XMMU J134736.6+173403: an eclipsing LMXB in quiescence or a peculiar AGN?

0.16 MB·

by  S. Carpano

Astronomy&Astrophysicsmanuscriptno.9027 (cid:13)c ESO2008
February4,2008
XMMU J134736.6+173403: an eclipsing LMXB in quiescence or a
peculiar AGN?
(ResearchNote)
S.Carpano1,B.Altieri2,A.R.King3,A.Nucita1,andP.Leisy4
8
0 1 XMM-NewtonScienceOperationsCentre,ESAC,ESA,POBox50727,28080Madrid,Spain
0 2 HerschelScienceCentre,ESAC,ESA,POBox50727,28080Madrid,Spain
2 3 DepartmentofPhysicsandAstronomy,UniversityofLeicester,LeicesterLE17RH,UK
4 IsaacNewtonGroupofTelescopes,ApartadodeCorreos321,E-38700Sta.CruzdeLaPalma,Spain;InstitutodeAstrofsicade
n
Canarias,E-38205LaLaguna,Tenerife,Spain
a
J
Submitted:8November2007;Accepted:8January2008

8
2 ABSTRACT

] Aims.WereportthediscoveryofapeculiarobjectobservedserendipitouslywithXMM-Newton.Wepresentitstimingandspectral
h
propertiesandinvestigateitsopticalcounterpart.
p
Methods.The light curve of the X-ray source, its spectrum, and the spectrum of the best optical counterpart are presented and
-
o analyzed.
r Results.TheX-rayfluxdecreasesbyafactorof6.5within1handstaysinalowstateforatleast10h,therebysuggestingthepresence
t ofaneclipse.Thespectrumisverysoft,apowerlawwithaslopeofΓ∼2.8,anddoesnotchangesignificantlybeforeandafterthe
s
a fluxdrop.Thesourceisspatiallycoincidentwithinfewarc-secondswithaSeyfert2galaxybelongingtoagalaxypair.
[ Conclusions.AlthoughthebackgroundAGNseemsthebestcounterpart,neitherthetemporalnorthespectralpropertiesoftheX-ray
sourcearecompatiblewithit.Weinvestigatethepossibilityofhavingaforegroundlow-massX-raybinaryinquiescence,wherethe

1 companionisnotdetectedintheopticalwavelength.
v
6 Keywords.X-rays:general–X-rays:binaries–Galaxies:Seyfert
4
2
4 1. Introduction 1996). A hot quasi-spherical flow is expected in the inner re-
.
1 gionsproducingX-rays(Narayanetal.1996).
In this paper we report the discovery of an X-ray source for
0
8 whichthefluxdropsabruptlybyafactorof6.5withinonehour, The spectrum of NS-LMXBs in quiescence displays a
0 andstaysinthislowstateforatleast10hours.Thenatureofthe soft(kT∼0.1keV)thermalcomponent(McClintocketal.2004),
: sourcedescribedinthispaperisveryambiguous.TheX-raylight probablyassociatedwiththestellarsurface.Indeed,theobserved
v
curvesuggeststhepresenceofaneclipseandhenceabinarysys- X-rayluminosities,temperatures,anddistanceofthesesystems
i
X tem. The detected opticalcounterpartis a pair of galaxies,one indicate that the thermal emission comes from a source with a
r of which is a Seyfert2 AGN. The X-ray spectrum is also very radiusof∼10km.Ontheotherhand,forBH-LMXBsinquies-
a softso as to be compatiblewith such an AGN. We suggestthe cence,nothermalemissionhasbeenreportedforthe15sources
possibilitythatwehaveaforegroundlow-massX-raybinaryin reportedbyMcClintock&Remillard(2004),sincetheypossess
quiescenceforwhichtheopticalcounterpartisbelowdetectabil- aneventhorizon.Theirspectraarewell-fittedbyasinglepower
ity,butwedonotexcludethepresenceofapeculiarAGN.Inthis lawwithaphotonindex1.5<Γ <2.1(McClintock&Remillard
section we introducethe generalX-ray propertiesboth of low- 2004).
massX-raybinariesinquiescenceandofSeyfert2galaxies.
