1990ApJ. . .361. .590K 4342 (MakinoandGingaTeam1987). yet, notunderstood. between ultrasoftspectraandblackholesisintriguingbut, as al. 1989)alsohaveultrasoftspectra.Thephysicalrelation (Makino andGingaTeam1988),GS2000+25(Tsunemi et sources, 4U1543-475(Kitamotoetal.1984),X1734-275 X-ray spectra.AswellasA0620-00,somesofttransient dates. Forexample,thehighstatesofCygX-landGX339-4, LMC X-l,X-3,andA0620—00allexhibitultrasoft keV, whicharepreferentiallyassociatedwithblackholecandi- class, calledultrasoftsources,withblackbodytemperature <1 to theirspectra.Theyarguedthatthereisanultrasoftspectral considered tobelow-massX-raybinaries. et al.1980),andsomeofthemwereopticallyidentifiedwithK or Mdwarfs(BradtandMcClintock1983).Therefore,theyare bursts (Koyamaetal.1981;Murakami1980;Matsuoka sources (AqlX-l,X1608-522,CenX-4,etc.)showX-ray Holt 1986).Ontheotherhand,someofsofttransient binary X-raysourceswitheccentricorbits(Priedhorskyand generally X-raypulsars,andtheyarerecognizedtobemassive al. 1977),accordingtotheirspectra.Hardtransientsourcesare hard andsofttransients(Cominskyetal.1978;Kaluzienski 1 The AstrophysicalJournal,361:590-595,1990October1 © 1990.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. In thispaper,wereportthediscovery andopticalidentifica- WedesignatedGS1354-64,previously ASM1354-64,inIAUCire,No. White etal.(1984)classifiedbinaryX-raysourcesaccording The transientX-raysourcesaredividedintotwocategories, -o5 912 1 Subject headings:blackholes—stars:individual(GS1354-64)novaeX-rays:binaries X-ray emissionregionortoadecreaseoftheinterveningabsorptioncolumndensity.Theposition source isconsistentwiththepositionsofbothMX1353-64andCenX-2.However,characteristicsGS spectrum duringthedecayphasewasobserved,whichmaybeattributedtoatemperaturedecreaseof the distancetoGS1354-64,plusahardtailwithpower-lawshape(photonindex=2.1).Softeningof perature of0.7keVandradius51(cos¿)'(D/10kpc)km,whereiistheinclinationdiskD spectrum iswell-describedbyanultrasoftcomponentwithadiskblackbodyspectrum,innertem- 1354-64 weremarkedlydifferentfromeitherofthesesources. the detectionlimitduringdecay.Themaximumfluxwasabout(2.9±0.1)x10“ergss“cm“in mag atdiscovery.TheASMhasmonitoredalmosttheentirelightcurvefromrisingphaseuntilitreached February 13.Anopticalcounterpartwasidentifiedon1987March28,whichhadavisualmagnitudeof16.92 1-10 keVband.Thedecaytimeconstantwasabout66days,assuminganexponentialdecay.energy OUTBURST, IDENTIFICATION,ANDX-RAYLIGHTCURVEOFGS1354-64(=MX1353-64? © American Astronomical Society • Provided by the NASA Astrophysics Data System The AllSkyMonitor(ASM)onboardtheGingasatellitediscoveredanX-raynova,GS1354—64,1987 Astronomical Institute“AntonPannekoek,”UniversityofAmsterdamandCenterforHigh-EnergyAstrophysics I. INTRODUCTION X-rays :sources Department ofPhysics,FacultyScience,OsakaUniversity Shunji KitamotoandHiroshiTsunemi Received 1989August3;accepted1990March30 Observatoire deHaute-Provence(CNRS) Copenhagen UniversityObservatory Michiel vanderKlis CENTAURUS X-2?) Sergio A.Ilovaisky Holger Pedersen ABSTRACT 590 AND 2o examination byoneofus(H. P.)ofallstarslyingwithinthe