-ray bi- nary one should be careful and learn which is the sub- PabloReig ject of investigation as the same name can be used to 1IESL, Foundation for Reseach and Technology-Hellas, 71110, mean different components. In massive X-ray bina- Heraklion,Greece. ries,the most massivestar is normally termed primary 2Physics Department, University of Crete, 71003, Heraklion, whereasthelessmassiveoneiscalledsecondary. Inlow- Greece. mass systems, the term primary refers to the neutron 2 IV or V) and supergiant X-ray binaries (SGXBs), if XRB they contain a luminosity class I-II star. In SGXBs, the optical star emits a substantial stel- Black Holes Neutron stars White dwarf larwind, removingbetween10−6 10−8 M⊙ yr−1 with − a terminal velocity up to 2000 km s−1. A neutron star - (Classical) novae HMXB LMXB HMXB LMXB - Recurrent novae in a relatively close orbit will capture some fraction of - Dwarf novae this wind, sufficient to power a bright X-ray source. - Polars BeXB SGXB atoll Z - VY Sculptoris If mass transfer occurs via Roche lobe overflow, then - AM Canum theX-rayemissionishighlyenhancedandanaccretion - SW Sextantis - Transient - disc-fed disc is formed around the neutron star. At present, - wind-fed - Persistent - SFXT there is known only one disc-fed SGXB in the Galaxy (CenX-3)andthreeintotal(SMCX-1andLMCX-4), Fig. 1 Classification of X-ray binaries. while there are about a few tens of wind-fed SGXBs. Because of their brightness and persistent X-ray emis- star while the word secondary is reserved for the late- sion,SGXBswerethefirsttobediscovered. Theywere type companion. Other names also used are ”optical” initially thought to represent the dominating popula- or ”donor”for the largerstar and ”compact”,”gainer” tion of HMXBs, whereas BeXB were considered atyp- or ”accreting” for the denser companion. ical cases. Hence, the name classical or standard was given to SGXBs. They admit several classification schemes depend- In BeXB, the optical companion is a Be star. Be ing upon whether the emphasis is put on the type of stars are non-supergiant fast-rotating B-type and lu- the compact companion or the physical properties of minosity class III-V stars which at some point of their the optical star. X-ray binaries divide up into black- lives have shown spectral lines in emission, hence the holesystems,neutronstarX-raybinariesorcataclysmic qualifier ”e” in their spectral types (Porter & Rivinius variables(ifthecompactobjectisawhitedwarf). Nev- 2003; Balona 2000; Slettebak 1988). The best stud- ertheless, the term ”X-ray binaries” is normally re- ied lines are those of hydrogen (Balmer and Paschen served to designate binaries with neutron stars. series) but Be stars may also show He, Fe in emission (seee.gHanuschik1996). Theyalsoshowanamountof 1.1 High-mass X-ray binaries IR radiationthan is larger than that expected from an absorption-line B star of the same spectral type. This Neutron-star X-ray binaries are divided up into high- extralong-wavelengthemissionisknownasinfraredex- mass (HMXBs) and low-mass(LMXBs) X-raybinaries cess. depending on the spectral type of the mass donor, as The origin of the emission lines and infrared excess this feature determines the mode of transferring mass in BeXB is attributed to an equatorial disc, fed from to the compact object and the environment surround- materialexpelledfromtherapidlyrotatingBestarina ing the X-raysource. HMXBs containearly-type(Oor mannerthatitisnotyetunderstood(Porter & Rivinius B) companions, while the spectral type of the optical 2003). During periastron,the neutronstarpasses close starinLMXBsislaterthanA.HMXBsarestrongemit- to this disc, sometimes may even go through it caus- ters of X-ray radiation. Sometimes they appear as the ing major disruption. A large flow of matter is then brightestobjects of the X-ray sky. The high-energyra- accreted onto the compact object. The conversion of diation is produced as the result of accretionof matter thekineticenergyofthein-fallingmatterintoradiation fromtheopticalcompanionontotheneutronstar. The powerstheXrays. BeXBhavelargeorbitalperiodsand termaccretionreferstothegradualaccumulationorde- by definition they are non-supergiant systems. Hence, position of matter onto the surface of an object under the Be star is well within the Roche lobe. However, theinfluenceofgravity. Iftheaccretingobjectisaneu- transientRochelobeoverflowmayoccurduringperias- tron star (or black hole), then matter falls down onto tron passage in systems with large eccentric orbits or anenormouswellofgravitationalpotentialandisaccel- during giant X-ray outbursts when a large fraction of erated to extremely high velocities. When the matter the Be star’s disc is believed to be accreted. reachesthe surfaceofthe neutronstar,itis rapidly de- Most BeXB are transient systems and present mod- celeratedandthe free-fall kinetic energy radiatedaway erately eccentric orbits (e > 0.3), although persistent as heat which is available