X-ray Binaries

2.2 MB·

by  Jorge Casares

X-Ray Binaries
J.Casares,P.G.Jonker,andG.Israelian
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5 Abstract ThischapterdiscussestheimplicationsofX-raybinariesonourknowl-
2 edgeofTypeIbcandTypeIIsupernovae.X-raybinariescontainaccretingneutron
stars and stellar–mass black holes which are the end points of massive star evolu-

]
E tion.Studyingtheseremnantsthusprovidescluestounderstandingtheevolutionary
processes that lead to their formation. We focus here on the distributions of dy-
H
namicalmasses,spacevelocitiesandchemicalanomaliesoftheircompanionstars.
.
h These three observational features provide unique information on the physics of
p
corecollapseandsupernovaeexplosionswithininteractingbinarysystems.Thereis
-
o suggestiveevidenceforagapbetween≈2-5M(cid:12) intheobservedmassdistribution.
r Thismightberelatedtothephysicsofthesupernovaexplosionsalthoughselections
t
s effectsandpossiblesystematicsmaybeimportant.Thedifferencebetweenneutron
a
starmassmeasurementsinlow-massX-raybinaries(LMXBs)andpulsarmassesin
[
high-mass X-ray binaries (HMXBs) reflect their different accretion histories, with

1 the latter presenting values close to birth masses. On the other hand, black holes
v inLMXBsappeartobelimitedto(cid:46)12M becauseofstrongmass-lossduringthe
0 (cid:12)
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J.Casares
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InstitutodeAstrof´ısicadeCanarias,E–38205LaLaguna,S/CdeTenerife,Spain.
0
DepartamentodeAstrof´ısica,UniversidaddeLaLaguna,E–38206LaLaguna,S/CdeTenerife,
.
1 Spain.
0 DepartmentofPhysics,Astrophysics,UniversityofOxford,KebleRoad,OxfordOX13RH,UK
7 e-mail:[email protected]
1
P.G.Jonker
:
v SRON,NetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CA,Utrecht,TheNether-
i lands.
X DepartmentofAstrophysics/IMAPP,RadboudUniversityNijmegen,P.O.Box9010,6500GL,Ni-
r jmegen,TheNetherlands.
a e-mail:[email protected]
G.Israelian
InstitutodeAstrof´ısicadeCanarias,E–38205LaLaguna,S/CdeTenerife,Spain.
DepartamentodeAstrof´ısica,UniversidaddeLaLaguna,E–38206LaLaguna,S/CdeTenerife,
Spain.e-mail:[email protected]
1
2 J.Casares,P.G.Jonker,andG.Israelian
windWolf-Rayetphase.Detailedstudiesofalimitedsampleofblack-holeX-raybi-
nariessuggestthatthemoremassiveblackholeshavealowerspacevelocity,which
couldbeexplainediftheyformedthroughdirectcollapse.Conversely,theformation
oflow-massblackholesthroughasupernovaexplosionimpliesthatlargeescapeve-
locities are possible through ensuing natal and/or Blaauw kicks. Finally, chemical
abundancestudiesofthecompanionstarsinsevenX-raybinariesindicatetheyare
metal-rich(allexceptGROJ1655-40)andpossesslargepeculiarabundancesofα-
elements.Comparisonwithsupernovamodelsis,however,notstraightforwardgiven
currentuncertaintiesinmodelparameterssuchasmixing.
1 Introduction
X-raybinariescontaincompactstellarremnantsaccretingfrom”normal”compan-
ionstars.Therefore,theyprovideidealopportunitiesforprobingthecore-collapse
ofmassivestarsinabinaryenvironmentandarethusabletoconstrainthephysicsof
TypeIbcandTypeIIsupernovae.Thesecompactremnantsarerevealedbypersis-
tent/transientX-rayactivitywhichistriggeredbymassaccretion.Observationally,
theycomeinthreeflavors–pulsars,neutronstarsandblackholes–thatarepaired
withcompanion(donor)starsofawiderangeofmasses.Historically,X-raybinaries
havebeenclassifiedaccordingtothedonormassaseitherLowMassX-rayBinaries
(LMXBs)orHighMassX-rayBinaries(HMXBs).Theformerarefueledbyaccre-
tion discs supplied by a (cid:46)1 M Roche-lobe filling star while HMXBs are mostly
(cid:12)
feddirectlyfromthewindsofa(cid:38)10M companion.TheydisplaydistinctGalactic
(cid:12)
distributions associated with Population I and Population II objects, with HMXBs
lying along the Galactic plane and LMXBs clustering towards the Galactic bulge
and in globular clusters [32] (Fig. 1 ). A handful of X-ray binaries with ≈1−3
M Roche-lobefillingcompanionsaresometimesreferredtoasIntermediateMass
(cid:12)
X-rayBinaries(IMXBs).ForacomprehensivereviewonX-raybinarieswereferto
[9].