Low-mass X-ray binaries (LMXBs) are composed of a The brightest and best-studied NS-LMXBs in quiescence
compact object, either a neutron star (NS) or a black hole are Cen X-4 and Aql X-1. Spectral and temporal analysis
(BH), accreting matter from a companion star with masses . of these sources during quiescence are reported, for exam-
1M⊙. These objects have typical X-ray luminosities of about ple, by Campanaetal. (2004) and Campana&Stella (2003),
1037–1038ergs−1 but can drop down to 1031–1033ergs−1 dur- respectively. Both sources display a thermal component and
ingquiescence.Athighluminosities,the diskisopticallythick a power law, as well as short-timescale temporal variability
and geometrically thin, while a hot optically thin, advection- (Campana&Stella2003;Rutledgeetal.2002).TheX-rayNova
dominatedaccretiondisk isexpectedforlowerluminosities.In V404Cygniis,ontheotherhand,thebrighteststellar-massBH
theadvection-dominatedaccretionflow (ADAF)model,thera- in quiescence. A recent review of its properties is reported by
diativeefficiencyisverylow(∼10−4–10−3)andmostofthegrav- Bradleyetal. (2007): the spectrum in quiescence is typically a
itationalenergyisstoredasinternalenergyandadvectedtowards powerlaw witha Γ ∼ 2,consistentwith with whatis expected
the compact object with little X-ray emission (Narayanetal. fortheadvection-dominatedaccretionflow.Inthelowstate,the
sourceisalsovariableonashorttimescaleuptoafactorof&20
Sendoffprintrequeststo:S.Carpano,e-mail:[email protected] duringa60ksobservation(Hynesetal.2004).
2 S.Carpanoetal.:XMMUJ134736.6+173403:aneclipsingLMXBinquiescenceorapeculiarAGN?(RN)
We review in this paragraph some of the X-ray properties Table 1. Results of the spectral fits for
ofSeyfert2galaxies.IntheunifiedmodelforAGN(Antonucci XMMU J134736.6+173403, using an absorbed power law
1993), Seyfert2 galaxiesare identicalto the Seyfert1, but ob- model(phabs*power,inXSPEC).
servedathighinclinationangle.ThenucleusofSeyfert2galax-
iesisthereforenotdirectlyvisible,andshowsmuchhighercol- HighState Lowstate
umn densities than in Seyfert 1 objects, blocking the soft X- NH(×1020cm−2) 4.73−+00..8930 3.13−+12..7152
rayfluxbelow2keV.Risalitietal.(1999)showthatallsources Γ 2.81+0.07 2.72+0.19
aonfdth7e5ir%saNmple≥o1f0S23eycfmer−t2.2TghaeliarxisepsechtaravearNeHre≥lati1v0el2y2chma−rd2,. F0.2−10keV×10−12(cgs) 1.33−+−000..00.0557 0.24−+−000..00.1235
H
Moranetal.(2001)showacompositespectrumfor29Seyfert2
galaxiesobserved withASCA. The continuumis described by
usingtheLONGpackage.Wefirstperformedthestandardreduc-
adoublepowerlawcomponent:anunabsorbedcomponent(en-
tions,i.e.abiassubstraction,aflat-fielding,andacosmicrejec-
ergyindexα=0.84)dominatesbelow3keV,andathigherener-
tion.Thenweextractedthearcsattheobjectposition,summed
gies the spectrum is dominated by a heavily absorbed compo-
themifmorethanone,andfromthemweidentifiedsomelines
nent(N = 3×1023cm−2,α=0.46).AnFeKαlineispresentat
H and automatically performed a wavelength calibration. All the
6.4keV, with an equivalent width of 420eV. Small-amplitude,
objectswerecalibratedinwavelength,correctedfromthemean
short-timescalevariabilityisobservedforSeyfert2galaxies,as
extinction,andcalibratedin fluxeswith 3 standardsstarstaken
Awakietal. (2006). These authors report variation of a factor
during the night. This instrumental response is very stable in
less than two for 13 Seyfert 2 galaxies observed with XMM-
time, regular checks are done monthly. Finally, from these 2D
Newton,onatimescaleofafewthousandseconds.
