Chandra observations of five X-ray transient galactic nuclei

0.21 MB·English

by  S. Vaughan

Mon.Not.R.Astron.Soc.000,1–5(2003) Printed2February2008 (MNLATEXstylefilev2.2)
Chandra observations of five X-ray transient galactic nuclei
S. Vaughan,1,2 R. Edelson3 and R. S. Warwick2
1InstituteofAstronomy;MadingleyRoad;Cambridge,CB30HA
2X-RayAstronomyGroup;UniversityofLeicester;Leicester,LE17RH
3AstronomyDepartment;UniversityofCalifornia;LosAngeles,CA90095-1562;USA
4
0
0
Accepted:14/1/2004;Submitted:23/12/2003;inoriginalform:12/11/2003
2

n
a ABSTRACT
J
WereportonexploratoryChandraobservationsoffivegalacticnucleithatwerefoundtobe

4 X-ray brightduring the ROSAT all-sky survey (with LX ∼> 1043 erg s−1) but subsequently
1 exhibitedadramaticdeclineinX-rayluminosity.Verylittleisknownaboutthepost-outburst

X-raypropertiesoftheseenigmaticsources.InallfivecasesChandradetectsanX-raysource
1
positionally coincident with the nucleus of the host galaxy. The spectrum of the brightest
v
source(IC3599)appearsconsistentwithasteeppower-law(Γ∼3.6).Theothersourceshave
4
toofewcountstoextractindividual,well-determinedspectra,buttheirX-rayspectraappear
6
2 flatter (Γ ∼ 2) onaverage.TheChandrafluxesare ∼ 102−103 fainter thanwas observed
1 during the outburst (up to 12 years previously). That all post-outburst X-ray observations
0 showed similarly low X-ray luminosities is consistent with these sourceshaving ‘switched’
4 toapersistentlow-luminositystate.Unfortunatelytherelativedearthoflong-termmonitoring
0 andotherdata meanthatthe physicalmechanismresponsibleforthisspectacularbehaviour
/ isstillhighlyunconstrained.
h
p
Keywords: galaxies:active–galaxies:nuclei–galaxies:Seyfert–X-rays:galaxies
-
o
r
t
s
a
: 1 INTRODUCTION rayluminous)AGNsuddenly‘switchesoff,’possiblyastheresult
v
ofthethermal-viscousaccretiondiskinstabilitiesknowntooperate
i Very large amplitude variations in the X-ray luminosity (greater
X inGalacticaccretingsources(Siemiginowskaetal.1996).SeeKo-
than afactor ∼> 100) emanating fromgalacticnuclei areunques- mossa(2002)andreferencesthereinforfurtherdiscussionofthese
r tionablyanindicatorofunusualandinterestingphenomena. Such
a andothermodels.
‘transient-like’ behaviour has been observed in only a handful of
ThisletterpresentsexploratoryChandraobservationsoffive
galaxiestodate(seeDonleyetal.2002andKomossa2002)through
ofthebest-studiedX-raytransientgalacticnuclei.Thesamplecom-
observations with ROSAT1. These soft X-ray bright galactic nu-
prises WPVS 007, IC 3599, RX J1242.6-1119, NGC 5905 and
cleidetectedduringtheROSATall-skysurvey(RASS)werefound
RXJ1624.9+7554.Verylittleisknownaboutthepost-outburstX-
insubsequentfollow-upobservationswiththesamesatellitetobe
raypropertiesofthesesourcesandthesenewobservationsalmost
fainter,byfactorsof70−400,thantheirinitialRASSdetection.
doublethetotalnumber ofpost-RASSdetectionsforthissample.
Suchspectacular long-term fading ismarkedly different fromthe
Therestofthisletterisorganisedasfollows.Section2describesthe
persistentvariabilityusuallyexhibitedbysoftX-raybrightActive
ChandraobservationsandX-raydataanalysis.Section3discusses
GalacticNuclei(AGN).
theopticalsourceclassificationandtheconstructionofX-raylight
Onepossibleexplanationisthatthesesourcesareintrinsically
curvesbycombiningthesenewChandradatawithearliermeasure-
quiescent objects (dormant or inactive galactic nuclei) that were
ments.Finally,section4discussestheimplicationsoftheseresults.
subjecttoaluminousoutburstnearthetimeofRASS,followedby
Throughoutthisletterthecosmologicalparametersareassumedto
a decline in luminosity over timescales of months or years. This beH0=70 kms−1 Mpc−1andq0 =0.5.