to power the X-ray source. sources also exist (Reig &∼Roche 1999). Persistent TheluminosityclassservestosubdivideHMXBsinto BeXB differ from transient BeXB in that they display Be/X-ray binaries (BeXB), when the optical star is a much less X-ray variability (no large outbursts are de- dwarf,subgiantorgiantOBestar(luminosity classIII, tected), lower luminosity (L <1035 erg s−1), contain x ∼ 3 inthisdiagram.Figure2displaysanupdatedversionanticorrelation,whileBeXBshowapositivecorrelationmasstransfer.SGXBsexhibitnocorrelationatallorangram(Corbet1986),whichreflectsthedifferenttypesofpositionsinthespinperiodversusorbitalperioddia-ThedifferenttypesofHMXBsoccupywell-definedtems(oftheorderof10000years).poweredphaseisrelativelyshortforOBsupergiantsys-SGXBsarelessnumerousthanBeXB.Theaccretion-tion,theevolutionarytimescalesinvolvedimplythattheirtriggeringmechanismbeingactivated.Inaddi-toobutthediscoveryofnewsystemsisalsorelatedtomissions.BeXBbenefitedfromthetechnicaladvancesthesensitivityoftheX-raydetectorsonboardspacesources,newdiscoveriescomefromtheimprovementoftransientemissioninBeXB.SinceSGXBsarepersistentenvelope),leadingtopersistentemissioninSGXBsandginoftheaccretedmass(stellarwindversusBestar’sBeXBforoneSGXB.Thereasonliesinthedifferentori-90’stherateofnewdiscoverieswasapproximatelyfourberofSGXBshadstabilised.Duringthe80’sandthenumberofBeXBwasgrowingfast,whilethenum-UntiltheadventoftheINTEGRALmissionin2002,belongindeedtoourGalaxy.wardstheGalacticplane,indicatingthatthemajorityconcentrationtowardstheGalacticcenterandalsoto-2010).Thedistributionofthesesourcesshowsaclearergyrange1–10keV(Liuetal.2006,2007;Birdetal.withfluxeswellabove10ergcmsintheen-1021−−−naries.Therearemorethan300brightX-raysourcesTable2givesthenumberofvarioustypesofX-raybi-beenshowntobetypicalofBeXB(seeSects.2and3).terpartsandwhoseX-rayandoptical/IRvariabilityhascludesonlythosesystemswithidentifiedopticalcoun-Table1.1liststheconfirmedBeXB.Thistablein-orbinwiderorbitsystems(P>200d).spinslowlyrotatingneutronstars(P>200s)andreside J2103.5+4545.to2S0114+65andOAO1657-41andtheopencircletoSAXPPdiagram.TheopentrianglescorrespondorbspinFig.2− orbP (d)11010010000.01 disc-fed SGXR 1 Pspin (100s)BeXB wind-fed SGXR 10000 Table 1 List of galactic BeXB. Only systems with known optical counterparts and well-established optical/X-ray behaviour are included X-rayname OpticalCtp. RA(J2000) Dec(J2000) Spec. type V J E(B-V) Pspin (s) Porb (d) e d (kpc) 4U0115+634 V635Cas 011831.90 +634424.0 B0.2Ve 14.8-15.5 10.8-12.3 1.55 3.6 24.3 0.34 8 IGRJ01363+6610 – 013618.00 +661036.0 B1IV-Ve 13.3 10 1.61 – – – 2 RXJ0146.9+6121 LSI+61235 014700.17 +612123.7 B1Ve 11.2 10 0.93 1412 330 – 2.3 IGRJ01583+6713 – 015818.20 +671326.0 B2IVe 14.4 11.5 1.46 19692 – – 4 RXJ0240.4+6112 LSI+61303 024031.67 +611345.6 B0.5Ve 10.7 8.8 0.75 – 26.45 0.54 3.1 V0331+530 BQCam 033459.89 +531023.6 O8-9Ve 15.1-15.8 11.4-12.2 1.9 4.4 34.3 0.3 7 4U0352+309(XPer) HD24534 035523.08 +310245.0 O9.5IIIe-B0Ve 6.1-6.8 5.7-6.5 0.4 837 250 0.11 1.3 RXJ0440.9+4431 LSV+4417 044059.32 +443149.3 B0.2Ve 10.8 9.2 0.65 203 – – 3.3 1A0535+262 V725Tau 053854.57 +261856.8 B0IIIe 8.9-9.6 7.7-8.5 0.75 105 111 0.47 2.4 IGRJ06074+2205 – 060724.00 +220500.0 B0.5V 12.3 10.5 1.88 – – – 5 XTEJ0658-073 – 065842.00 –071100.0 O9.7Ve 12.1 9.7 1.19 160.4 – – 3.9 4U0726-260 V441Pup 072853.60 –260629.0 O8-9Ve 11.6 10.4 0.73 103.2 34.5 – 6 RXJ0812.4-3114 LS992 081228.84 –311452.2 B0.2III-IVe 12.4 11.2-12.0 0.65 31.89 80 – 9 GS0834-430 – 083555.00 –431106.0 B0-2III-Ve 20.4 13.3 4.0 12.3 105.8 0.12 5 GROJ1008-57 – 100946.00 –581730.0 O9e-B1e 15.3 10.9 1.9 93.5 247.5 0.66 2 RXJ1037.5-5647 LS1698 103735.50 –564811.0 B0III-Ve 11.3 – 0.75 862 – – 5 1A1118-615 Hen3-640 112057.21 –615500.3 O9.5III-Ve 12.1 9.6 0.92 405 – – 5 4U1145-619 V801Cen 114800.02 –621224.9 B1Vne 9.3 8.7 0.29 292.4 187.5 >0.5 3.1 4U1258-613 GX304-1 130117.20 –613607.0 B2Vne 13.5 9.8 2.0 272 132.5 >0.5 2.4 2S1417-624 – 142112.80 –624154.0 B1Ve 17.2 13.3 2.0 17.6 42.12 0.45 10 GS1843+00 – 184536.90 +005148.2 B0-2IV-Ve 20.9 13.7 2.5 29.5 – – >10 XTEJ1946+274 – 194539.30 +272155.0 B0-1IV-Ve 16.8 12.5 2.0 15.8 169.2 0.33 8-10 KS1947+300 – 194930.50 +301224.0 B0Ve 14.2 11.7 1.09 18.76 40.4 0.03 9.5 EXO2030+375 V2246Cyg 203215.20 +373815.0 B0e 19.7 12.1 3.8 41.8 46.03 0.41 5 GROJ2058+42 – 205900.00 +414300.0 O9.5-B0IV-Ve 14.9 11.7 1.38 192 110 – 9 SAXJ2103.5+4545 – 210335.71 +454505.5 B0Ve 14.2 11.8 1.35 358.6 12.67 0.4 6.8 4U2135+57 CepX-4 213930.72 +565910.0 B1-B2Ve 14.2 11.8 1.3 66.3 – – 3.8 SAXJ2239.3+6116 – 223920.90 +611626.8 B0-2III-Ve 15.1 11.5 1.4 1247 262.6 – 4.4 4 Table 2 Statistics on HMXBsin theMilky Way Number of neutron-star X-ray binaries1 327 Number of suspected HMXB1 131 Number of suspected BeXB2 63 Number of confirmed BeXB3 28 1Sources intheLiuetal.(2007)andLiuetal.