Probing stellar winds and accretion physics in high-mass X-ray binaries and ultra-luminous X-ray sources with LOFT

6.7 MB·English

by  M. Orlandini

Probing stellar winds and accretion physics in
high-mass X-ray binaries and ultra-luminous
X-ray sources with LOFT
5
1 White Paper in Support of the Mission Concept of the
0
2
Large Observatory for X-ray Timing

n
a
J

6
Authors
1

] M.Orlandini1,V.Doroshenko2,L.Zampieri3,E.Bozzo4,A.Baykal5,P.Blay6,
E
M.Chernyakova7,R.Corbet8,A.D’Aì,T.Enoto9,16,C.Ferrigno4,M.Finger10,
H
D.Klochkov2,I.Kreykenbohm11,12,S.C.Inam13,P.Jenke14,J.-C.Leyder15,N.Masetti1,
.
h
A.Manousakis16,T.Mihara17,B.Paul18,K.Postnov19,P.Reig20,P.Romano21,A.Santangelo2,
p
- N.Shakura19,R.Staubert2,J.M.Torrejón22,R.Walter4,J.Wilms11,12,C.Wilson-Hodge23
o
r
t
s
a 1 INAF/IASF-Bologna,viaGobetti101,I-40129Bologna,Italy 13DepartmentofElectricalandElectronicsEngineering,Baskent
[ 2 InstitutfürAstronomieundAstrophysik,Sand1,72076Tübingen, University,Ankara06810,Turkey
Germany 14UniversityofAlabamainHuntsville,301SparkmanDrive,
2
3 INAF-AstronomicalObservatoryofPadova,I-35122Padova,Italy Huntsville,Alabama,USA
v 4 ISDC,UniversityofGeneva,Chemind’Écogia16,1290Versoix, 15ResearchFellowwithintheScienceOperationsDepartment,Eu-
7
Switzerland ropeanSpaceAgency(ESA),EuropeanSpaceAstronomyCentre
7 5 PhysicsDepartment,MiddleEastTechnicalUniversity,Ankara (ESAC),E-28691VillanuevadelaCañada,Madrid,Spain
7 06531,Turkey 16CentrumAstronomiczneim.M.Kopernika,Bartycka18,PL-00716
2 6 ImageProcessingLaboratoryUniversityofValenciaPOBOX Warszawa,Poland
0 22085,E-46071,Valencia,Spain 17MAXIteam,RIKEN,2-1Hirosawa,Wako,Saitama,351-0198,
. 7 DublinInstituteforadvancedstudies,31FitzwilliamPlace,Dublin Japan
1 2,Ireland 18RamanResearchInstitute,Bangalore560080,India
0 8 UniversityofMaryland,BaltimoreCounty,Baltimore,MD21250, 19MoscowM.V.LomonosovStateUniversity,SternbergAstronomi-
5 USA calInstitute,119992Moscow,Russia
1 9 NASAGoddardSpaceFlightCenter,AstrophysicsScienceDivi- 20IESL,FoundationforResearchandTechnology-Hellas,GR-71110
: sion,Code662,Greenbelt,MD20771,USA Heraklion,Crete,Greece
v 10NationalSpaceScienceandTechnologyCenter,320Sparkman 21INAF,IstitutodiAstrofisicaSpazialeeFisicaCosmica–Palermo,
i Drive,Huntsville,AL,USA viaU.LaMalfa153,90146Palermo,Italy
X 11Dr.KarlRemeis-SternwarteBamberg,Sternwartstrasse7,96049 22InstitutoUniversitariodeFísicaAplicadaalasCienciasylasTec-
r Bamberg,Germany nologías,UniversidaddeAlicante,E03080Alicante,Spain
a 12ErlangenCentreforAstroparticlePhysics,Erwin-Rommel-Str.1, 23AstrophysicsOffice,ZP12,NASAMarshallSpaceFlightCenter,
91058Erlangen,Germany Huntsville,AL35812,USA
ProbingstellarwindsandaccretionphysicsinHMXBsandULXswithLOFT
Preamble
TheLargeObservatoryforX-rayTiming,LOFT,isdesignedtoperformfastX-raytimingandspectroscopy
with uniquely large throughput (Feroci et al., 2014). LOFT focuses on two fundamental questions of
ESA’s Cosmic Vision Theme “Matter under extreme conditions”: what is the equation of state of ultra-
dense matter in neutron stars? Does matter orbiting close to the event horizon follow the predictions of
generalrelativity? ThesegoalsareelaboratedinthemissionYellowBook(http://sci.esa.int/loft/
53447-loft-yellow-book/)describingtheLOFT missionasproposedinM3,whichcloselyresembles
theLOFT missionnowbeingproposedforM4.