scenario is explained by the tidal disruption model in which the
X-rayoutburstistheresultofasupermassiveblackholeinthenu-
cleus of a inactive galaxy capturing and accreting a passing star
2 OBSERVATIONSANDDATAANALYSIS
(e.g.Gurzadian&Ozernoi1980;Rees1988).Alternatively,avery
differenttypeofmodelisthatinwhichanotherwisenormal(i.e.X- 2.1 Observations
Thefivetargetswereeachobserved closetotheaim-point of the
1 Piroetal.1988reportedafactorof∼>20decreaseinthe0.5−4.5keV back-illuminatedACISchipS3(ACIS-S3).AstheexpectedX-ray
luminosity of E1615+061 between HEAO-1 A2 and Einstein observa- fluxesfortheseobjectswereuncertainbyanorder ofmagnitude,
tions;however, ASCAmeasuredrapid,persistentvariability inthisobject fouroftheobservationswereperformedwiththeACISCCDsus-
(Guainazzietal.1998)suggestingthatitdoesnotfittheabovedefinition. ingthequarter-framesubarray.ThisreducedtheCCDreadouttime
2 Vaughan,Edelson&Warwick
Figure1. ContourplotsoftheX-rayintensityderivedfromfull-band(0.3−7.0keV)ACIS-S3imagesofthefivetargetsources.Eachimagespans60′′×60′′
andhasbeenadaptivelysmoothedatthe2σlevel(Ebeling,White&Rangarajan2003).Crossesmarktheopticalpositionsofthegalacticnuclei.Thenucleus
ofeachmemberofthegalaxypairisindicatedinthecaseofRXJ1242.6-1119.
(to1.07 s) and thereby reduced any possible effectsfromphoton sitionsweretaken fromthereferences giveninSection3andthe
pile-up (Ballet 1999) if the sources were brighter than expected. NASA/IPACExtragalacticDatabase2(NED).
TheobservationofRXJ1242.6-1119wasperformedinfull-frame InthecaseofRXJ1242.6-1119thepositionalerrorcircleof
mode. theRASSX-raysourcecontainsapairofinactivegalaxies,labelled
The data were processed from the level-1 events files using AandBbyKomossa &Greiner(1999). FromtheROSATdatait
CIAO v2.3.Onlyeventscorrespondingtogrades0,2,3,4and6 wasnotclearwhichofthetwogalaxiesshouldbeidentifiedwith
were used in the analysis of processed data. Flares in the back- theX-raysource.TheX-raysourcedetectedbytheChandraobser-
ground level were identified by examining the light curve from vationisclearlycoincidentwithgalaxyA(Fig.1).Assumingthat
the whole ACIS-S1 chip. These showed that the observations of thisisthesameX-raysourceasdetectedbyROSATthentheX-ray
WPVS007, IC3599andRXJ1242.6-1119 werefreefromback- outburstshouldbeassociatedwithgalaxyA.AlltheX-raysources
ground flares. The observation of RX J1624.9+7554 showed an appearconsistentwithbeingpoint-likewiththepossibleexception
increase in the background level in the final ∼ 200 s of the ob- of NGC 5905, which displays aslightly asymmetricshape inthe
servation;datatakenduringthistimeintervalwereremovedprior smoothed image. However, with only ∼ 25 counts in the source
toanalysis.TheobservationofNGC5905sufferedfromahigher andanenhanced backgroundlevelitisdifficulttobemoreconfi-
background level (compared to the other observations) such that dentofthiswithoutamoresensitiveobservation.Itisinterestingto
theremovaloftheperiodsofhighbackgroundwouldleaveinsuf- notethatofthefivetargetsNGC5905isthenearest(1′′or2pixels
ficientdataforanalysis.Therefore,thefullexposureofNGC5905 correspondstoaspatialscaleof∼ 0.2kpcatthesourceredshift)
was analysed accepting that the background level was enhanced. and thus might be most likely to show extended emission in the
Table1liststhebasicpropertiesofthefivetargetsourcesandtheir Chandraimages.
Chandraobservations.
2.3 CountRatesandSoftnessRatios
Source counts were estimated by performing photometry on the
2.2 X-rayImaging raw (unsmoothed) images with a circular aperture of radius of
2′′centredonthesource.Thebackgroundlevelwasestimatedfrom
Figure 1 shows the X-ray contour plots derived from the ACIS a concentric annulus with inner and outer radii of 3′′ and 60′′,
images for the five targetsafter adaptive smoothing has been ap-
respectively (excluding nearby sources where present). The net
plied.Clearlyinallcasesthereisanexcessofphotonscoincident
withtheopticalpositionofthegalacticnucleus(i.e.withintheex-
pected ∼ 2′′ uncertainty in the optical position). The optical po- 2 http://nedwww.ipac.caltech.edu/
Chandraobservationsof fiveX-raytransientgalacticnuclei 3
Table1. Sourceobservationlogandproperties.