(2006)catalogs plusupdate fromthe4thIBIS/ISGRI softgamma-raysurveycatalog (Birdetal.2010) 2Sourcesintheupdatedon-lineversionoftheRaguzova &Popov(2005)catalog(http://xray.sai.msu.ru/∼raguzova/BeXcat/) 3Systemswithknownoptical counterpartandverifiedX-raybehaviour (fromTable1.1) of Corbet’s diagram. Disc-fed SGXBs (squares) show of a neutron star and a Be star. However, there seems short orbital periods and short spin periods and dis- to be no apparent mechanism that would prevent the playananticorrelationintheP P diagram. The formation of Be stars with black holes (BH) or white orb spin − small orbital separation and evolved companions make dwarfs(WD).Surprisingly,notasingleBeXBisknown Rochelobeoverflowthemostlikelymasstransfermech- to host a black-holecompanionin our Galaxy,whereas anism. Wind-fedSGXBs(triangles)showlongspinpe- the interpretation of γ Cas as a Be+WD system still riods and short orbital periods, occupying a more or remains very controversial. less flat region in the P P diagram. Two sys- Two ideas have been put forward to explain this orb spin − tems (open triangles) prevent the region from being a apparent lack of Be/BH binaries. Zhang et al. (2004) horizontal line: OAO1657-41, which might be making extended the application of the viscous decretion disc a transition to the disc-fed SGXB and 2S 0114+65,for model of Okazaki & Negueruela (2001) to compact which the association of the 104 s pulsations with the companionsofarbitrarymassandshowedthatthemost spin period of the neutron star remains controversial effective Be disc truncation would occur in relatively (Koenigsberger et al.2006). The spin and orbitalperi- narrowsystems. Usingthe populationsynthesisresults ods of BeXB (filled circles) exhibit a clear correlation. of Podsiadlowskiet al. (2003), which state that binary TheopencircleinFig.2representsSAXJ2103.5+4545, black holes are most likely to be born in systems with apeculiarsystemwhoseX-raypropertiesaresimilarto narroworbits(P <30days),thereasonforthelackof orb those of wind-fed systems but whose optical/IR emis- these systems can b∼e attributed to the difficulty to de- sion resemble that of BeXB (Reig et al. 2010a). tectingthem. Be/BHbinariesareexpectedtobeX-ray The observed correlation in Be/X systems is ex- transients with very long quiescent states. In contrast, plained in terms of the equilibrium period, defined as Belczynski & Ziolkowski(2009)showedthatBe/BHbi- the period at which the outer edge of the magneto- naries do not necessarily have narrow orbits. These sphererotateswiththeKeplerianvelocity(Davidson & Ostraiuktehrorsarguedthatthe predictedratioofBe/NSbina- 1973; Stella et al. 1986; Waters & van Kerkwijk 1989). ries to Be/BH binaries (FNS−to−BH 30 50) in our ∼ − If the the neutron star (and hence the magnetosphere) Galaxy, based on current population synthesis models rotates faster that the equilibrium period, then matter andevolutionaryscenarios,isconsistentwiththeobser- is spun away by the propeller mechanism; only when vations: there are60knownBe/NSbinaries,hence one thespinperiodislargerthantheequilibriumperiodcan would expect 0–2 Be/BH binaries, consistent with the matterbeaccretedontotheneutronstarsurface. This observedgalacticsample. BothZhang et al.(2004)and results in angular momentum transfer to the neutron Belczynski & Ziolkowski (2009) agree in that Be/BH star,increasingitsrotationvelocity(decreasingthespin binaries should exist in the Galaxy. period). Theequilibriumperioddependsmainlyonthe mass flux (or accretion rate) because it determines the 1.2 Other types of high-mass X-ray binaries sizeofthemagnetospherewhichisassumedtocorotate with the neutron star. In turn, the accretion rate de- The traditional picture of two classes of HMXBs, pends on the separation of the two components of the namely SGXB — subdivided into low-luminosity or binary systems, hence on the orbital period. wind-fed systems and high-luminosity or disc-fed sys- The compact object in all confirmed BeXB (Ta- tems — and BeXB — transient and persistent — is ble 1.1) is a neutron star. In fact, many times the neu- giving way to a more complex situation where newly tron star is taken as a defining property of BeXB. It is discoveredsystems may not fit in these categories. common to read in the literature that a BeXB consists 5 1.2.1 Low eccentricity BeXB There is a group of so far five BeXB (X Per, GS 0834– 430,KS1947+300,XTEJ1543–568,and2S1553-5421), characterised by P > 30 d and very low eccentricity orb (e<0.2). Their low e∼ccentricity requires that the neu- tro∼n star received a much lower kick velocity at birth than previously assumedby currentevolutionary mod- els (Pfahl et al. 2002). Most popular models for neutron star kicks involve Fig. 3 SpectralenergydistributionofLSI+61303. From a momentum impulse delivered around the time of the Chernyakovaet al. (2006) core collapse that produced the neutron star. These . modelsassumethatneutronstarsarebornwithspeeds in excess of 100-200 km s−1. Such velocities imply a INTEGRAL has unveiled a population of highly ob- probability close to zero that the post-supernova