TheextensiveassessmentstudyofLOFT asESA’sM3missioncandidatedemonstratesthehighlevel
ofmaturityandthetechnicalfeasibilityofthemission,aswellasthescientificimportanceofitsunique
core science goals. For this reason, the LOFT development has been continued, aiming at the new M4
launch opportunity, for which the M3 science goals have been confirmed. The unprecedentedly large
effectivearea,largegrasp,andspectroscopiccapabilitiesofLOFT’sinstrumentsmakethemissioncapable
of state-of-the-art science not only for its core science case, but also for many other open questions in
astrophysics.
LOFT’s primary instrument is the Large Area Detector (LAD), a 8.5m2 instrument operating in the
2–30keVenergyrange,whichwillrevolutionisestudiesofGalacticandextragalacticX-raysourcesdown
totheirfundamentaltimescales. ThemissionalsofeaturesaWideFieldMonitor(WFM),whichinthe
2–50keVrangesimultaneouslyobservesmorethanathirdoftheskyatanytime,detectingobjectsdownto
mCrabfluxesandprovidingdatawithexcellenttimingandspectralresolution. Additionally,themissionis
equippedwithanon-boardalertsystemforthedetectionandrapidbroadcastingtothegroundofcelestial
brightandfastoutburstsofX-rays(particularly,Gamma-rayBursts).
This paper is one of twelve White Papers that illustrate the unique potential of LOFT as an X-ray
observatoryinavarietyofastrophysicalfieldsinadditiontothecorescience.
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ProbingstellarwindsandaccretionphysicsinHMXBsandULXswithLOFT
1 Summary
HighmassX-raybinaries(HMXBs)constituteasizeablefractionofthebrightX-raysourcesintheMilkyWay
andinothernearbygalaxies. Intheseobjects,thebulkoftheX-rayemissionisduetotheaccretionofmaterial
lostbyamassivecompanion(&10 M )ontoeitheraneutronstar(NS)orablackhole(BH).Dependingmainly
(cid:12)
onthenatureofthecompanion,accretioncanbemediatedbyadiskorthroughastellarwind.
Supergiant,hypergiant,andWolf-Rayetstarshavethedensest,fastest,andhighlystructuredwinds. These
windscangiverisetoanextremeX-rayvariabilitywhenaccretedontoacompactobject. Accretionfromthe
milder winds of moderately massive red giants (&1 M ) also produces a substantial X-ray variability. Our
(cid:12)
understandingoftherelationbetweentheX-rayvariabilityandstellarwindpropertieshasbeenlimitedsofarby
thelackofX-rayfacilitiesfeaturingsimultaneouslyalargecollectingareaandgoodenergyandtimingresolution.
Accuratespectralandtimingvariabilitystudiesofwind-fedbinariesrequire,atpresent,longintegrationtimes,
leavingthestudyattypicaldynamicaltosub-dynamicaltimescaleslargelyunexplored.
YoungNSs(typicallyfew106 yr)hostedinHMXBsareknowntopossessstrongmagneticfields(&1012 G)
whichchanneltheaccretingmaterialfromdistancesaslargeas 103-104 kmdowntotheirsurfaces. Thisleads
∼
totheformationofextendedaccretioncolumnsandcomplexpulseprofileswitharemarkableenergydependence.
Ourunderstandingofthephysicsofthis“magnetosphericaccretion”hasbeenhamperedsofarbytheinability
to reveal changes in the source spectral and timing properties within integration times shorter than a single
sourcepulseperiod. Gainingaccesstosuchshortintegrationtimeswillpermitinvestigationofthephysicsofthe
accretioncolumns,mapthetopologyoftheNSmagneticfield,andultimatelyrevealthestateofmatterunder
suchextremeconditions.
Thedynamicsofplasmaatthediskinnerboundaryofdisk-fedNSHMXBswasalsopoorlyinvestigatedso
far,asinthesesystemsthediskistruncatedatsizeabledistancesfromthecompactobject(&1000km)andthe
timing/spectralsignaturesproducedbythemovingplasmaareweaklydetectable. Asignificantimprovementin
thecollectingareasofcurrentlyavailableX-rayfacilitiesisneededtoopen-upstudiesinthisfieldandtransform
currenttentativedetectionsindiagnostictools.
Largeaccretiontorques,thatalternativelyslow-downorspin-uptheNS,havebeenmeasuredonbothwindand
disk-fedHMXBsontimescalesthatrangefromyearstodecades. Thesetorquesandtheirreversalsresultfrom
theyetunknowncouplingbetweentheNSmagneticfieldandtheaccretingmaterial. Severalmodelsarestill
debated. PursuingthelongtermmonitoringofHMXBsisthusmandatorytosecurecontinuityinthesestudies.
Ultra-luminousX-raysources(ULXs)arealsooftenfoundinyoungstellarenvironments(see,e.g.,Liuetal.,
2007;Griséetal.,2008,2011)andinatleasttwocasestheyhavebeendirectlyassociatedwithHMXBs(Liu
et al., 2013; Motch et al., 2014). These sources are a challenge of modern X-ray astronomy, as the physical
mechanismsproducingtheirunusuallyhighX-rayluminosityisstilldebated. Thisispartlyduetothelackof
highSNRX-raytiming/spectraldata.