The type of X-ray activity observed is determined by (i) the mass transfer rate
fromthedonor,(ii)themagneticfieldofthecompactstar,and(iii)theX-rayheating
oftheaccretiondiscbytheaccretionluminosity.Theinterplaybetweenthesethree
quantitiesexplainswhyblackholeremnantsaremostlyfoundintransientLMXBs,
neutronstarsinpersistentLMXBsandpulsarsinHMXBs.Inrecentyearswehave
seen the discovery of pulsars with millisecond spin periods in transient LMXBs.
These are considered a missing link in X-ray binary evolution, with neutron stars
beingspunupbysustainedaccretiontobecomerecycledpulsars[1,85].Adetailed
review of X-ray binary evolution with the variety of evolutionary paths and end
productscanbefoundinChap.7.13ofthisbook.
X-ray binaries present ideal laboratories for examining the physics of the su-
pernovaexplosionswhichformedtheircompactobjects.Theorbitalmotionofthe
stellar companions can be used to weigh the masses of the supernova remnants.
X-RayBinaries 3
Abundance anomalies are often seen in the companion star atmospheres, demon-
strating chemical pollution by the supernova ejecta. And the spatial motion of the
binarypossessesinformationonthekickvelocityimpartedbytheexplosionitself.
Thesethreetopics(dynamicalmasses,kickvelocitiesandchemicalanomalies)and
theirimpactonourunderstandingofTypeIbcandTypeIIsupernovaearethescope
ofthischapterandwillbepresentedinturn.
Fig.1 GalacticdistributionofHMXBs(top)andLMXBs(bottom).OpencirclesindicateLMXBs
inGlobularClusters.From[81].
2 RemnantMasses
Thedistributionofmassesofcompactremnantscontainstheimprintsofthephysics
of the supernova explosions. Various aspects, such as the explosion energy, mass
cut, amount of fallback or the explosion mechanism itself are important for the
finalremnantmassdistribution.Bybuildingthemassspectrumofcompactobjects
in X-ray binaries we can therefore obtain new insights onto the physics of core-
collapse in Type Ibc and Type II supernovae. In principle, precise masses can be
extractedfromeclipsingdouble-linespectroscopicbinariesusingsimplegeometry
andKepler’slawsbutthisisnotoftenthecaseinX-raybinaries.Notethatneutron
star masses in binary radio pulsars are already covered in Chap. 7.4 of this book
andhencearenotdiscussedhere.Itshouldalsobenotedthattheaccretionprocess
4 J.Casares,P.G.Jonker,andG.Israelian
responsibleforlightinguptheX-raybinariescaninprinciplechangesignificantly
theneutronstarmassinsystemswheresufficienttimeisavailablesuchasneutron
starsinoldLMXBs.Ontheotherhand,theaccretedmassistoolowtoaltertheBH
mass significantly and similarly, the neutron star mass in short lived HMXBs can
alsonotbechangedsignificantly.
2.1 PulsarMassesinHMXBs
Pulsarsineclipsingbinariespresent,inprinciple,thebestprospectsforaccuratede-
terminationofremnantmasses.TheDopplershiftofthedonor’sphotosphericlines,
combined with timing delays of the neutron star pulse, allows us to measure the
projectedorbitalvelocitiesofthetwobinarycomponents(K andK respectively)
opt X
thus making them double-lined binaries. If the pulsar is eclipsed by the massive
donor(a≈40%chanceinincipientRoche-lobeoverflowingsystems)thenthein-
clinationangleiisgivenby
(cid:112)
1−(R /a)2
opt
sini= (1)
cosθ
whereθ istheeclipsehalf-angle,athebinaryseparationandR thestellarradius.