imagesweextractedtheobjectswithanoptimizedspectrumex-
The paper is organized at follows. Section 2 describes the
traction batch and a line-fitting to measure the line intensities
XMM-Newton and optical observation and data reduction. In
andtheredshift.
Sect. 3, we present timing and spectral analysis of the source,
reservingdiscussionforSect.5.
3. Timingandspectralanalysisof
XMMUJ134736.6+173403
2. Observationanddatareduction
The combined EPIC-MOS and pn background-subtractedlight
2.1.XMM-Newtondata
curve is shown in Fig. 1 (top), with a time bin size of 500sec.
XMMU J134736.6+173403 was observed serendipitously by Thecorrespondinghardness-ratiolightcurveisgivenatthebot-
XMM-Newton on 2003 June 24 for 64ksec, the target being tom, with a time bin size of 2000sec. Hard and soft bands are
Tau Boo. The EPIC-MOS (Turneretal. 2001) were operated givenbyH=2–10keVandS=0.2–2keV,respectively.Timesare
in large window mode and EPIC-pn (Stru¨deretal. 2001) cam- giveninhoursfromthestartoftheobservation.Periodsofhigh
erasin fullframemodewiththe thickfilter forallinstruments. background at the end of the observation were excluded from
The optical monitor was blocked throughout the full observa- the data. The straight lines show the average of the respective
tion.AfterscreeningtheMOSdataforprotonflaresusingstan- portionof the curves.In the firstcurvethe flux dropsby a fac-
dard procedures1, a total of 60 and 56ksec of low-background torof6.5within1hr.Wesearchedforperiodicitiesbetween10s
emissionremainedfortheMOSandpn,respectively. and6hrinthelightcurvebeforethedipwherethefluxisvisibly
Using theXMM-NewtonSoftware AnalysisSystem (SAS) variable. We found a significant modulation at 1.8h, but more
edetect chain task, which performs maximum-likelihood data are needed to confirm the periodicity. On the other hand,
sourcedetection,XMMUJ134736.6+173403,observedat∼ 8′ no periodic signal was found in the low state. The fractional
off-axis, was detected with a maximum likelihood of 3 ×104. root mean-squarevariability amplitude,defined for example in
Thebest-fitcoordinateswereα = 13h47m36s.6andδ = Vaughanetal. (2003), F , is 13.7±1.2% before the flux drop
J2000 J2000 var
+17◦34′02′.′8 with a statistical errorof 0′.′1. Since the sourceis and 13.2±3.8%after it, showing that the short-term variability
off-axis, however, the uncertainty on the statistical error might is roughly constant during the observation. The hardness-ratio
be much greater than what is provided by edetect chain. lightcurvedoesnotvaryafterthefluxdeclineeither.
Combiningthiserrortothesystematicalshiftexpectedbetween Figure2showsthe pnand MOSspectraof the source.The
X-raydataandopticalcounterpart(typicallyfewarcsec),theto- dataarebinnedtohaveatleast25countsineachenergybin.The
talestimatederrorisabout3′.′ sourceisdescribedwellbyapowerlawmodel,yieldingχ2/ν=
ν
1.18inthehighstateandχ2/ν=0.85inthelow.Addinganother
ν
componentdoesnotimprovethefit.Thebest-fittingparameters
2.2.Opticalcounterpartspectra
of the absorbed power law model are shown in Table 1. Here,
Low-resolution spectroscopy was performed with ISIS at the NH is theequivalentcolumndensityof neutralhydrogenandΓ
WilliamHerschelTelescope(ING,LaPalma)inthenight2007 the photonindex.Thecorresponding0.2–10keV flux is shown
June 14 in service mode. The slit was aligned east-west to get inthelastrow.Uncertaintiesaregivenata90%confidencelevel.