Thankstotheuniquecombinationofcollectingareaandenergyresolutioninawideenergyband(2-50keV),
togetherwithanunprecedentedlargefield-of-viewimagingmorethan1/3oftheskyatonce,theinstruments
on-boardLOFT (Ferocietal.,2014)willdramaticallyopen-upperspectivesforresearchinallabovementioned
fields. Inparticular:
the LAD will grant for the first time access to accurate spectral and timing variability studies on
•
dynamicalandsub-dynamicaltimescalesforwind-fedanddisk-fedHMXBs. Thiswillovercomethe
longstandingdifficultyindisentanglingphysicalprocessesoccurringinstellarwinds,accretioncolumns,
andthestronglymagnetizedenvironmentsofyoungpulsars. LADobservationswillmapthemagnetic
fieldtopologyofyoungNSsandinvestigatetheirmicrosecondvariability. Detailedorbitalmonitoring
observationswillbepossibleforalargenumberofγ-ray-loudandγ-ray-quiteHMXBs,astheLADcan
measurechangesintheiremissionpropertieswithinintegrationtimesasshortas1-10%comparedto
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ProbingstellarwindsandaccretionphysicsinHMXBsandULXswithLOFT
thoserequiredwiththecurrentlyandpastX-rayfacilities.
UnderstandingthetimingpropertiesandsearchingforperiodicitiesinbrightULXs. TheLADwillbe
•
abletoperformadetailedtiminganalysisofalimitedbutnon-negligiblegroupofbright( afewtensof
≥
mCrab)sources. Linkingtimingpropertiestoothersourceproperties(e.g. spectral/fluxvariability)orto
eachother(highvs. lowfrequencyQPOs)isofprimaryscientificimportancetounderstandhowtheycan
beproducedinanon-standardaccretionregime,acrucialsteptousethemforestimatingBHmasses.
theWFMwillpromptlydetectfastflaresandtheonsetofoutburstfromHMXBs,aswellasdiscovernew
•
transientsourcesinthisclassandnearbyULXs( 4Mpc). Itwillbepossibletomonitorcontinuously
≤
a large fraction of bright accreting HMXBs, providing for them data with good spectral and timing
resolutiontostudytheirlongtermvariabilityandaccretiontorques. Synergiesareenvisagedbetween
LOFT and the Square Kilometer Array to jointly monitor ULX radio/X-ray variability (Wolter et al.
2014).
2 Probing stellar winds in wind-fed binaries
2.1 Transient structures
Massivestarsgeneratedensefastoutflowsthattriggerstarformationanddrivethechemicalenrichmentand
evolutionofGalaxies(Kudritzki,2002). Theamountofmasslostthroughtheemissionofthesewindshasa
large impact on the evolution of the star (see, e.g., Meszaros & Rees, 2014, for a recent review). In the past
decades,observationalevidencehasbeengrowingthatwindsofmassivestarsarepopulatedbydense“clumps”.
Thepresenceofthesestructuresaffectthemasslossratesderivedfromthestudyofstellarwinds,thusleadingto
uncertaintiesinourunderstandingoftheirevolutionarypaths(Pulsetal.,2008).
HMXBswerelongconsideredaninterestingpossibilitytoprobeclumpiness(see,e.g.,Sakoetal.,2003,and
referencestherein). AsubstantialfractionofthesesystemshostacompactobjectorbitinganO-Bsupergiant
andaccretingmaterialfromthestellarwind(see,e.g.,Chaty,2013;Paul&Naik,2011,forrecentreviews). As
theX-raysreleasedbytheaccretionprocesscantracethemassinflowratearoundthecompactobject,suchan
objectprovidesanaturalprobetomeasureinsituthephysicalpropertiesofthemassivestarwindandclumps.
Theso-called“SupergiantFastX-rayTransients”(SFXTs)showedsofarthemostconvincingevidenceforthe
accretionoflargeclumpsontothecompactobject(in’tZand,2005;Walter&ZuritaHeras,2007). InX-rays,
theroleofclumpsistwo-fold. Clumpspassinginfrontofthecompactobjectcause(partial)obscurationofthe
X-raysource,anddisplaythesignaturesofphotoelectricabsorptionandphoto-ionization(D’Aíetal.,2007;
Rampyetal.,2009). Inaddition,clumpscanleadtotemporarilyincreasedaccretionandX-rayflares(Ducci
etal.,2009;Oskinovaetal.,2012). Anumberofhours-longflaresdisplayedbytheSFXTscouldbeconvincingly
associatedtotheaccretionofdenseclumps,themoststrikingcasebeingthatofamoderatelybrightflarefrom
IGRJ18410 0535observedwithXMM-Newton(Bozzoetal.,2011). WiththeEpic-pncamera,integrationtimes
−
ofseveralhundredstothousandssecondswereneededtogetaroughestimateoftheclumppropertiesandan
averagepictureoftheclumpaccretionprocess.