opt
Thelattercanbeapproximatedbysomefractionβ ≤1oftheeffectiveRochelobe
radius R , also known as the stellar ”filling factor”, while R /a is purely a
Lopt Lopt
functionofthebinarymassratioQ=M /M =K /K andthedegreeofstellar
X opt opt X
synchronizationΩ (usuallyΩ ≈1).Thestellarmassescanthenbesolvedfromthe
massfunctionequations
K3P(1−e2)3/2
M = X (1+Q)2 (2)
opt 2πGsin3i
K3 P(1−e2)3/2(cid:18) 1(cid:19)2
opt
M = 1+ (3)
X 2πGsin3i Q
wherePstandsforthebinaryperiodandetheorbitaleccentricity.Thismethodhas
producedninepulsarmasseswithrelativelyhighprecisionwhichwelistinTable1.
Themajorsourceofuncertaintyarisesfromthecombinedeffectofvariablestellar
wind,tidalpulsationsandX-rayirradiationwhichdistorttheabsorptionprofilesand
hence the radial velocity curve of the optical companion (e.g. [62, 68]). Although
notapulsar,wehavealsoincludedinthissectionaremnantmassdeterminationfor
the eclipsing HMXB 4U 1700-37. With a mass significantly higher than the nine
HMXBs pulsars, the nature of the compact object in this system is unclear and a
low-mass black hole cannot be dismissed. In any case, the quoted mass should be
regarded as somewhat less secure because it rests upon the spectroscopic mass of
theopticalcompanion(see[11]fordetails).
X-RayBinaries 5
Interestingly, the largest known population of pulsar X-ray binaries (over 100)
belongtothesubclassofBe/X-raybinaries[66].Thesesystemsgenerallyhavevery
wideandeccentricorbits,withtheneutronstaraccretingmaterialfromthecircum-
stellarBediscduringperiastronpassagesorthroughepisodicdiscinstabilityevents.
Unfortunately, the scarcity of eclipsing systems and the very long orbital periods
makesreliablemassdeterminationinBe/X-raybinariesextremelydifficult.
Table1 PulsarandNeutronStar(NS)massesinX-rayBinaries†
Object X-rayBinaryClass Remnant Mass(M(cid:12)) References
OAO1657-415 HMXB/persistent X-raypulsar 1.42±0.26 [43]
SAX18027-2016 ,, ,, 1.2-1.9 [42]
EXO1722-363 ,, ,, 1.55±0.45 [41]
4U1538-52 ,, ,, 1.00±0.10
SMCX-1 ,, ,, 1.04±0.09
VelX-1 ,, ,, 1.77±0.08
LMCX-4 ,, ,, 1.29±0.05
CenX-3 ,, ,, 1.49±0.08
4U1700-37 ,, ? 2.44±0.27 [11]
HerX-1 IMXB/persistent X-raypulsar 1.07±0.36
CygX-2 LMXB/persistent NS 1.71±0.21 [6]
V395Car ,, ,, 1.44±0.10
ScoX-1 ,, ,, <1.73 [44]
XTEJ2123-058 LMXB/transient ,, 1.46+0.30 [78]
−0.39
CenX-4 ,, ,, 1.94+0.37 [71]
−0.85
4U1822-371 ,, X-raypulsar 1.52-1.85 [53]
XTEJ1814-338 ,, msec,, 2.0+0.7 [83]
−0.5
SAXJ1808.4-3658 ,, ,,,, <1.4
HETE1900.1-2455 ,, ,,,, <2.4
†Adoptedfrom[89]and[63],unlessotherwisestatedinthereferencecolumn.