thespectraofthegalaxyandtheAGNinthesameframes.The WesearchedforX-raydataforthesourcefromotherX-ray
blue and red spectra were acquired simultaneously on two dif- satellites:noobservationhasbeenperformedwithChandra,and
ferentCCDs, thanksto adichroicsystem.Two shortexposures thesourceisdetectedinneithertheall-skysurveynorinashort
of10mneachweremadetoavoidcosmicrays. (600s) PSPC observationfromtheROSATsatellite, wherethe
Thedatareductionwasperformedinastandardwayforboth upperlimitfluxwasof3×10−13and8×10−13ergs−1cm−2,re-
theblueandredspectrumparts.Wewroteadedicatedandsemi- spectively.Thesourcewas,however,visibleina∼80ksASCA
automaticMIDASbatchesforISISlong-slitspectraessentially observation and reported in the catalogue of ASCA sources
(Uedaetal.2005).Theirfluxandhardness-ratiosarecompatible
1 http://xmm.esac.esa.int/external/xmm user support/documtoenthteatmioeann/svaasluuesmg/eUaSsGu/redbyXMM-Newton.Weextractedthe
S.Carpanoetal.:XMMUJ134736.6+173403:aneclipsingLMXBinquiescenceorapeculiarAGN?(RN) 3
0.10
c
e
s
s/
nt
u
o
C
0.01
0 5 10 15
−0.6
)
S
+
H −0.8
(
)/
S
−
H
−1.0
( Fig.3. Color-composite image of the optical counterpart from
theSloanDigitalSkySurvey(SDSS).Theredcolorisassociated
−1.2
with theRband,thegreencolorwith theG band,andtheblue
0 5 10 15
colorwiththeUband.ThecircleiscenteredontheX-raysource
Time (hours)
andtheradiusis3′′.
Fig.1. Mean XMM-Newton EPIC-MOS and pn background-
subtracted light curve of XMMU J134736.6+173403 (top),
OIII
and the correspondinghardness-ratiolightcurve(bottom).The
400
straight lines show the average of the respective portion of the
curves.
300
normalized counts/sec/keV 100.010.1−3 Flux120000 Hβ
10−4
0
2 4000 5000 6000 7000 8000 9000 10000
wavelength (A)
0
χ
−2 Fig.4. Optical spectrum of the best optical counterpart of
−4 XMMU J134736.6+173403.The width of the H line, and the
β
0.5 1 2 5 ratiobetweenO overH (<3)isconsistentwiththespectrum
channel energy (keV) III β
ofaSeyfert2galaxy.
Fig.2. XMM-Newton EPIC-pn and MOS spectra of
XMMU J134736.6+173403 in the high state, fitted with
Fig.4).Atthisdistancethesourcesareseparatedby10kpc(cor-
anabsorbedpowerlaw.Bottom:residualsexpressedinσ.
responding to 10′′), suggesting that they form a pair of galax-
ies,maybeininteraction.TheSeyfert2galaxyisalsoassociated
ASCAlightcurve,andtheredoesnotseemtobeanyfluxdrop with a radio source with an intensity of 17.91 mJy at 1.4Ghz
like the one observed in the XMM-Newtondata, although the (Beckeretal.2003).
countrateisverylow(5ctsks−1).