Similar observations performed with the LAD on-board LOFT will dramatically improve our present un-
derstanding of clumpy wind accretion and winds in massive stars in general. By taking advantage of the
unprecedentedlylargeeffectiveareaofthisinstrument,itwillbepossibletostudyspectralandintensity
variability of the source on time scales as short as few to tens of seconds. This will permit a study of the
dynamicsoftheclumpaccretionprocessindetailandobtainmorereliableestimatesoftheclumpmass,radius,
density,velocity,andphoto-ionizationstate. Thelattercanbeprobedalsobyrevealingchangesinthecentroid
energyofthefluorescentironlinebetween6.4-6.6keV,commonlyproducedinwind-fedbinaryasaconsequence
oftheX-rayirradiationofthewindandclumpmaterial(see,e.g.,Torrejónetal.,2010,forarecentreview).
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ProbingstellarwindsandaccretionphysicsinHMXBsandULXswithLOFT
1− 4
s

2− 3 Figure1:Left:Sketchofthe
m
g c 2 accretion of a clump (cred-
er 1 its: ESA). Right: Changes

10− 0 in the flux and spectral pa-
0
,1 1 rameters during the flare
Fx
2.4 recorded by XMM-Newton
2.2
fromIGRJ18410 0535. Vi-
2.0 −
1.8 XMM olet boxes represent the
Γ 1.6 measurementsobtainedwith
1.4
XMM-Newton, while black
1.2 LAD
1.0 points represent values ob-
tained from the simulated
50 LADspectrawithexposure
220 40 timesasshortas50s(uncer-
1×30 tainties are sometimes too
H
N 20 small to be visible on the
10 scale of the figure). The
6.8 LADsensitivitytochanges
6.7 in the centroid energy of
6.6 thefluorescentironline(6.4-
EFe6.5 6.6keV)isaddressedinthe
6.4
bottompanel.
6.3
6.2
4000 6000 8000 10000 12000 14000 16000
Time, s
BasedonthespectralparametersmeasuredinXMM-NewtonobservationofIGRJ18410 0535,wesimulated
−
severalLADspectraoverthecourseoftheflarebyusingintegrationtimesasshortas50sandinterpolating
thespectralpropertiesofthesourcebetweentheindividuallyavailableXMM-Newtonpoints. Asisshownin
Fig.1,duringsimilarevents,theLADisabletorevealspectralchangesinthecontinuumaswellasintheiron
lineenergywithabetteraccuracythancurrentfacilitiesbyusingintegrationtimesafactorof 10-100shorter.
∼
Similarstudieswillbepossiblewithothersub-classesofwind-fedX-raybinariesaswell,testingwindsand
clumpsparametersindifferenttypesofstars.
2.1.1 ThecaseofsymbioticX-raybinaries
Brightwind-accretingX-raybinariescanalsobefoundamonglowmassX-raybinaries(LMXBs),inparticular
intheso-calledSymbioticX-raybinaries(SyXBs). Theseareinteractingsystemsinwhichthecompactobject
movesinawideorbit(periodsmonthstoyears)aroundaredgiantandaccretesmaterialfromitsslowanddense
wind(Masettietal.,2006;Nespolietal.,2010;Hynesetal.,2014;Bahramianetal.,2014). Observationally,
SyXBs are characterized by a highly variable X-ray emission, displaying bright flares similar to those of
other wind-fed X-ray binaries (Masetti et al., 2007). X-ray pulsations are unusually slow, but demonstrate
unambiguouslythepresenceofaccretingNSsinthesesources(spinperiodsrangefromhundredsto 18400s;
∼
Corbetetal.,2008). AboutadozenSyXBsareknowntodate,andthemajorityofthesesystemsarecharacterized
byaverageX-rayfluxesof10 11-10 10 ergcm 2s 1.
− − − −
LADobservationscanthusbecarriedoutasinthecaseofHMXBstoprobethepropertiesofthepoorly
known winds from red giants (which are dust rather than radiatively accelerated) and probe the physics of
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ProbingstellarwindsandaccretionphysicsinHMXBsandULXswithLOFT
Figure2: Thequasi-sphericalaccretionmodel
isparticularlywellsuitedtowindaccretingsys-
teminwhichthemassaccretionrateislow(fig-
urefromEnotoetal.,2014).Inthismodelahot
quasi-staticshellformsabovetheNSmagneto-
sphereandplasmaentrythroughthemagnetic
fieldlinesisregulatedbyinefficientradiative
plasmacoolingorComptoncooling(Shakura
etal.,2013). Theapplicationofthismodelto
thecaseof3A1954+319,aSyXBprototype,
wouldsolvetheproblemofthepeculiarlylarge
valueofthemagneticfieldabovethemagnetar
range(1016G),requiredbythestandardwind
accretion theory (see Enoto et al., 2014, and
references therein). We note that the quasi-
spherical accretion model is also applicable
tootherclassesofwind-fedsources(e.g.,the
SFXTs),andthusabetterunderstandingofthis
accretionregimeisofwideinterest(Shakura
etal.,2014,2013).