2.2 NeutronStarMassesinLMXBs
NeutronstarsinLMXBsdonotusuallypulse(with4U1822-371andahandfulof
millisecondpulsarsastheonlyexceptions)andtheirradialvelocitycurvesarethus
notavailable.Onlythemassfunctionofthecompactstarisattainablethroughthe
radialvelocitycurveoftheopticalcompanion(eq.3).Inthesecasesitisstillpos-
sible to derive reliable masses by exploiting the fact that the low-mass donor star
overflowsitsRochelobeandissynchronizedinacircularorbit(whichinturnim-
pliesΩ =1,e=1).Thisisreasonableassumptiongiventhelonglifetimesandshort
circularizationtimescalesexpectedinLMXBs[87].Onthisbasis,thebroadeningof
thedonorabsorptionlinesVsinidependsonbinarymassratioq=Q−1 according
to[82]
6 J.Casares,P.G.Jonker,andG.Israelian
Vsini/K (cid:39)0.462q(1/3)(1+q)(2/3) (4)
opt
while its orbital light curve (governed by tidal distortions) correlates with incli-
nation. Therefore, the detection of the faint donor star in LMXBs ensures a full
dynamical solution which makes this technique feasible for transient LMXBs in
quiescence(i.e.whenaccretionishaltedandX-rayemissionisweak)orpersistent
LMXBswithlongorbitalperiodsandthusluminouscompanionstars.
In the case of persistent LMXBs with short periods (cid:46)1 d the companion star
is totally overwhelmed by the accretion luminosity. However, some constraints on
stellarmassescanstillbederivedthroughtheBowentechniquewhichemploysflu-
orescencelinesexcitedontheX-rayheatedfaceofthedonorstar[75].Theradial
velocitycurvesoftheBowenlinesarebiasedbecausetheyarisefromtheirradiated
faceofthestarinsteadofitscenterofmass.Therefore,aK-correctionneedstobe
appliedinordertorecoverthetruevelocitysemi-amplitudeK .TheK-correction
opt
parametrizesthedisplacementofthecenteroflightwithrespecttothedonor’scen-
terofmassthroughthemassratioanddiscflaringangleα,withthelatterdictating
thesizeofthediscshadowprojectedovertheirradiateddonor[52].Extrainforma-
tiononqandα isthusrequiredtomeasuretherealK .Furtherlimitstoneutron
opt
starmassescanbesetifthebinaryinclinationiswellconstrainedthrougheclipses
(e.g. 4U 1822-371). It is interesting to note that the Bowen technique, despite its
limitations, has enabled the first dynamical constraints in persistent LMXBs since
theirdiscovery50yearsago.ThebestneutronstarmassesinLMXBsobtainedby
meansofthesetechniquesarealsolistedinTable1.
2.3 BlackHoleMasses
ThegreatmajorityofaccretingblackholesarefoundintransientLMXBs/IMXBs,
as proved by dynamical studies. Relatively precise masses have been measured in
quiescence through exploiting the photometric and spectroscopic detection of the
companion star. These are listed in Table 2. Only in the case of GX 339-4 is the
constraintontheblackholemassaidedbytheBowentechnique.Insomecasesun-
certaintiesarequitelargeowingtopossiblesystematicsaffectingthedetermination
ofthebinaryinclinationangle.Inothers,onlyarobustlowerlimittotheblackhole
mass is secured by the spectroscopic mass function combined with the absence of
X-rayeclipses.
FiveadditionalblackholeshavenowbeenestablishedinHMXBs.Althoughthe
companionstarunderfillsitsRochelobeinthesesystems,extrainformationonthe
stellarradiusisgrantedbyveryprecisedistancedeterminations((cid:46)5%e.g.through
VLBI parallax for Cyg X-1) coupled with the observed apparent brightness and
effective temperature. Furthermore, in the case of M33 X-7, eclipses of the X-ray
sourcebythedonorstarprovideadditionaltightconstraintsontheinclinationwhich
resultsinoneofthelargestaccuratelyknownblackholemasses.