Source R.A. Dec NHa Observation Exposure
Name (J2000) (J2000) z (1020cm−2) Date (ks) Countsb SRc Fluxd LXe
WPVS007 003915.8 -511703 0.029 2.6 2002Aug2 9.3 9.8±4.2 2.4±1.7 0.89 2×1040
IC3599 123741.2 264229 0.022 1.3 2002Mar7 10.2 247.8±16.8 5.2±0.9 17 2×1041
RXJ1242.6-1119f 124238.5 -111921 0.05 3.6 2001Mar9 4.5 17.9±5.3 1.3±0.6 2.8 2×1041
NGC5905 151523.4 553057 0.011 1.4 2002Oct4 9.6 25.3±6.6 1.3±0.6 1.8 6×1039
RXJ1624.9+7554 162456.5 755456 0.064 3.8 2002Sep15 10.1 3.5±3.2 0.3±0.4 0.24 3×1040
aGalacticcolumndensityfromDickey&Lockman(1990).bLargerofthetwo1σerrorboundsusingtheapproximationofGehrels(1986).cSRisthe
0.3−1.0/1−7keVsoftnessratio.dFluxinthe0.3−7.0keVband(10−14ergs−1cm−2).eTheestimatedunabsorbedX-ayluminosityinthesame
band(ergs−1).fOpticalpositionofgalaxyA(Komossa&Greiner1999).
countsassociatedwitheachsourcearegiveninTable1.Thebright-
estobject,IC3599,has∼>200photonsinitsX-rayimage,enough
for crude spectral analysis (seebelow). The other four detections
are based on far fewer counts. In the case of RX J1624.9+7554
thereare only 4counts inthe source aperture, i.e. a∼ 2σ confi-
dencedetection(withnegligiblebackground),butthecoincidence
withtheopticalpositionmakesthisalikelydetectionofthenuclear
X-rayemission.
Softness ratios were calculated for use as a crude indica-
tor of the spectral slopes of the faint sources. The softness ratio
was defined as the ratio of counts in the bands 0.3−1.0/1.0 −
7.0 keV. These aregiven inTable 1and show IC 3599 to havea
verysoftspectrumwiththeotherfoursourcesshowingsomewhat
harder emission. For the threeharder sources (RX J1242.6-1119,
NGC5905andRXJ1624.9+7554) thesoftnessratiocorresponds
toapower-lawphotonindexΓ62.5.Themeasuredsoftnessratio
Figure2. ACIS-S3spectrumofIC3599(crosses)fittedwithapower-law
ofIC3599requiresΓ∼4(seebelow). model(histogram).Thedatahavebeenrebinnedfordisplaypurposes.
In all cases the 0.3−7.0 keV flux was estimated using the
PIMMScalculatorattheChandraX-rayCenter3.Forthispurpose
the spectrum was assumed to be a power-law (with a photon in- bremsstrahlung continuum or a mekal plasma model) gave no-
dexΓ= 3)modifiedbyGalacticabsorption.Theestimatedfluxes ticeablylargerdata/model residuals.Thebest-fittingtemperatures
arelistedinTable1.IftheunderlyingX-rayspectradiffersubstan- werekT ∼0.16keVforablackbodyand∼0.26keVforamekal
tiallyfromtheassumedspectralformthenthesefluxestimatesmay plasmamodel.
changebyafactorofafew.The0.3−7.0keVunabsorbedlumi-
nositieswereestimatedassumingthesamespectralmodelandare
alsolistedinthetable.
3 COMPARISONWITHOTHEROBSERVATIONS
3.1 X-Raylightcurvesandsoftnessratios
2.4 X-raySpectrumofIC3599
In order to better understand how these sources have changed
Spectrawereextractedfromthesourceandbackgroundregionsof sincetheirX-rayoutbursts,longtermlightcurveswereconstructed
theobservationofIC3599.Aresponsematrixandanancillaryre-
for each of the five sources, as shown in Fig. 3. For this pur-
sponsefileweregeneratedwithmkrmfandmkwarf,respectively. posetheChandra 0.3−7.0keV count rateswereconverted into
The low number of counts meant that applying standard binning 0.3−2.0keVfluxesusingPIMMS,assumingtheenergyspectrum
(i.e.N >20countsperenergybin)wouldresultintoofewbinsfor isa steep power-law (with Γ = 3) modified by Galacticabsorp-
spectralfitting.Thereforetheunbinnedspectrumwasfittedbymin- tion. The 0.3 −2.0 keV band was chosen as both the Chandra
imisingtheC-statistic(Cash1979),appropriateforsituationswhen ACIS and ROSAT PSPCcover thisenergy range with reasonable
spectra contain few counts. The fitting was performed in XSPEC
effectivearea.TheROSATdatapointswerederivedfromboththe
v11.2(Arnaud1996).