windaccretionfromslowflowsatlowmasslossrates(<10 10 M yr 1 comparedtothetypical10 5-10 6 M
− − − −
yr 1 observed in supergiant stars). The latter are particularly int(cid:12)eresting to test the quasi-spherical accretion(cid:12)
−
model onto magnetized NSs (see Fig. 2; Davies et al., 1979; Ikhsanov, 2007; Shakura et al., 2012), and the
X-rayvariabilitymodelviaredgiantwindaccretionsuggestedbyFilippovaetal.(2014). Thecontinuoussky
monitoring performed with the WFM will increase the number of SyXBs by, e.g., discovering long period
pulsations(seeSect.4).
2.1.2 ThecaseofwindcollidingbinariesandEta-Carinae
Another interesting class of binaries for which LOFT observations could help probing the properties of star
windsisthatofthecolliding“windbinaries”(see,e.g.,Stevensetal.,1992). Amongthese,weconsiderbelow
specificallythecase ofηCar, anearbymassivebinary system(distanceof2.3kpc)hosting aluminousblue
variableandaWolf-Rayetoralate-typenitrogen-richOstar,ina5.5yrsorbit. Thesourceishighlyabsorbed,
embeddedinathickcocoonduetoepisodesofmassiveejections(see,e.g.,Davidson&Humphreys,1997). The
X-rayemissionabove2keVisdominatedbyapoint-likeopticallythinthermalcomponentwithatemperatureof
4keVwhichismostprobablyduetothecollisionbetweenthetwostellarwinds. Regularweeklymonitoring
∼
bytheRXTEPCAhasrevealedaperiodicvariabilitylinkedtotheorbitalmotionwithamaximumatperiastron
lastingforafewmonths,followedbyastrong3-monthlongsuppressionofsoftX-rayemission. Aphaseknown
asdeepminimumseesthealmostcompletesuppressionofthesoftX-raycomponent,possiblyduetooccultation
bythestellarphotosphere(seeCorcoran,2005,andreferencestherein). Shortertermvariabilityisalsopresent
anditsoriginmightbelinkedtowindclumpiness(Moffat&Corcoran,2009). Anexcesspower-lawemission
hasbeenreportedabove 10keV(Viottietal.,2004;Sekiguchietal.,2009;Hamaguchietal.,2014;Leyder
∼
etal.,2010)withanindexof1-1.8anda10-50keVfluxof1-2 10 11erg/s/cm 2. Thiscomponentisthoughtto
− −
×
extenduptotheGeVrange,whereitcanbematchedtotheFERMIdetection(Farnieretal.,2011);itmightbe
interpretedasasignatureofparticleaccelerationattheshockbetweenthewinds,butitsoriginisdebated.
LOFT willbeabletoperformbothmonitoringanddetailedX-rayspectralanalysisowingtothelargeeffective
areaoftheLAD.Wehavesimulateda3kslongsnapshotassumingthespectralparametersdeterminedwith
Suzaku(Hamaguchietal.,2014)fortenorbitalphaseswithaconstantpower-lawemissionalongtheorbitbut
Page6of21
ProbingstellarwindsandaccretionphysicsinHMXBsandULXswithLOFT
1024
2m)−
(cNH1023
MNS=1.5 Mfl
MNS=1.8 Mfl
MNS=1.95 Mfl
LOFT/WFM
LOFT/LAD
XMM
1022
0.0 0.2 0.4 0.6 0.8 1.0
φ - Phase
Figure 3: Left: An example of hydrodynamical simulations from Manousakis et al. (2012). The wind of the primary
isstronglyperturbedbythegravitationalfieldoftheNS.AdensestreamofmattertrailstheNSandabsorbsitsX-ray
emission. Right: AbsorptioncolumndensityasafunctionoftheorbitalphaseandNSmass,asmeasuredinthecaseof
IGRJ17252 3616(averageX-rayfluxofF =5.5 10 11ergcm 2s 1inthe3-30keVenergyrange;Manousakis&Walter,
x − − −
− ×
2011). ThegreenpointsaretheoriginalXMM-Newtonmeasurements(&10ksexposureforeachpoint),whileredpoints
representsthesimulatedLADobservationsbyusingonly1ksofexposure. Wealsoshowtheresultsobtainedfromthe
simulatedWFMspectra(blackpoints)forabrightersourcewithafluxsimilartoVelaX-1(F =5.5 10 9ergcm 2s 1).
x − − −
×
TheWFMachieves,forsuchfluxes,agoodaccuracyinthemeasurementofthespectralparametersalreadyin.20ks.