ec- scuredHMXBs withsupergiantcompanionsanda new centricity is less than 0.2. Hence, the discovery of type of source displaying outbursts which are signif- these low-eccentricity BeXB with wide orbits was un- icantly shorter than typical for BeXB and which are expected. Tidal circularizacion is ruled out since this characterized by bright flares with a duration of a mechanism requires that the star almost fills its Roche few hours and peak luminosities of 1036 1037 erg lobeandthattherebeaneffectivemechanismfordamp- s−1. These new systems have been termed−as Super- ing the tide. Tidal torques should have little effect on giant fast X-ray Transients (SFXTs, Smith et al. 2006; the orbit of a HMXB with P = 10 days, as long as orb Negueruela et al. 2006; Walter & Zurita-Heras 2007; the secondary is not too evolved and the eccentricity Negueruela et al. 2008). is not so large that the tidal interaction is enhanced Both obscured HMXBs and SFXTs display X-ray dramatically at periastron (as it is the case of typical and IR spectra typical of SGXBs. In some cases, BeXB). the X-ray sources are pulsed and orbital parameters Pfahl et al. (2002) developed a phenomenological typical of persistent SGXBs have been found (e.g., modelthatsimultaneouslyaccountsforthelong-period Bodaghee et al. 2006; Zurita Heras et al. 2006). The (>30 d), low-eccentricity (<0.2) HMXBs and which heavily-absorbedsourceshadnotbeendetectedbypre- d∼oes not violate any previo∼us notions regarding the vious missions due to high absorption, which renders numbers and kinematics of other neutron star popu- their spectra very hard. The current understanding is lations. They propose that a neutron star receives a that the entire binary system is surrounded by a dense relativelysmallkick(<50kms−1)iftheprogenitor,i.e., and absorbing circumstellar material envelope or co- thecoreoftheinitiall∼ymoremassivestarinthebinary, coon, made of cold gas and/or dust (Chaty 2008). rotates rapidly. This condition may be met when the SFXTs differ from SGXBs because they are only progenitor star experienced case B or C mass trans- e e detected sporadically, during very brief outbursts ferinabinarysystem2. Ifthehydrogen-exhaustedcore (Romano et al. 2010). A promising model to ex- of an initially rapidly rotating massive star is exposed plainSFXTsinvokeshighlystructured(clumpy)stellar following case Be or Ce mass transfer in a binary, then winds. The outburstoccurs as a resultof the accretion the core is also likely to be a rapid rotator. ofoneoftheclumpsofdensematterfromthewind. An 1.2.2 Obscured sources & Supergiant fast X-ray alternativemodel(”Be-type”model)assumesaveryel- transients lipticalorbitforthebinary. Inthismodeloutburstsare triggered when the compact object travels through its Since the launch of INTEGRAL in October 2002, the periastron. Other possibilities imply that SFXTs con- situation is also changing among the SGXB group. tain strongly magnetised neutron stars. The outbursts resultfromthe overcomingofcentrifugalandmagnetic 1The optical counterparts of XTE J1543–568 and 2S 1553-542 barriers (see Grebenev 2010, and references therein). arenotknown,hencetheydonotappearinTable1.1. 2In case B, mass transfer occurs during the shell hydrogen- 1.2.3 γ-ray binaries burning phase, but prior to central helium ignition, while case C evolution begins after helium has been depleted in the core. γ-ray binaries are HMXBs that emit most of their ra- Cases B and C are naturally divided into an early case (Be or diative output in the MeV-TeV range. Currently, only Ce), where the envelope of the primaryis mostly radiative, and fourHMXBsarewell-establishedmembersofthisgroup a late case (Bl or Cl), where the primaryhas adeep convective envelope. of high-energy sources: LS I +61 303, LS 5039, PSR 6 B1259-63, and Cygnus X-3, while other two are firm pulsar. In the pulsar wind scenario, the rotation of a candidates: Cygnus X-1, and HESS J0632+0573. The young pulsar provides stable energy to the nonthermal optical counterpart is either a luminous O-type star relativistic particles in the shocked pulsar wind mate- (Cygnus X-1, LS 5039), a Be star (LS I +61 303, PSR rial outflowing from the binary companion. As in the B1259-63, HESS J0632+057) or a likely Wolf-Rayet microquasar-jetmodels, the γ-rayemissioncanbe pro- star in Cygnus X-3. In addition to the variable γ-ray duced by inverse Compton scattering of the relativis- emission these systems share in common a resolved ra- tic particles from the pulsar wind on stellar photons dio counterpart with a jet or jet-like structure, multi- (Torres 2010, and references therein). In the pulsar wavelengthorbitalmodulationandspectralenergydis- wind scenario the resolved radio emission is not due tribution(Fig.3). Thewiderangeoforbitalparameters to a relativistic jet akin to those of microquasars, but (Paredes 2008) and the non-unique nature of the com- arises instead from shocked pulsar wind material out- pact