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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- ble tothe Chandra flux, implying no further fading has occurred REFERENCES onatimescaleof∼ 10years.Unfortunately,inallfivecases,itis ArnaudK.,1996,inJacobyG.,BarnesJ.,eds,AstronomicalDataAnalysis notclearwhetherthequiescentsourceisaresiduallow-luminosity SoftwareandSystems,ASPConf.SeriesVol.101,p17 nuclearsourceorunrelated,backgroundgalacticemission. BadeN.,KomossaS.&DahlemM.,1996,A&A309,L35 Thequestionofwhetherthe‘switchoff’markedtheendofa BalletJ.,1999,A&AS,135,371 single,isolatedaccretionepisode(suchasatidaldisruptionevent) BollerTh.,BrandtW.N.,FinkH,.1996,A&A,305,53 orarapiddecreaseintheluminosityofapersistentAGN(Seyfert BrandtW.N.,PoundsK.A.,FinkH.,1995,MNRAS,273,L47 galaxy to LLAGN) remains largely open. The peak luminosities BurderiL.,KingA.R.,SzuszkiewiczE.1998,ApJ,509,85 wereLX ∼>1043 ergs−1,comparablewithbrightSeyfert1galax- CashW.,1979,ApJ,288,939 ies,whilethequiescentluminositiesareonlyLX ∼1040−1041erg CrenshawD.M.,KraemerS.B.,BoggessA.,MaranS.P.,MushotzkyR. s−1. The latter are rather high compared to the nuclear emis- F.,WuC.-C.,1999,ApJ,516,750 DickeyJ.M.,LockmanF.J.,1990,ARA&A,28,215 sionexpectedfromnormal/inactiveorstarburstgalaxies(seeFab- DonleyJ.L.,BrandtW.N.,EracleousM.,BollerTh.,2002,AJ,124,1308 biano1989)butquitecomparabletothoseoflow-luminosityAGN EbelingH.,WhiteD.A.,RangarajanF.V.N.,2003,MNRAS,submitted (LLAGN: Ptak et al. 1999; Roberts & Warwick 2000; Ho et al. EdelsonR.,VaughanS.,WarwickR.,PuchnarewiczE.,GeorgeI.M.,1999, 2001; Ptak2001). Thustheevidence does favour thepresence of MNRAS,307,91 LLAGN in these galaxies. The existence of long-lasting, high- FabbianoG.,1989,ARA&A,27,87 ionisation optical line emission from the nuclei of WPVS 007, FabbianoG.,WhiteN.E.,2003,inLewinW.,vanderKlisM.eds.,“Com- IC3599andNGC5905furthersuggeststhesemayharboursome pact Stellar X-ray Sources”, Cambridge University Press, in press kindoflong-livedAGN.However,itisdifficulttomakeanystrong (astro-ph/0307077) claims about their prior X-ray activity since no suitable observa- FraserG.W.,etal.,2002,Proc.SPIE,4497,115 tionsexist. GehrelsN.,1986,ApJ,303,336 GezariS.,HalpernJ.P.,KomossaS.,GrupeD.,LeighlyK.M.,2003,ApJ, The remaining two galaxies, RX J1242.6-1119 and 592,42 RX J1624.9+7554, show no evidence for a luminous AGN GoodrichR.W.,2000,NewAR,44,519 in their optical spectra and remain the best candidates for tidal GrupeD.,BeuermannK.,MannheimK.,BadeN.,ThomasH.-C.,deMar- disruption events, athough their (relatively) high residual X-ray tinoD.,SchwopeA.,1995a,A&A,299,L5 luminositymayindicatesomeresidualnuclearactivity(asargued Grupe D., Beuermann K.,Mannheim K.,Thomas H.-C.,Fink H. H., de above). The optically inactive galaxy RX J1242.6-1119 ispartic- MartinoD.,1995b,A&A,300,L21 ularly interesting as this was the only one of the target sources GrupeD.,ThomasH.-C.,LeighlyK.M.,1999,A&A,350,L31 to have been observed (although not detected) just prior to its GrupeD.etal.,1999,A&A,350,805 outburstdetection(seeKomossa&Greiner1999).Thissuggestsa GuainazziM.,etal.1998,A&A,339,337 risetimefortheoutburstoflessthantwoyears. GurzadianV.G.,OzernoiL.M.,1980,A&A,86,315 HaldersonE.L.,MoranE.C.,FilippenkoA.V.,HoL.C.,2001,AJ,122, Transientgalacticnucleirepresentarelativelynewandexcit- 637 ing avenue of X-ray astronomy research (see Komossa 2002 for HoL.,etal.,2001,ApJ,549,L51 a review). So little is known about these objects that any future HonmaF.,MatsumotoR.,Kat,S.,1991,PASJ,43,147 X-rayobservationsarepotentiallyofgreatimportance.Forexam- KomossaS.,BadeN.,1999,A&A,343,775 ple, a longer observation of NGC 5905 could reveal whether the Komossa S., 2002, Reviews in Modern Astronomy, 15, 27 X-ray emission is extended (and hence due to the circumnuclear (astro-ph/0209007) starburst, not a LLAGN). Deeper observations of IC 3599 would MagorrianJ.etal.,1998,AJ,115,2285 betterdefinetheX-rayspectrumandtherebyhelpclarifytheorigin PiroL.etal.1988,ApJ,325,L25 oftheremaininglow-luminosityX-rayemission.Futuremonitor- PtakA.,SerlemitsosP.,YaqoobT.,MushotzkyR.,1999,ApJS,120,179 ingofthesesourcesisneededtoseewhethertheyarepersistently PtakA.,2001,inWhiteN.E.,MalagutiG.,PalumboG.G.C.,eds,X-ray astronomy:stellarendpoints,AGN,andthediffuseX-raybackground, variable (whichwould imply on-going accretion) and, inparticu- AIPconferenceproceedingsVol.599,p326(astro-ph/0008459) lar,toseewhetheranyshowrepeatoutbursts.Aseverehindrance ReesM.J.,1988,Nature,333,523 tosuchaprojectisthedearthofknownsources.Futurelarge-area RobertsT.R.,WarwickR.S.,2000,MNRAS,315,98 monitoringmissionssuchasLobster(Fraser2002)arewellsuited SembayS.,WestR.G.,1993,MNRAS,262,141 to finding transient galactic nuclei and providing the most likely SiemiginowskaA.,CzernyB.,KostyuninV.,1996,ApJ,458,491 routetoareasonablysizedsampleofsuchobjects(seediscussions TerashimaY.,IyomotoN.,HoL.C.,PtakA.F.,2002,ApJS,139,1 inSembay&West1993andDonleyetal.2002)onwhichacon- WinklerH.,StirpeG.M.,SekiguchiK.,1992,A&AS,94,103 certedprogrammeoffollow-upobservationsmightbebased. ThispaperhasbeentypesetfromaTEX/LATEXfilepreparedbythe author. ACKNOWLEDGEMENTS We thank Steve Allen for help with CIAO, Tim Roberts for use- fuldiscussionsandananonymousrefereeforahelpfulreport.SV

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