variablesoftX-rayemission. Wehaveverifiedthatthepower-lawcanbefirmlydetectedevenatthesoftX-ray
maximum,anddeterminingtheindexmorepreciselythanSuzakurequiresonly<10%ofitsobservingtime. If
the LAD instrument observes η Car every few weeks during 3 (satellite) revolutions, it will help understand
themysteriousnatureofthehardX-rayemission,anditwillallowtosearchforitsvariability. ThehardX-ray
emissioncanbeusedtoconstrainthecharacteristicsoftheparticleaccelerationinthecollidingwindregion,
such as the particle density and the magnetic field. These monitoring observations could also be useful
to quantify the mass loss rates of the stellar winds and their clumped nature, by a detailed study of the
relevantspectral/variabilityparameters(seealsoSect.2). Weremarkthatthiscannotbeeasilydoneatother
wavelengthsastheradiationisheavilyabsorbed.
2.2 Persistent structures
Many wind-fed HMXBs display prominent variations of their average X-ray spectral properties at different
orbitalphases. ThisvariabilityoccursonamuchlongertimescalewithrespecttothatdiscussedinSect.2.1and
providedifferentimportantinsightsinpersistentstructuresthatformaroundthecompactobjectwhenthestellar
windisfocusedbyitsgravitationalfield(Blondinetal.,1991).
RecentlyManousakis&Walter(2011)investigatedthevariationsoftheX-rayemissionpropertiesfromthe
wind-accretingsupergiantX-raybinaryIGRJ17252 3616. ByusingXMM-Newtonobservations,theseauthors
−
foundthattheabsorbingcolumndensityhasapeculiardependenceontheorbitalphase(seeFig.3). Manousakis
etal.(2012)usedhydrodynamicsimulationstoshowthattheincreaseoftheabsorptioncolumndensityismost
likelycausedbyanaccretionwakethatformaroundtheNS.Thephaseprofileoftheabsorptioncolumndensity
could be well reproduced in the simulation, suggesting that the mass of the compact object plays a key role.
If extended to other wind accreting systems, these studies could provide a new independent way to measure
NS masses. A first attempt to extend the applicability of this study to the supergiant X-ray binary prototype
VelaX-1waspresentedbyDoroshenkoetal.(2013). However,formostwind-fedbinaries,asufficientlyaccurate
Page7of21
ProbingstellarwindsandaccretionphysicsinHMXBsandULXswithLOFT
Figure4: An “off-state” observed from the
sourceGX301-2withtheRXTE/PCA(Gög˘üs¸ 0.10
et al., 2011) and a simulation of LOFT capa- LOFT WFM RXTE PCA
2.6
bilities to study similar events. The grey area LOFT LAD
representsthesourcelightcurveasobservedby 0.08
thePCA.Theblueboxesshowtheaccuracyand 2.4
integrationtimeofthesourcespectralslopemea-
s
suredbythePCA(Γisthepower-lawspectral 2.2 0.06cts/
index). The black points show how the LAD e,
Γ at
cinouglrdeabteerudseetdaitlobyfotlalkowingthaedvsapnetcatgraelovfairtisatliaorgnes 2.0 0.04WFM r
collectingareaandgoodspectralresolution(we
simulatedLADspectrain50sintegrationtimes 1.8
0.02
andinterpolatedthephotonindexvaluesamong
the PCA measurements). The red points and
1.6
thecorrespondingerror-barsrepresenttheWFM
0.00
lightcurveofthesourcesimulatedwithatime- 1000 2000 3000 4000 5000 6000 7000 8000 9000
Time, s
binof685s(rightscale). Notethatdespitethe
significantdropinflux, thesourcecanstillbe
detectedbytheWFMduringtheoff-state.
monitoringoftheabsorptioncolumndensity(aswellasotherrelevantspectralparameters)isnotyetavailable
assuchobservationsrequiresrelativelylongexposuretimeswiththecurrentX-rayfacilities.
ThelargeeffectiveareaandgoodbroadbandcoverageoftheLADmakeitperfectlysuitedtocarryoutthese
investigations,astherequiredmeasurementaccuracycanbeobtainedwithmuchshortertimescalescompared
to,e.g.,XMM-NewtonorMAXI(see,e.g.,Islam&Paul,2014,forarecentstudyontheorbitalphaseresolved
spectroscopyofGX301-2). InFig.3wepresentacomparisonoftheresultsobtainedforIGRJ17252 3616
−
withtheEpic-pnon-boardXMM-NewtonandasimulationofthesameobservationswiththeLAD.TheLAD
can achieve the same accuracy in the spectral parameters of the source by using .1/10 of the Epic-pn
exposuretime(notethatforthemajorityofthesystemsofinterest NH&1022 cm−2). Forbrightersourceslike
VelaX-1,WFMdatawouldalsobeabletoprovidetherequiredlong-termspectroscopicinformation,without
theneedofdedicatedLADcampaigns(seeFig.3).