companion (unknown in most systems but with a flowing from the binary (Dubus 2006). confirmedneutronstarinPSRB1259-63andafairlyse- cureblackholeinCygX-1)representachallengeforthe 1.2.4 γ-Cas like objects theoreticalmodellingofthesesystems. Twoalternative scenarios may explain the variable γ-ray emission: the A growing number of early Be stars discovered in X- microquasaroraccretion-poweredscenarioandthepul- ray surveys exhibit strong X/optical flux correlations sar wind scenario. and X-ray luminosities intermediate between those of All confirmed γ-ray binaries show a jet or jet-like normal stars and those of most BeXB in quiescence radio structure, which would indicate the presence of (Lopes de Oliveira et al. 2006). The optical properties relativistic particles. In black-hole binaries the radio are very similar to those of BeXB: i) the spectral type jet can account for their broad-band spectrum, from is always in the range B0-B1 III-Ve, and ii) show Hα radio to X-rays (Markoff et al. 2003) as well as for the equivalent widths stronger than 20 ˚A. However, they − origin of most of the timing variability (Kylafis et al. differ from the typical BeXB in their X-ray properties: 2008). Therefore, it is reasonable to think that the i) they show harder X-ray spectra that can be best jet may also be the origin of the very high-energy γ- fittedbyathinthermalplasmawithT 108 K,rather ∼ rays. Inthecontextofrelativisticjets,themostefficient thanapowerlawasseentypicallyinHMXBs, ii) there gamma-raymechanismwouldbeinverseComptonscat- is no evidence for coherent pulsations in any of these tering,bywhichrelativisticparticlescollidewithstellar systems but strong variability on time scales as short and/orsynchrotronphotonsandboosttheirenergiesto as100sisusuallyobserved,andiii)theydonotexhibit the VHE range. Two flavors of the microquasar model large X-ray outbursts. can be found in the literature depending on whether Theprototypeofthisgroupofsourcesisγ Cas. Two hadronic or leptonic jet matter dominates the emission models have been put forward to explain this type of at such an energy range. Among leptonic jet mod- system: accretion onto a compact object (most likely els, there are inverse Compton jet emission models in a white dwarf) and magnetically heated material be- which X-rays and γ-rays result from synchrotron self- tween the photosphere of the B star and the inner Compton processes (Atoyan & Aharonian 1999), or in part of its disc (Robinson et al. 2002). In the mag- which the seed photons come from external sources, netic corona model the hard X-rays result from high- i.e., companion star and/or accretion disk photons energyparticlesthatareemittedduetomagneticrecon- (Kaufman Bernad´o et al. 2002; Georganopoulos et al. nection, while the optical variability is due to changes 2002). In the hadronic scenario, the gamma-ray emis- in the density structure of the inner disc as a conse- sion arises from the decay of neutral pions created quence of turbulence generatedby changesin the mag- in the inelastic collisions between relativistic protons netic field. In support of this scenario there is the in the jet and either the ions in the stellar wind of fact that X-ray fluxes show random variations with or- the massive companion star (Romero et al. 2003) or bital phase, thereby contradicting the binary accretion nearby high-density regions (i.e. molecular clouds) model, which predicts a substantial modulation. In (Bosch-Ramo´n et al. 2005). favourofthebinarymodelisthesimilarityoftheX-ray Alternatively, relativistic particles can be injected spectra with those of cataclysmic variables, the rapid in the surrounding medium by the wind from a young variability andlarge numbers of Be + white dwarfsys- tems predicted by the theory of evolution of massive binaries. 3The Be star HD 215227, likely counterpart of the gamma-ray In the remaining part of this report I shall concen- sourceAGLJ2241+4454,hasbeensuggestedasanewcandidate (Wiliamsetal.2010). trate on the properties and variability of BeXBO˙nly 7 (Teff, log g) = (30000, 3.5) X Per 1.05 10-9 1 y -2-1m A)10-10 Intensit000..89.955 no disc X Per c -1g s 10-11 Flux (er10-12 ndois cdisc ntensity34000000 disc Hα He 10-13 I X Per 2000 1000 10000 6400 6500 6600 6700 6800 Wavelength (A) Wavelength (A) Fig. 4 IRexcess and Hαemission are thetwo main observational characteristics of Be stars. when for the sake of the discussion a comparison with on the geometry (size, shape) and dynamics (veloc- the behaviour of other type of X-ray binaries may be ity and density laws) of the envelope (Hanuschik 1986, illustrative, will these other type of binaries be men- 1995; Dachs 1992). tioned. Figure 4 illustrates graphically these two properties for the system X Persei. When an equatorial disc is present the near-IR emission exceeds that predicted 2 Optical/IR properties of BeXB by model atmosphere (shown is a Kuruzc model with T = 30000 K and logg = 3.5) and emission