RASSandpointedobservationsofeachsource(seebelowforref-
Thespectralfittingwasrestrictedtothe0.6−7.0keVband erences).The0.1−2.4keVPSPCcount rateswereconvertedto
sincetheACIScalibrationisuncertainbelow0.6keV.Apower-law 0.3−2.0keVfluxesassumingthesamespectrumasabove.Inaddi-
modelwithGalacticabsorptiongaveabest-fittingphotonindexof tionasingleROSATHRIobservationofNGC5905,takenin1996
Γ = 3.56+−00..3374 (90 per cent confidence limits) which is steeper October,isincluded.Sincethelargestuncertaintyassociatedwith
than that normally seen in Seyfert galaxies. This fit is shown in thesefluxestimatesisduetothemodel-dependenceofthecounts-
Figure 2. Fitting with alternative spectral models (blackbody or to-fluxconversionthefluxeswerealsocalculatedassumingΓ=2
andΓ=4.Thisresultedinfluxestimatesdifferingbyfactors∼>2
fromthefiducialmodelandtheerrorbarsshowninFig.3reflect
3 http://asc.harvard.edu/ thislargeuncertainty.
4 Vaughan,Edelson&Warwick
Chandra/ACISspectraandthesmallnumberofsourcephotonsin
theChandradata.
3.2 OpticalSpectraandClassification
WPVS007: TwoyearspriortoitsRASSdetectionanopticalspec-
trum was obtained by Winkler, Stirpe & Sekiguchi (1992). They
classifiedthesource(#7intheir notation) asaSeyfert1.Obser-
vations by Grupe et al. (1995b) taken two years after the RASS
detectionidentifiedWPVS007asaNarrow-lineSeyfert1galaxy
(NLS1;Boller,Brandt&Fink1996).AnHST/FOSultravioletspec-
trumtakenin1996alsoshowedbroadpermittedlinesaswellasin-
trinsic,ionisedabsorption(Crenshawetal.1999;Goodrich2000).
IC3599: Theoptical spectrum takenfivemonthsafter itsinitial
RASSdetectionshowedIC3599tobeaNLS1(Brandtetal.1995).
However,furtherobservations14monthsaftertheRASSdetection
showedtheopticalspectrumtohavechangedtoresemblethatofa
Seyfert1.9(Grupeetal.1995a).Observationsinsubsequentyears
confirmedthis(Grupeetal.1995a;Komossa&Bade1999).
NGC5905: ThissourcewasidentifiedasanHII/starburstgalaxy
six years after its RASS detection (Bade, Komossa & Dahlem
1996;Komossa&Bade1999).However,higherspatialresolution
spectroscopywithHSTrevealednarrow,high-ionisationlinesorig-
inatingfromthenucleus(Gezarietal.2003).Theseimplythepres-
enceofalow-luminositySeyfert2nucleusthatwasswamped by
thesurroundingHIIemissioninthepreviousobservations.Gezari
Figure3.HistoricallightcurvesofsoftX-ray(0.3−2.0keV)fluxcompiled et al. (2003) used the correlation between Hα and soft X-ray lu-
fromROSATPSPCobservations(lefthandsidefluxpoints)andnewChan- minositydescribedbyHaldersonetal.(2001)toestimatethesoft
draobservations (datapointsonfarright).Inaddition, thefluxesderived X-rayluminosityofthenucleustobeL0.1−2.4 ∼4×1040ergs−1,
fromoneROSATHRIobservationofNGC5905(1996.9)andoneXMM- afactor∼<6largerthantheX-rayluminosityactuallyobservedby
Newtonobservation ofRXJ1242.6-1119(2001.5)arealsoshown.Upper
Chandra.
limits on the flux, derived from non-detections with ROSAT are marked
RXJ1242.6-1119andRXJ1624.9+7554: Theseappearedasoth-
witharrows.
erwiseinactivegalaxieswhenobservedseveralyearsaftertheirini-
tialROSATdetections(Komossa&Greiner1999;Grupe,Thomas
& Leighly 1999). Gezari et al. (2003) report no detectable non-
XMM-Newton observed RX J1242.6-1119 during 2001 June stellarcontinuumorhigh-ionisationlineemissionintheirHSTob-
21-22,foradurationof30.1ks,anddetectedtheX-raysource(with servations.