corresponding Bplateofthe ESOquick-bluesurvey.Visual the ESO1mSchmidttelescope on1987March27withthe sible afterX-raydiscoveryby comparingaBplatetakenwith A searchforanopticalcounterpart wasmadeassoonpos- is (1950.0equinox) (FWHM), tiltedby+8°,24°,or42°tothemeridianof the satellite. Detailsaredescribedelsewhere(Tsunemietal.1989). cm ineach.ThefieldofvieweachdetectorisIx 45° with Xe+CO(1.2atm)havinganeffectiveareaofabout 70 (Makino andGingaTeam1987).TheASMconsistsofsix pro- where. portional countersequippedwithBewindow(50fim)andfilled (Makino andAstro-CTeam1987)on1987February 13 and theopticallightcurveevolutionwillbereportedelse- mation obtainedwiththeLargeAreaCounter(LAC)onGinga Danish telescopes.Theshort-termanddetailedspectralinfor- data wereobtainedwiththeAllSkyMonitor(ASM)on Ginga X-raysatelliteandtheESO1mSchmidt1.54 X-ray lightcurveandspectralbehavioraredescribedhere.The tion ofanultrasofttransientX-raysource,GS1354-64.Its 2 The celestialpositiondeterminedbyASMforGS1354— 64 GS 1354-64wasdiscoveredwiththeASMonboardGinga n. OBSERVATIONSANDRESULTS hm Ô =—64°24'±18'. a =1353+2”4, 1990ApJ. . .361. .590K h ASM. Errorboxescorrespondingto possible detectionsbyothersatellitesarealsoplotted. 2.4 x4insize,(b)Thepositionofthe opticalcounterpartofGS1354—64(dot)plottedinequatorialcoordinates togetherwiththeX-rayerrorboxderivedfrom optical counterpartismarkedbya cross. ThevisualmagnitudeatdiscoverywasV=16.92.Northisthe top,andeasttotheleft.Thefieldisapproximately trum, kindlytakenbyJ.BergeronusingEFOSContheESO bility, andwillbereportedinaseparatepaper(Ilovaiskyetal. 3.6 mtelescopesimultaneouslywiththeCCDphotometryat showing aslowdeclinewithsuperposedlarge-amplitudevaria- surements wereobtainedinthefollowingdaysandweeks, typical accretiondiskinalow-massX-raybinary(vanParadijs measurements ontheCCDframestakenthatnight,when transformed intotheUBVsystemusingobservationsof Landolt (1983)standards,giveF=16.92,R-F=+1.08, UT 1987March28)withthe1.54mDanishtelescopeatLa survey plate.ACCDimagetakenonthefollowingnight(4 Silla confirmedthediscovery(Pedersenetal1987).Magnitude 1990). Ginga errorboxrevealedanobjectthatwasnotpresentinthe 1983), reddenedbyE(B—V)=1mag.Furthermagnitudemea- U —B=—0.13.Thesevaluesareconsistentwiththoseofa h Fig. 1.—(a)A5minuteVbandCCD imageoftheGS1354-64fieldobtainedon1987March29at4UTwith the1.54mDanishtelescopeatLaSilla,Chile.The A 10minutelow-dispersion(13ÂFWHMresolution)spec- © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. la X-RAY LIGHTCURVEOFGS1354-64 h measured coordinatesis±0'.'5,asderivedfromtheresidualsof the Perthstandards.Thederived1950.0coordinatesforGS which werecalibratedwithrespectto42fundamentalPerth70 frame relativetonearbysecondaryastrometricreferencestars the MAMAReticonscanningmachineatObservatoirede standards, measuredontheESO/SRCglassplateJ-097using mass X-raybinaries,therebyconfirmingtheidentification.In § III.Adetailedstudyofthecharacteristicsoptical equatorial coordinatestogetherwiththeX-rayerrorbox. very faintimageatthepositionofcounterpart,implying Paris (Guibert1990).Theabsoluteastrometricaccuracyofthe