lines are eff The optical and infrared flux of a BeXB is completely prominently seen. dominatedby the Be starcompanion. While the X-ray emissionfromBeXBprovidesinformationonthephysi- 2.1 Hα line profiles calconditionsinthevicinityofthecompactobject,the optical and infrared emission reveals the physical state The Hα line is the prime indicator of the circumstel- ofthe massdonorcomponent. Since the fuelthatpow- lar disc state. Hα emission lines can be morphologi- ers the X-rayemission, namely matter from a powerful callydividedintwoclasses(Hummel & Vrancken1995; stellar wind orfroma equatoriallyconcentrateddenser Silaj et al. 2010): class 1 are symmetric and includes disc, comes from the massive companion, observations symmetric double-peak, wine-bottle and shell profiles of BeXB in the optical/IR are crucial to understand andclass2, whichareasymmetricandshowvariability the physicalconditions underwhichthe neutronstaris ontimescalesofyears. Eachclassdoesnotrefertodif- accreting. ferentgroupsofsourcesbecauseindividualBestarscan The two main observational characteristics of Be change from one to the other. These changes are nor- stars are the emission spectral lines, as opposed to the mally slow (of the order of years to decades in isolated normal absorption photospheric lines and an excess of Be stars and months to years in BeXB). Symmetric IR emission. Both properties, line emission and IR ex- profilesarebelievedto be generatedinquasi-Keplerian cess,originatefromextendedcircumstellarenvelopesof discs (see e.g. Hummel 1994), while asymmetric pro- ionisedgassurroundingtheequatoroftheBstar. They files are associated with distorted density distributions result from free-free and free-bound emission from the (Hanuschik 1995; Hummel & Hanuschik 1997). disc,i.e. recombinationoftheopticalandUVradiation Most BeXB show asymmetric split Hα profiles. The from the central star (Woolf et al. 1970; Gehrz et al. peaks adopt the names of the relative position of their 1974). The assumption of a common origin for the centralwavelengths. Inaspectrumwithmonotonically line emission and IR excess is strongly supported by increasing wavelength values, the left peak is known thecorrelationbetweentheintensitiesofcontinuousIR as the ”blue” (or violet) peak, while the right peak is emission,asmeasuredeitherbycolourindices(J M), named the ”red” peak. V/R variability refers then to − (J K) or fluxes at a certain wavelength and the in- the variation of the relative strength of the blue to the − tensity ofthe Hαline, asmeasuredeither asequivalent redpeak. Thereforethe V/Rratio,definedasthe ratio widthorfluxes(Dachs & Wamsteker1982;Dachs et al. of violet-side to red-side peak intensities above contin- 1988; Neto & de Freitas Pacheco 1982). By studying uum in units of continuum intensity represents a mea- thelinevariabilityonecanobtainimportantconstraints sure of the asymmetry of the line. A more convenient 8 quantity is the logarithmic of this ratio, log(V/R), be- cause in this case, positive values of log(V/R) corre- spond to a blue-dominated profile and negative values to a red-dominated line. The top panel of Fig. 5 dis- plays the V/R variability of the BeXB LS I +61 235 (Reig et al. 2000). V/R variability can be explained in terms of a non- axisymmetrical equatorial disc in which a one-armed perturbation (a zone in the disc with higher density) propagates(Okazaki1991,1996,1997;Papaloizouet al. 1992; Savonije & Heemskerk 1993). Double-peak sym- metricprofilesareexpectedwhenthehigh-densitypart is behind or in front of the star, while asymmetric pro- files are seen when the high-density perturbation is on 1 odnenesistiydepaorfttohfetdhiescd.iscMpoerretuprrbeactisioelny,iswlhoecnatetdheonhigthhe- log(V/R)-00-..0551 side of the disc where the rotational motion is directed 9.6 away from us, we see enhanced red emission (V < 9.8 J 10 R), while when the high-density part is moving toward 10.2 the observer, blue-dominated profiles V > R are ex- 0.4 0.3 pected (Telting et al. 1994). For systems seen at high- J-K0.2 0.1 inclinationangle,thetwosymmetriccasescanbe read- -20 ily distinguished since the central depression between αH )-15 the two symmetric peaks would be more pronounced W(-10 E -5 (reachingorgoingbeyondthe continuum, the so-called 48500 49000 49500 50000 50500 51000 51500 MJD shellprofile),whenthe perturbationis betweenthe ob- serverandthestar. Thereasonisthatshellprofilesare Fig. 5 Top: V/R variability observed in the Hα line of thoughttobeduetoaperspectiveeffect,namely,when LS I +61 235. From Reig et al. (2000). Bottom: Evolution the line of sight toward the star probes the equato- of the V/R ratio, J magnitude, J −K colour and the Hα equivalentwidth in LS I +61 235. From Reig et al. (2000). rialdiscandself-absorptionisproduced(Rivinius et al. 2006). If the density perturbation revolves around the star in the same direction as the material in the disc stars, which are found in the range 2-11 years with an (progradeprecession),thentheV/Rsequencewouldbe averageof 7 years (Okazaki 1997). accordingtoTelting et al.