2.3 Probing the origin of off-states in wind-fed binaries
An intriguing and poorly understood phenomenon in wind-fed binaries is that of the so-called “off-states”
(Kreykenbohmetal.,2008;Gög˘üs¸ etal.,2011;Doroshenkoetal.,2012). Thesearerapid(fewseconds)and
sporadic drops of the source X-ray flux, lasting typically several pulsation cycles, which origin is currently
unknown. The study of such events has so far been hampered by their short duration, which requires long
monitoringobservationstobediscoveredindifferentsources,andtheinabilitytoobtainsystematicallyaccurate
spectralandtiminginformationduringtheingressandegressphases(Doroshenkoetal.,2011). Differentspectral
changeswouldbeexpectediftheoff-statesaredueto: (i)accretionfromregionscharacterizedbyararefied
stellar wind (i.e., the intra-clump medium; Kreykenbohm et al., 2008; Oskinova et al., 2012); (ii) switches
inmagnetosphericaccretionmodes(Bozzoetal.,2008;Doroshenkoetal.,2011);(iii)switchesbetweenthe
Comptonandradiativelyinefficientcoolingmodesinthesettlingaccretionregime(Shakuraetal.,2013);(iv)
hydrodynamicinstabilitiesclosetothecompactobjectmagnetosphere(Manousakis&Walter,2014). Changes
inthesourcepowerspectrumduringtheingressandegressphasescouldalsohelprevealingswitchesbetween
differentaccretionmodesordynamicalconfigurationoftheaccretionflow.
ThelargeeffectiveareaoftheLADwillprovideforthefirsttimedetailedspectralandtiminginforma-
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ProbingstellarwindsandaccretionphysicsinHMXBsandULXswithLOFT
Figure 5: Example of the LAD capability to reconstruct the spectral shape of an accreting X-ray pulsar in very short
integrationtimes. SimulationswereperformedusingtheresultsofaBeppoSAXobservationofVelaX-1(Orlandinietal.,
1998). Eachcurvegivesthesignificanceofaspectralparameter(definedastheratiobetweenthebestfitparameterand
itsuncertainty)asafunctionoftheexposuretime. Thecurvesarecolorcodedaccordingthetheassumedflux(black:
500mCrab,red: 150mCrab,green: 15mCrab). Middle: SameasbeforebutshowingtheLADcapabilitiesinthecase
of4U0115+634. ForthissourcewealsoshowthesignificanceoftheCRSFdetectionat12.8keVusingthespectral
model proposed by (Santangelo et al., 1999). “Norm” is the CRSF normalization and “Sigma” its width. We do not
plotthesignificanceoftheCRSFcentroidenergy,asitisalwaysdeterminedataveryhighsignificance. TheLADcan
indeedmeasuretheCRSFcentroidenergyatasignificancelevelof100,20,and3σforfluxesof500,150,and15mCrab,
respectivelyinonly0.1s. Right: Sameastheleftpanel, butforAthena/WFI.Thisshowthatsimilarstudiesareonly
possibleiftheLADlargecollectingareaisavailable. OthercurrentlyplannedX-rayinstrumentswouldneedtoolong
exposuretimestoachieveaccuratespectralmeasurements.
tionwithinintegrationtimesshorterthantheoff-stateingressesandegresses. ThisisillustratedinFig.4
by using the example of the off-states in GX301-2 discovered with the RXTE/PCA (Gög˘üs¸ et al., 2011). In
thefigurewealsoshowthatoff-statescanbeeasilydetectedwiththeWFM,bytakingadvantageofitsgood
sensitivityandtheavailabilityoflongtermmonitoringobservationsofmanywind-fedbinariesduringthecourse
ofthemission(seealsoSect.4).
3 Probing the physics of magnetospheric accretion
3.1 Pulse phase resolved spectroscopy and cyclotron features
ClosetothemagneticpolesofayoungNS,X-rayphotonsareforcedtointeractwiththemagneticfieldbefore
beingabletoescapeinthedirectionoftheobserver. Asthequantizationoftheelectronmotioninthedirection
perpendiculartothemagneticfieldhastheconsequencethatthescatteringcrosssectionsofX-raysareanisotropic
and energy dependent, X-ray photons with frequencies close to the gyro-magnetic (Larmor) frequency are
scatteredoutofthelineofsight,givingriseto“cyclotronresonantscatteringfeatures”(CRSFs). Thesefeatures
are often observed in the X-ray spectra of highly magnetized NSs and provide a unique tool to measure the
compact object magnetic field strength (Canuto & Ventura, 1977). Because the CRSF parameters strongly
depend on the viewing angle, a spectral analysis as a function of the pulse phase (pulse phase spectroscopy,
PPS)canbeusedtoreconstructthetopologyoftheNSmagneticfieldandprobetheconditionofmatterclose
totheemittingregions. SuchstudieshavebeenhamperedsofarbythefactthatPPScouldonlybeperformed
by averaging the source spectrum at a given phase over many individual pulses. This provides information
onlyontheaveragedlargescalemagneticfieldconfigurationandspectralformationmechanisms,limitingthe
development of theoretical models (see, e.g., Schönherr et al., 2014, for a recent review). These topics thus
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ProbingstellarwindsandaccretionphysicsinHMXBsandULXswithLOFT
Figure6:CyclotronlineenergyE asafunctionoflumi- 31
cyc
nosityinV0332+53. Measurementsathighluminosities
30
(black open circles) are from Tsygankov et al. (2010).