addition, therearestronginterstellarabsorptionlinesat4430, counterpart willappearelsewhere. et al.1984)arealsoplotted.Wewilldiscussthesedetectionsin obtained on1987March29atUT4isshowninFigurela.In Inspection oftheRplatefromESO/SRCsurveyshowsa 6284, (unidentified),and5890Â(Nai). tinuum withstrongemissionatHen24686,typicaloflow- the 1.54mDanishtelescope,revealedasmooth,reddenedcon- response. Thefirstthreepointswereobtainedwithaconstant vations onFebruary13weremadeduringpreliminarysystem with theexceptionofdatapointFebruary13.Theobser- keV and3-10thehardnessratio(3-10keV/1-3keV) satellites (Markertetal.1977,1978;Seward1976;Wood Error boxescorrespondingtopossibledetectionsbyother Figure lb,thepositionofopticalcounterpartisplottedin that, whenquiescent,itmaybenearR=22.TheCCDimage spacecraft attitude,andthesourcewasnearcenterof testing. Thenormalobservationswerestartedattheendof the datapointswereobtainedfromsinglescansofASM of GS1354—64afterthediscoveryon1987February13.All Figure 2indicatetheeffectsofstatisticsandcollimator February (MakinoandGingaTeam1987).Theerrorbarsin 1354 —64counterpartare The positionoftheobjectwasmeasuredinCCDV Figure 2showstheX-raylightcurvesinenergyband1-3 d — 6429'29"0. hm 135427!53 , RIGHT ASCENSION Fig. lb 591 1990ApJ. . .361. .590K -912 A0620-00 (Rickettsetal.1975).Therefore,GS1354-64 and power-law modelwasabout4.7,whichissimilartothat of model satisfactorilyfitsthedata,whichisconfirmedby the LAC observation.However,thephotonindexofbest-fit best-fit parametersarelistedinTable1.Nosingle-component thermal bremsstrahlung,power-law,anddiskblackbody; Mitsuda etal.1984)werefittedtotheobservedspectrum. The 25, isshowninFigure3.Foursimplemodels(blackbody, response functions. points maybeduetoanunderestimateoftheerrorin ratio withadecaytimeconstantof202±0.5days. energy range1-3keVand61.5±0.3daysinthe 3-10 keV.Wecanalsoseeagradualdecreaseofthehardness decay timeconstantisderivedtobe66.3±0.2daysinthe believed tobequitereliable.Thescatterofthesubsequent field ofview.Thustherelativeintensitiesthesepointsare August. IfweassumeanexponentialdecayafterApril,the gradually decreasedtothedetectionlimitofASMin (2.9 ±0.1)x10ergsscmduring1987Marchand 2; 3-10keV).Thefluxreachedamaximumofabout -The lightCUrVeSftheenergyrange1-3keVand3-10hardnessratio/310 592 Disk blackbody Blackbody Thermal bremsstrahlung Power-law scribed °(-keV)//(l-3keV).Best-fitexponentiallines,after April,arealso The energyspectrum,integratedfromFebruary25toJuly Our observationsappeartoincludetherisingphase(seeFig. © American Astronomical Society • Provided by the NASA Astrophysics Data System Model Best-Fit ParametersfortheSpectraIntegratedfromFebruary25 toJune25 65 6 261 3 48 2 R,„ =(2.3+0.4)x10(cos¡)“°-(ß/10 kpc)cm r =(4.8+0.6)x10(D/10kpc)cm nV =(8.3±1.9)x10(D/10kpc) cm“ Scale =(1.5±0.8)x10(D/10kpc) at1keV KITAMOTO ETAL. lAULfc, i Best-fit Parameters keV. Thebest-fitparametersareshowninFigure4.Wefound that theobservedsofteningduringdecayphasemay be disk blackbody,restrictingtheenergyrangefrom1keV to 6 Therefore, weusedasingle-componentmodel,blackbody