(1994): V =R(perturbation behind the star) V > R V =R (perturbation 2.2 EW(Hα) vs infrared colours −→ −→ in front of the star, shell profile) V <R. −→ One prediction of the model is that no changes in If the infrared excess observed in Be stars is due to the slope of the infrared continuum are expected be- the same processes as those responsible for Balmer cause the V/R variations are not the result of changes line emission, namely absorption and subsequent re- in the radial gradient of the circumstellar gas. This is emission of the optical and UV light from the underly- exactly the behaviour that it is found in LS I +61 235 ingstarinthecircumstellarenvelope,thenacorrelation (bottom panelofFig.5). While the individualinfrared between IR colours and the strength of the hydrogen photometric bands changed (∆J ∆H ∆K 0.3 lines shouldbe expected. Suchcorrelationhas been re- ≈ ≈ ∼ magnitudes)theinfraredcolours(e.g. J K)remained portedforisolatedBestars(Dachs & Wamsteker1982; − unchanged (Reig et al. 2000). Neto & de Freitas Pacheco 1982; Dachs et al. 1988) V/R variability is also seen in other lines, like HeI andBeXB(Coe et al.1994,2005,althoughmixedwith 6678˚A.Since helium lines are generatedatsmaller disc isolated Be stars). Figure 6 shows the first such dia- radii than the hydrogen lines (Hummel & Vrancken gram made from BeXB only. It displays the IR colour 1995; Jaschek & Jaschek 2004), the asymmetry of the index(J K) asafunctionoftheHαequivalentwidth, 0 HeI line profiles indicates that the internal changes of − EW(Hα),forthesourceslistedinTable3. Theinfrared the disc are global, affecting its entire structure. colours were corrected for interstellar extinction. Only V/R quasi-periods in BeXB range from 1-5 yr (Ta- contemporaneous data (when the time difference be- ble 5) and are shorter than those seen in isolated Be tween the IR and optical observations was less than 9 100 eitherdiffuseinterstellarbands,disc-lossepisodesorby disentanglingthe circumstellarandinterstellarredden- ing (see Fabregat & Torrej´on 1998). Under these conditions, namely, simultaneity of the 10 optical and infrared data and used of the extinction- α) W(H corrected (J − K) index, the EW(Hα) − (J − K)0 E becomes a useful tool to estimate the extra reddening 1 caused by the circumstellar disc. Once a value of the EW(Hα) is known, one can look for the corresponding intrinsic (J K) in Fig. 6. By comparing this value − with that expected according to the spectral type, an -0.4 -0.2 0 0.2 0.4 0.6 estimatedofthedisccontributionontheindex(J K) (J-K)0 − can be obtained. The relatively large scatter, however, Fig.6 CorrelationbetweentheequivalentwidthoftheHα may limit the usefulness of the diagram. line andtheinfrared colour J−K. Different colours repre- It should be noticed that although the Hα and in- sent different systems. This correlation implies a common fraredemissioncorrelateasexpectedforacommonori- origin for theinfrared excessandthelineemission, namely, gin in the disc, the spatial extension and precise lo- thecircumstellar disc round theBe star. calisation in the disc are different. From long-baseline interferometric observations in the K′ band Gies et al. one month) were used. The second and third columns (2007)foundthatthe angularsize ofthe infraredemis- in Table 3 represent the amplitude of change in the sion is consistently smaller than that determined for infrared colour (J K) and the EW(Hα) over the ob- the Hα emission. In other words, the near-IR emis- − served range, whereas Fig. 6 plots the actual values. sion forms closer to the star than does the Hα emis- In Fig. 6 we have included only those sources for sion. Furthermore there is good evidence in several whichweareconfidentthatthecolourexcessE(B V) cases (Clark et al. 2001; Grundstrom et al. 2007) that − is free of circumstellar effects. Due to the surround- anincreaseindiscbrightnessoccursfirstinthenear-IR ing envelope, the use of photometric magnitudes and fluxexcessandlaterinHα asexpectedforanoutwardly colours to derive astrophysicalparameters, like E(B progressing density enhancement. This difference can − V), may be misleading because they are contaminated be explained if the dependence with disc radius of the by disc emission. The effect of the disc is to make Hα optical depth is less steep than that of the infrared the photometric indices to appear redder than a non- optical depth (Gies et al. 2007). emitting B star of the same spectral type. The disc emission makes a maximum contribution to the op- 2.3 Identification of the optical counterparts tical (B V) colour of a few tenths of a magnitude − (Dachs et al. 1988; Howells et al. 2001). The values of With the improved sensitivities of the currently opera- E(B V) used to produce Fig. 6 were obtained from tionalspacemissionsmanynewX-raysourcesarebeing − discovered. AboutonethirdofthetheX/γ-raysources inthe 4thINTEGRAL catalogue(Bird et al.2010)are Table 3 List of BeXB used in Fig. 6 with colour excess thoughttobeX-raybinaries,ofwhichhalfarebelieved and amplitudeof change in (J−K)0 and EW(Hα). to contain early-type companions. An optical identi- X-raysource E(B−V) ∆(J−K)0 ∆EW(Hα) fication is necessary to facilitate a complete study of (mag) (mag) (˚A) these systems. Without a knowncounterpart, observa- 4U 0115+63 1.65 0.8 20 tions are limited to X-ray energies, and hence our un- RX J0146.9+6121 0.93 0.7 10 derstandingofthestructureanddynamicsofthosesys- V 0332+53 1.88 0.4 6 tems that remain optically unidentified is incomplete. 