5. Discussion
4. Theopticalcounterparts
WereportedthediscoveryofanX-raysourcethatpresentsaflux
Figure 3 shows the optical counterpart image of the X-ray dropofa factorof6.5within1handstaysina lowstate forat
source.ThecircleiscenteredonthepositionoftheX-raysource least10h,suggestinganeclipse.Fromopticalarchivedimages,
andtheradiusis3′′.Bothsourcesareextra-galacticobjectswith there are two counterparts:the first an extendednormalgalaxy
a redshift of z=0.045. From the image and the spectrum, the andthesecondapointsource.Aradiosourceisalsoassociated
source on the left is a normalgalaxywith some emission lines to this last one.We gotthe opticalspectrumof the two objects
(notshowninthispaper).Theobjectontherighthasaspectrum from the ING Telescope in La Palma. The extended object is
typicalofaSeyfert2galaxy(narrowH lineandO /H <3,see indeedaredshiftedobjectwithemissionlinesconsistentwitha
β III β
4 S.Carpanoetal.:XMMUJ134736.6+173403:aneclipsingLMXBinquiescenceorapeculiarAGN?(RN)
normalorslightlyactivegalaxy.Surprisingly,thesecondobject University of Portsmouth, Princeton University, the United States Naval
appearedatthesameredshift(z=0.045)andhasaspectrumtyp- Observatory,andtheUniversityofWashington.
icalfora Seyfert2 galaxy.Atthatdistancetheobjectsaresep-
arated by 10kpc, suggesting that they form a pair of galaxies,
References
maybeininteraction.
Within this context, the X-ray source would be associated Antonucci,R.1993,ARA&A,31,473
withtheSeyfert2galaxy(generallystrongX-raysources)rather Awaki,H.,Murakami,H.,Ogawa,Y.&Leighly,K.M.2006,ApJ,645,928
Becker,R.H.,Helfand,D.J.,White,R.L.,Gregg,M.D.&Laurent-Muehleisen,
thanthenormalgalaxy.Atthisdistance,theobservedluminosity
S.A.2003,VizieROnlineDataCatalog,8071
in the0.2-10keV bandis 6.5×1042ergs−1 inthe highstate and
Bradley,C.K.,Hynes,R.I.,Kong,A.K.H.,Haswell,C.A.etal.2007,ApJ,
1×1042ergs−1inthelowstate.Thiskindofluminosityistypical 667,427
ofSeyfert2galaxies.Thetemporalandspectralpropertiesofthe Campana,S.&Stella,L.2003,ApJ,597,474
X-rayobjectareunexpected,however,forsuchanAGN.Aflux Campana,S.,Israel,G.L.,Stella,L.,Gastaldello,F.&Mereghetti,S.2004,ApJ,
601,474
dropofafactorof6.5within1hhasneverbeenreportedsofar,
Cox,A.N.2000,Allen’sastrophysicalquantities,4thed.Publisher:NewYork:
especially for Seyfert 2 galaxies where the galaxy is observed AIPPress;Springer
edge-on.Thespectrum,furthermore,is extremelysoftforsuch Fabbiano,G.,King,A.R.,Zezas,A.,Ponman,T.J.,Rots,A.&Schweizer,F.