and significant changesoftheparametersbecausepoorstatistics. were investigated.Thetwo-componentmodelsdidnotyield energy spectrainthreeintervals(fromFebruary25toMarch 20, fromMarch24toApril21,andJune15 25) Table 2. to theobserveddatawell.Thebest-fitparametersareshownin law, and(ii)adiskblackbodypower-law.Bothmodelsfit models werefittedtothedata:(i)ablackbodyandpower- al. 1981;Makishimaet1986).Thus,twotwo-component consisting ofablackbody(ordiskblackbody)component and ahard-tailcomponentwithpower-lawshape(Bedfordet Generally, ultrasoftsourcesarewell-describedbythespectrum component modelaremainlyduetoahigh-energyexcess. X-ray color-colordiagram(Whiteetal.1984). This isconfirmedbyplottingthedataforGS1354-64on A0620 —00belongtothesamecategory(ultrasoftsources). To checkthespectralchangeduringdecayphase, The residualsoftheobserveddatafrombest-fitsingle- kT =(0.71±0.03)keVlog(JV)(22.13 ±0.13) kT =(1.63+0.13)keVlog(N)(22.58 ±0.05) Photon index=4.74+0.23log(;V ) =(22.80+0.04) H eH H = (0.89±0.03)keVlog(JV)(22.39 ±0.06) H 2 X/dof 46/8 56/8 34/8 18/8 Vol. 361 1990ApJ. . .361. .590K _91 No. 2,1990 density. region ortoadecreaseoftheinterveningabsorptioncolumn attributed toatemperaturedecreaseoftheX-rayemission maximum intensityis must belessthantheEddingtonluminosity,upperlimitof where Disthedistancetosource.Sincethisluminosity The upperlimit(90%confidence)ontheamplitudeofmodula- with afoldingmethod.Nosignificantperiodicitywasdetected. gravitational energy,whichisreleasedbythematterfalling where Misthemassofcompactstar.Ifenergysource M =(4.1±0.1)x10(r/10kmXM/Mo)' onto thecompactstar,requiredmassaccretionrateat the distanceisgivenas and 10.0keVisestimatedfromthemaximumfluxas tion was45%ofthemeanintensity. conversion ofgravitationalenergytoradiationenergy. The where ristheradiusofX-raystarandrjefficiency for best-fit modelspectra,composedofadiskblackbodyandpower-law,are also shown(histograms). Disk blackbody+power-law: Blackbody +power-law: 6_o 5 452 462 6 The maximumluminosityintheenergyrangebetween1.0 A searchforperiodicityfrom5to40dayswascarriedout Fig. 3.—TheenergyspectrumintegratedfromFebruary25toJuly25.The Disk blackbody....R=(5.1(+1.7, -2.0))x10(cos0(£>/10kpc)cm Power-law Scale=(6.45( +316,-6.36))x10(D/10kpc)at1keV Power-law Scale=(1.01( +34.0,-0.99))x10(D/10kpc)at1keV Blackbody r=(8.7(+ 2.2, -2.5))x10(D/10kpc)cm in 372-1 Model Lx =(3.5±0.1)10(D/10kpc)ergss, ma © American Astronomical Society • Provided by the NASA Astrophysics Data System 5 (D/10 kpc)<1.93±0.03(M/M)°, o Best-Fit ParameteroftheTwo-ComponentModelsforSpectrum IntegratedfromFebruary25toJune j i. 1 10 ENERGY (KEV) 2-1 x (D/10kpc)*/Myr, 0 X-RAY LIGHTCURVEOFGS1354-64 Best-fit Parameters TABLE 2 -lo12 The outburstsofCenX-2andMX1353—64seemtohavea than ourobservation,whilethatofMX1353—64wassimilar. (Seward etal1976),andHEAO1alsodetectedafaintsource, similar duration,oneortwomonths.ThesourcesA1354—64 indicate therecurrenceofthissource,assuggestedbyMarkert missed thestar;thismightbeduetoinfluencesofanearby region ofthepositionreportedhere,Ariel5detectedafaint burst