4U 0352+30 0.39 1.1 18 RX J0440.1+4431 0.65 0.3 5 If the only available information is that providedby 1A 0535+262 0.75 1.0 15 an isolated X-ray detection then potential HMXBs are 4U 0728-25 0.73 0.1 2 selected as those exhibiting properties of a magnetised RX J0812.4-3114 0.65 0.2 5 neutron star, namely, X-ray pulsations and/or an ab- GRO J1008-57 1.90 1.0 15 sorbed power-law continuum spectrum with an expo- 1A 1118-616 0.90 1.5 25 nential cutoff at 10-30 keV and cyclotron absorption 4U 1145-619 0.29 2.8 40 features. Extrainformationonthenatureofthesource EXO 2030+375 3.8 1.1 15 SAXJ2103.5+4545 1.35 0.1 1 can be obtained if the long-term X-ray variability is 10 -5 SAX J2103.5+4545 -5.2 -5.4 αH R--5.6 -5.8 -6 0.5 1 1.5 2 2.5 B-V -4 IGR J01363+6610 -4.5 αH R- -5 -5.5 -6 1 1.5 2 2.5 B-V Fig. 7 Colour-colour diagrams and V-band images of the field around SAX J2103.5+4545 (top) and IGR J01363+6610 (bottom). The position of the confirmed optical counterpart in the colour-colour diagram is marked with a filled circle. Goodcandidatesarethosethatoccupytheupperpartofthecolour-colourdiagramandlieinsideorveryclosetotheX-ray uncertainty region. For example, the stars that are close to SAX J2103.5+4545 in the colour-colour diagram lie far away from thesatellite error circles. known (information provided by all-sky monitors, like dates. Furthermore, uncertainty regions are given to ASM RXTE or BAT SWIFT). The presence of regular a certain confidence level, hence it is possible that the and periodic outbursts or unexpected giant outbursts true optical companion is located close but outside the reachingEddingtonluminosityattheirpeaksmayindi- X-ray error circle. catethe presenceofaBestar. Likewise,erraticflaring, Photometric detection of Be stars can be performed i.e., X-ray variability with changes in the X-ray inten- by selecting colours directly relatedto the excess emis- sity by a factor 3-5 over a few minutes might indicate sionboth in the Hα line (”red” colour)andthe contin- thepresenceofastrongstellarwindfromanearly-type, uum(”blue”colour). Observingthroughanarrowfilter probably evolved, companion. centred on the Hα line and a wider filter also contain- However,a systemwith X-raydata only willremain ing this line, e.g. Johnson R filter (Reig et al. 2005), in the category of suspected HMXBs until a confirmed Sloan-r (Guti´errez-Soto 2006) would account for the optical identification is performed. The most obvious emission line, while Johnson B V (Reig et al. 2005) − observational features to look for is Hα emission and or Stro¨mgren b y (Guti´errez-Soto 2006) can be used − near-infrared excess emission. While the detection of asreddeningindicators. TheuseofStro¨mgrenb,y,and these two observational features does not guarantee narrow-bandHα photometrythrough(b y, y)colour- − that the source is indeed an OBe type star, it defi- magnitude and (b y, y Hα) colour-colourdiagrams − − nitely narrows the number of candidates to a handful isparticularlysuitabletoidentifyBestarsinopenclus- of sources. ters (Grebel et al. 1992; McSwain & Gies 2005). The size of the uncertainty in the location of the Stars with a moderately or large Hα excess can be X-ray source, the so-called error circle, determines the distinguished from the rest because they deviate from typeofobservationaltechniquetouse. Iftheerrorcircle thegeneraltrendandoccupytheupperleftpartsofthe is small (a few arcseconds) then the number of visible diagram. Be star are expected to show bluer colours, starsinthe regionisexpectedtobesmallanditispos- i.e.,low(B V)or(b y),because theyareearly-type − − sible to perform narrow-slit spectroscopic observations stars(althoughthey normallyappearredderthannon- and look for early-type, Hα emitting line stars. If the emittingB starsdue tothe circumstellardisc)andalso errorradius is large (>1 arcminute), then it is likely to larger(i.e., less negative) R Hα colours because they − includealargenumbe∼rofsourcesandhencenarrow-slit showHαinemission. Oncethenumberofcandidatesis spectroscopy becomes impractical. In this case colour- reduced to a few systems, narrow-slitspectroscopy be- colour diagrams can be used to identify good candi- comes feasible. The final step is to obtain a spectrum

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