X-RayBinaries 7
Table2 BlackHoleMassesinX-rayBinaries†
Object X-rayBinaryClass Mass(M(cid:12)) References
GRS1915+105 LMXB/transient 12.4+2.0 [65]
−1.8
V404Cyg ,, 9.0+0.2
−0.6
BWCir ,, >7.0
GX339-4 ,, >6.0
XTEJ1550-564 ,, 7.8−15.6
H1705-250 ,, 4.9−7.9
GS1124-684 ,, 11.0+2.1 [88]
−1.4
GS2000+250 ,, 5.5−8.8
A0620-00 ,, 6.6±0.3
XTEJ1650-500 ,, 4.0−7.3
GRS1009-45 ,, >3.6 †Adopted
XTEJ1859+226 ,, >5.42
GROJ0422+32 ,, >1.6
XTEJ1118+480 ,, 6.9−8.2
XTEJ1819.3-2525 IMXB/transient 6.4±0.6 [40]
GROJ1655-40 ,, 5.4±0.3
4U1543-475 ,, 2.7−7.5
CygX-1 HMXB/persistent 14.8±1.0
LMCX-1 ,, 10.9±1.4
LMCX-3 ,, 7.0±0.6 [58]
M33X-7 ,, 15.7±1.5
MWC656 HMXB/transient(?) 3.8−5.6 [7]
from[8]unlessotherwisestatedinthereferencecolumn.LowerlimitsforBWCir,GRS1009-45,
XTEJ1859+226andGROJ0422+32arebasedontheabsenceofeclipses,combinedwithupdated
determinationsofthemassfunctionandq(whenavailable).ThelowerlimitonGX339-4isbased
onthelackofX-rayeclipsesplusconstraintsprovidedbytheK-correction.
NotethatwehaveexcludedmassmeasurementsforthetwoextragalacticHMXBs
NGC300X–1andIC10X–1.Thisisbecausetheserelyonradialvelocitycurves
oftheHeIIλ4686windemissionlineandanassumedmassfortheWolf-Rayetstar.
Differentgroupshavereportedconflictingresultswhichrangefromcanonicalneu-
tronstarstothelargestblackholemassesmeasuredsofarandarehenceunreliable.
ThetableincludesMWC656,thefirstblackholecompaniontoaBestar[7].Here
theblackholemassreliesonthespectroscopicmassoftheopticalcompanionand
theradialvelocitycurvesofthetwostars,extractedfromthedynamicsofcircum-
stellargaseousdiscs.Acriticalreviewofblackholemassdeterminations,including
potentialsystematiceffects,ispresentedin[8].
2.4 TheMassSpectrum.ImplicationsforSupernovaeModels
Figure 2 presents the observed masses of neutron star and black hole remnants in
X-raybinaries,asinTables1and2.Notethatwehavehereexcludedneutronstar
8 J.Casares,P.G.Jonker,andG.Israelian
88
66
44
22
00
00 55 1100 1155
Fig.2 Top:compactremnantmassesmeasuredinX-raybinaries.Neutronstarsandblackholesare
indicatedinblackandredcolours,respectively.4U1700-37isplottedindotted-stylelinebecause
thenatureofthecompactstarisuncertain.ThehorizontaldottedlinedividesLXMBs/IMXBsfrom
HMXBs.Bottom:Observeddistributionofneutronstarsandblackholemasses.
massesinradiopulsarsandbinarymillisecondpulsarsasthesearecoveredbyChap.
7.4. The bottom panel displays the number distributions of neutron stars (black)
and black holes (red) masses, excluding upper/lower limits. Three main features
seem to be drawn from the plot, namely (1) neutron star masses tend to be larger
in LMXBs/IMXBs (mass average of 1.54±0.16 M ) than in HMXBs (1.34±0.26
(cid:12)
M ),(2)adearthofremnantsorgapappearsbetween∼2-5M ,and(3)themost
(cid:12) (cid:12)
massiveblackholes(∼15M )arefoundinHMXBs.
(cid:12)
Feature(1),althoughtentative,couldbeamanifestationofthepulsarrecycling
scenario.Thedifferenceinneutronstarmasses,ifconfirmed,wouldstemfromdif-
ferentbinaryevolutionhistories,withneutronstarsinLMXBshavingexperienced
significant accretion over extended periods of time. This interpretation would be
further supported by indications that pulsar mass decreases with spin period [89].
NeutronstarsinHMXBsarelittlemodifiedbyaccretionand,thus,theirmassesare
expectedtolieclosertotheirbirthvalues.Andindeed,boththemeananddispersion
X-RayBinaries 9
oftheneutronstarmassdistributioninHMXBsarefoundtoagreewiththeoretical
expectationsofcore-collapsesupernovae[60].Constraintsonneutronstarforming
supernovae do seem to be provided by two distinct populations of X-ray pulsars
in Be/X-ray binaries; short P pulsars with short orbital periods and low eccen-
spin
tricitieswouldbeproducedbyelectron-capturesupernovaewhilelongP pulsars
spin
with long orbital periods and high eccentricities in iron-core-collapse supernovae
[33].Theformerpulsarsarenaturallyexpectedtobelessmassive((cid:46)1.3M )but,
(cid:12)
unfortunately, this cannot yet be tested because of the lack of precise neutron star
massdeterminationsinBe/X-raybinaries.