≈ 286counts).AspectrumwasextractedfromtheEPICpndata
(using standard procedures) and fitted in XSPEC. This was well-
fittedwithasimpleabsorbedpower-lawmodel(Γ=2.42±0.23,
4 DISCUSSION
consistentwiththeestimatesfromsection2.3).The0.3−2.0keV
X-rayfluxwaswell-constrainedat≈ 1.8×10−14 ergs−1cm−2, Asisclearfromthelightcurves,inallfivecasesthefluxrecorded
consistent withtheChandrameasurement. Thisfluxpointisalso byChandrafalls2−3ordersofmagnitudebelowthemaximum
showninFig.3. flux observed by ROSAT. Although the light curves are sparsely
Ascanbeseen,forallfivesourcestheChandrafluxeslieat sampledthisdoesstronglysuggest thattheX-raylightcurvesare
leasttwoordersofmagnitudebelowthebrightestmeasuredROSAT bestcharacterisedbyasingle,dramaticdeclineinluminosityona
fluxes.Nonehaveshownasignificantincreaseinfluxatanytime timescaleof∼<2yearsfollowedbyaperiodofrelativequiescence.
during the (albeit poorly-sampled) period since the huge decline This is consistent with the X-ray outbursts being non-recurring
influxwasfirstestablished.Theseresultsareallconsistentwitha events (at least on timescales ∼< 10 years). In the most extreme
declinefromhighluminositytoapersistentlowluminosity(LX ∼ example,RXJ1624.9+7554fadedbyafactor∼> 1000betweenits
1040−1041ergs−1)overatimescale∼<2years. RASSobservationin1990OctoberanditsChandraobservationin
TheChandrasoftnessratiosalsoindicatechanges.Forallfive 2002September.Theopticalspectraseemtoindicateawidevariety
objectstheROSATX-rayspectrumwasextremelysoftduringout- ofsourcetypes,rangingfromgenuineAGNtoinactivegalaxies.
burst.InfactWPVS007wasthesoftestspectrumAGNobserved It is difficult to make a simple comparison with the fad-
duringtheRASS(Grupeetal.1995b)withaneffectivephotonin- ingpredicted inthe tidal disruption scenario (Rees1988): LX ∝
dexΓ∼8.TheotherobjectsshowedslopesintherangeΓ=3−5 (t − t0)−5/3 (where t0 is the time of the outburst event). The
(Brandt, Pounds&Fink1995; Komossa &Greiner 1999; Grupe, Chandra X-ray luminosities could contain a significant contribu-
Thomas&Leighly1999;Bade,Komossa&Dahlem1996).Inthe tionfromunrelatedgalacticemissionsuchasstarformingregions,
case of IC 3599 the spectrum remains rather soft. For the other brightX-raybinaries,diffuseemissioncomponents,etc.Indeed,in-
four objects the Chandra softness ratios suggest harder spectra dividualUltra-LuminousX-ray(ULX)sourcesinnearbygalaxies
(Γ ∼< 2.5). However, it is difficult to make a more quantitative canreachX-rayluminosities∼ 1040 ergs−1 (Fabbiano&White
comparison due to the different energy ranges of the ROSAT and 2003).This‘background’emissionisanunknownquantityand,if
Chandraobservationsof fiveX-raytransientgalacticnuclei 5
dominatedbyafewbrightX-raybinaries,couldalsobevariable. acknowledges financial support from PPARC. This research has
ThustheX-raylightcurvefollowingstellardisruptionshouldbe: madeuseoftheNASA/IPACExtragalacticDatabase(NED)which
LX ∼ N(t−tx)−5/3+C whereC istheunknownbackground is operated by the Jet Propulsion Laboratory, California Institute
galactic emission. The model therefore has three unknowns (N, of Technology, under contract withthe National Aeronautics and
tx andC)butthelightcurvesunfortunatelyhaveonly2−5data SpaceAdministration.
pointsmakingthetestrathermeaningless.Inthebestsampledlight
curves(IC3599andNGC5905)thelastROSATfluxiscompara-
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ACKNOWLEDGEMENTS
We thank Steve Allen for help with CIAO, Tim Roberts for use-
fuldiscussionsandananonymousrefereeforahelpfulreport.SV