29
LAD measurements are simulated by carrying out PPS
onone20ks-longobservationatlowerfluxes(redfilled 28
cbiarrcsleasr)easnmdaalnleorththearnatthheigmhearrkfleurxseisze(girneethnissqcuaasree)s.;LerAroDr E,keVcyc27
observationsathigherandlowerfluxesare(clearly)also 26
doablebutnotshownhere. Atransitionbetweendifferent
25 V0332+53,Tsygankovetal. (2010)
E -fluxdependenciescanbeprobedforthefirsttimeon
cyc LOFT/LAD20ksatlowflux
awiderangeofluminosities. 24
LOFT/LAD20ksatmoderateflux
23
100 101
LX/(1037erg/s)
remain highly debated (Staubert et al., 2007; Schönherr et al., 2007; Nishimura, 2011; Becker et al., 2012;
Poutanenetal.,2013).
ThehighthroughputoftheLADwillbeableforthefirsttimetoaccuratelyconstrainspectralproper-
ties of accreting X-ray pulsars by using integration times much shorter than a single NS pulse. This is
illustratedinFig.5fortwoX-raypulsarsprototypes: VelaX-1and4U0115+634. Itisworthmentioningthat,
giventheoptimalresponseandeffectiveareaoftheLADat6-8keV,PPSanalyseswillnotbelimitedtoCRSFs,
butnaturallyencompassanyspectralfeature,e.g.,thefluorescenceiron-Kemissionline(simulationsshowthat
thetypicalironlineobservedinVelaX-1canbeconstrainedbytheLADwithanintegrationtime.10sec).
TheLADwillalsobeabletoperformPPSanalysesatlowluminosities,aregimeneverinvestigatedsofar
due to the lack of simultaneous broad-band energy coverage, energy resolution, and collecting area of past
andpresentX-rayfacilities. Especiallyintriguingisthevariationofthecyclotronlineenergywithluminosity
observedinseveralsources(see,e.g.,Klochkovetal.,2011b). Suchavariationmostprobablyreflectsavertical
displacementoftheemittingregionintheinhomogeneousmagneticfieldoftheNS,andcouldbeinvestigated
sofaronlyatluminosities&1037ergs 1 withrelativelylongpointedRXTEandINTEGRALobservations(see,
−
e.g.,Klochkovetal.,2011a,andreferencestherein). Atluminositiesbelow 1037ergs 1,thepolaremission
−
∼
regionsoftheNSwouldbeexpectedtoswitchtoadifferentconfigurationleadingtoadifferentdependenceof
thecyclotronlineenergy E onflux(Beckeretal.,2012). SimulationsinFig.6showthattheLADwillbe
cyc
abletomeasurespectralparametersofthecontinuumofaccretingpulsarsathighsignificance(&5σ)in
lessthanafew100sevenatluminositiesaslowasfewtensofmCrab.
3.2 Inside the NS magnetosphere: microsecond variability in HMXBs
In wind-accreting NSs, the plasma is believed to penetrate the compact object magnetic barrier through the
Rayleigh-Taylorinstability,intheformofaccretingblobs(Arons&Lea,1976;Shakuraetal.,2012). These
blobs,afterashortradialinfall,arechanneledbythemagneticfieldlinestowardtheNSpolarcaps,wherethey
releasetheirkineticenergyintoX-rayradiation. Acertain“granularity”intheobservedX-rayemissionisthus
expected(Orlandini&Morfill,1992). Itwasshownthattheenergyreleaseofeachshotoccursonmicrosecond
timescales(Orlandini&Boldt,1993). ThepassageofthemicrosecondpulsesthroughtheNSmagnetosphere
willtendtobroadenthem,butphotonsmovinginadirectionparalleltothatofthemagneticfieldandenergies
muchlessthanthecyclotronenergywouldemergeunscattered(becausetheirscatteringcoefficientaredrastically
reduced;Herold,1979). FromthetypicalcyclotronresonanceenergiesobservedinXRPs,itisexpectedthatfor
E . 5keV,µspulsesfromthesurfaceofNSsshouldbedetectablerelativelyundistorted.
Thetechniqueweareplanningtousetodetectsuchµsvariabilityismultipledetectorcoincidence. The
LAD is therefore the best suited instrument for this kind of studies, given its time resolution and high
number of independent detection modules, N . The presence of events at µs time scales can be probed by
d
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