Feature (2) is a statiscally robust property of the mass spectrum (see [15] and
referencestherein).Thelackofcompactremnantsbetween∼2-5M contrastswith
(cid:12)
numericalsimulationsofsupernovaexplosionsby[16]thatleadtocontinuousdis-
tributionsandtypicalexponentialdecays.Thesesimulations,however,arebasedon
single star populations with a heuristic treatment of binarity through Wolf-Rayet
winds following common envelope evolution. In order to accommodate the evi-
denceofamassgap,adiscontinuousdependenceofexplosionenergywithprogen-
itor mass seems unavoidable. In this context, it has been proposed that convection
(Rayleigh-Taylor)instabilities,growingwithin200msaftercorebounce,cansuc-
cessfullyrevivethesupernovashockandtriggertheexplosion,therebycausingthe
gap (see Fig. 3, but see also [79] for a different interpretation based on neutrino-
driven explosion models). Alternatively, a gap can be produced if red super-giant
starsof≈17-25M sufferafailedsupernovaexplosion,leavingaremnantwiththe
(cid:12)
mass of the He core while ejecting the weakly bound H envelope [34]. This inter-
pretationappearsattractivebecauseinturnitprovidesanexplanationforthedeficit
ofhigh-massprogenitorsseeninpre-explosionimagesofTypeIIpsupernovae[73].
On the other hand, it fails to account for the peculiar abundance of α-elements
detectedinthecompanionstarswhichdemandssignificantcontaminationfromsu-
pernovaejecta(seeSect.4).Itisalsounclearhowverywidebinarieswithsuchred
supergiantscanevolvetoformthecompactblackholebinariesthatweseetoday.
Feature (3) most likely reflects different binary evolutionary paths, with black
holes in LMXBs being limited to (cid:46)12 M by severe mass loss from the Wolf-
(cid:12)
Rayetprogenitorafterthecommonenvelopephase[16].Conversely,blackholesin
HMXBscangrowfrommoremassivestars,especiallyinlow-metallicityenviron-
mentssuchasinthecaseofM33X-7.Furthermore,itispossiblethattheprogenitor
starevolvesthroughtheHeburningphasestillembeddedintheHenvelope(caseC
masstransfer),thussufferinglesswindmass-loss[4].However,itshouldbenoted
that some aspects of binary and massive stellar evolution (e.g. radial expansion,
windmass-lossrates,efficiencyofcommonenvelopeejection)arestillquiteuncer-
tain, which certainly limits our understanding of the formation of X-ray binaries.
Atthispoint,theimpactofsystematicuncertaintiesinthedeterminationofbinary
inclinationanglesshouldalsobestressed,asexemplifiedbythelargedispersionof
valuesreportedbyindependentgroupsonindividualsystems(see[8]).Ignoringthe
role of systematics can lead to overestimated black hole masses and hence a bias
intheobserveddistribution[35].Inaddition,thesampleofX-raybinarieswithdy-
10 J.Casares,P.G.Jonker,andG.Israelian
Fig.3 ObservedmassdistributionofcompactobjectsinX-raybinaries(top),comparedtotheo-
reticaldistributionscomputedfordifferentsupernovamodels(bottom).Themassgapcanbere-
producedonlyifturbulentinstabilitiesgrowrapidly.Slowgrowinginstabilitiesleadtosignificant
fallbackthatwouldfillthemassgap.From[2].
namicalmassdeterminationsisalmostcertainlypronetocomplexselectioneffects
(both evolutionary and observational) not completely understood. For instance, it
maybepossiblethatlow-massblackholesbecomeunboundbythesupernovaex-
plosionorarehiddeninveryfaint(butpersistent)X-raybinaries.ThesystemMWC
656mightbeitselfamemberofahiddenpopulationofveryfaintlow-massblack
holes. More observational work is required to enlarge the sample of secure black
holemassesbeforetheobserveddistributioncanbedefinitivelyusedtoilluminate
thepropertiesofthesupernovaengine.