agalaxyanddoesnotpresentanyKαlineat6.4keV. 2003,ApJ,591,843
Hynes,R.I.,Charles,P.A.,Garcia,M.R.,Robinson,E.L.etal.2004,ApJ,611,
Wethusbelievethatthesourceisaforegroundobjectwithin
L125
the line-of-sightof this pair of galaxies. The detection limit of McClintock,J.E.,Narayan,R.&Rybicki,G.B.2004,ApJ,615,402
the Sloan Digital Sky survey is 23.3 mag and 23.1 mag in the McClintock,J.E.&Remillard,R.A.2004,inCompactStellarX-RaySources,
green and red bands, respectively (Willmanetal. 2002). Since ed.W.H.G.Lewin&M.vanderKlis(cambridge:CambridgeUniv.Press),
astro-ph/0306213
thesizeoftheGalaxytowardsthesourceisabout5kpc,thecom-
Moran,E.C.,Kay,L.E.,Davis,M.,Filippenko,A.V.&Barth,A.J.2001,ApJ,
panionmustbetypeM2,M =9.9(Cox2000)orlatertoexplain
V 556,L75
thenon-detectability.Assumingtheobjectisat5kpc,thelumi- Narayan,R.,McClintock,J.E.&Yi,I.1996,ApJ,457,821
nosityis4×1033ergs−1andbecomes6×1032ergs−1ifthesource Risaliti,G.,Maiolino,R.&Salvati,M.1999,ApJ,522,157
isatonly2kpc(implyingthereforeaM4/M5dwarfcompanion). Rutledge,R.E.,Bildsten,L.,Brown,E.F.,Pavlov,G.G.&Zavlin,V.E.2002,
ApJ,577,346
This is well within the rangeof whatis expectedfor low-mass
Stru¨der,L.,Briel,U.,Dennerl,K.,etal.2001,A&A,365,L18
X-raybinariesinquiescenceandisconsistentwiththeseobjects Turner,M.J.L.,Abbey,A.,Arnaud,M.,etal.2001,A&A,365,L27
having K or M companionstars. We excludethe presence of a Ueda,Y.,Ishisaki,Y.,Takahashi,T.,Makishima,K.&Ohashi,T.2005,ApJS,
cataclysmic variable, since their orbital periods are only a few 161,185
Vaughan,S.andEdelson,R.andWarwick,R.S.andUttley,P.2003,MNRAS,
hours,whilehere,ifthefluxdropisassociatedwithaneclipse,
345,1271
itmustbeatleast∼1day.Wenotethatthespectrumisabittoo Willman,B.,Dalcanton,J.,Ivezic´,Zˇ.,Jackson,T.,Lupton,R.,Brinkmann,J.,
soft(Γ∼ 2.8)foranBH-LMXBinquiescenceandthatapower Hennessy,G.&Hindsley,R.2002,AJ,123,848
lawcontinuuminunexpectedforathermalemissionfromaneu-
tronstarsurface.
IftheX-raysourceisaforegroundobject,thentheX-rayflux
of the background AGN should also be detected and both are
probablyconfused. A deepChandraobservation of the system
might represent the only solution for disentangling the LMXB
picture or the peculiar AGN scenario. Thanksto its better spa-
tialresolution,wemightbeabletoresolve2hypotheticalpoint
sourcesinthefirstscenario,whileonesinglesource,associated
withtheAGN,shouldbeobservedinthesecondcase.
Acknowledgements. ThispaperisbasedonobservationsobtainedwithXMM-
Newton, an ESA science mission with instruments and contributions directly
funded by ESA Member States and NASA, and on observations made with
the WHTtelescope operated on the island ofLaPalma bythe Isaac Newton
GroupintheSpanishObservatoriodelRoquedelosMuchachosoftheInstituto
deAstrofisica de Canarias. Weacknowledge the following persons (inalpha-
betic order) fortheir helpinthe interpretation ofthesource nature: L.Ballo,
A.-L.Longinotti,M.Guainazzi,A.Pollock,A.Read,R.Saxton,M.D.Trigo.
Funding for the SDSS and SDSS-II has been provided by the Alfred
P. Sloan Foundation, the Participating Institutions, the National Science
Foundation,theU.S.DepartmentofEnergy,theNationalAeronauticsandSpace
Administration, the Japanese Monbukagakusho, the Max Planck Society, and
the Higher Education Funding Council for England. The SDSS Web Site is
http://www.sdss.org/.
The SDSS is managed by the Astrophysical Research Consortium for
the Participating Institutions. The Participating Institutions are the American
Museum of Natural History, Astrophysical Institute Potsdam, University of
Basel, University ofCambridge, CaseWesternReserve University, University
of Chicago, Drexel University, Fermilab, the Institute for Advanced Study,
the Japan Participation Group, Johns Hopkins University, the Joint Institute
for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and
Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences
(LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for
Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New
Mexico State University, Ohio State University, University of Pittsburgh,