in1966November(e.g.,Francey1971).Nearthesky et al1977,1979),andthatofCenX-2,whichshowedanout- outburst areestimatedas total energy,E,andtheaccretedmass,Mduringthis energy rangeofseveralkeV,butChodiletal(1968)reported reported. CenX-2isreportedtoshowapower-lawindexof and 1H1359—645areabout50timesweakerthanGS maximum intensityofCenX-2wasabout50timesstronger et al(1977).CenX-2isoneofthestrongestsofttransient source, 1H1348—633;Remillard1990.Theseobservations source, A1354—64,during1974and1976observations medium intensityandhard(MX1353—64);faint(or lard (1990)alsoreportedthatthespectrummaybeharderin but thatthespectrumisvariable.Onotherhand,spec- This impliesthatCenX-2belongstotheultrasofttransients, that thespectrumofCenX-2wassofterthanSeoX-l. sources (CookeandPounds1971;Cominskyetal1978).The different source.Wenotethatspectralstatesfora “off”) andprobablyhardstates.However,itispossiblethat variable) (CenX-2);mediumintensityandsoft(GS1354—64); GS 1354—64.Ifweassumethatalldetectionsareofsame the HEAO1detection,whenitwasabout50timesweakerthan Crab Nebula,whichisquitedifferentfromourresults.Remil- that thespectrumofMX1353—64washarderthan tral informationreportedbyMarkertetal(1979)indicates times inatleasttwoothersources,4U1630—47(Parmaret al given ultrasoftX-raytransienthavebeenobservedseveral some oftheearlyrocketsightingsCenX-2mayconcerna source, thenitshowsatleastfourstates;intenseandsoft(or sidered tobeblackholecandidates.Thefactthatthespectra to theultrasoftsources(Whiteetal1984),whichare con- 1.15-2.8 invariousobservations(seeFrancey1971)the 1354 —64,andnodetailsontheirtemporalbehaviorhavebeen 1H 1359-645(Woodetal1984).(TheHEAO1errorbox 1353 —64observedbyOSO7during1971and1972(Markert 1986) andGX393-4(Ricketts1983;Ilovaiskyetal1986). tot 442 M -7.3x10(r/10km)(M/M)D/10kpc)*/. toto0 E ~1.9x10(D/10kpc)ergs, tot The positionofGS1354—64isconsistentwiththatMX Our observedspectrumindicatesthatGS1354—64belongs Photon index=2.1(+1.9,-1.7)) kT =(0.57(+0.07,-0.09))keV kT =(0.69(+0.21,-0.13))keV Photon index=2.2(-f1.7,—1.5)) in III. DISCUSSION 22.46( +0.22,-0.16)3.7/7 22.57( +0.14,-0.09)4.5/7 log (N) h 2 Z/dof 593 1990ApJ. . .361. .590K p of (fl)blackbodanddsk Cooke, B.A.,andPounds,K.A.1971, NaturePhys.Sei.,229,144. Cominsky, L.,Jones,C,Forman,W., andTananbaum,H.1978,Ap.J.,224,46. Francey, R.J.1971,NaturePhys.Sei., 229,228. Coe, M.J.,Engel,A.R.,andQuenby, J.1976,Nature,259,544. Chodil, G.,Mark,H.,Rodrigues,R., and Swift,C.D.1968,Ap.J.(Letters),152, Bradt, H.V.D.,andMcClintock,J.E.1983,Ann.Rev.Astr.Ap.,21,13. and isalsosimilartothatofotherbrightsofttransientsources : Bedford, D.K.,Carpenter,G.F.,Goodall,C.V.,Pollack,A.M.T.,Cole, R. E., Bath, G.T.,andPringle,J.E.1981,M.N.R.AS.,194,967. similar tothe1966outburstofCenX-2(Chodiletal.1968), hole candidatessuchasCygX-landGX339-4. to aso-calledlow(orhard)statesimilarthoseseeninblack region. IfMX1353—64isthesamesource,itmightcorrespond be observedherebecauseofpoorstatisticsinthehard tail and A0620-00(Coeetal.1976;Ricketts1975),couldnot ponent, whichisobservedintheblackholecandidatesCygX-l can beconsistentlyexplained. ually disappearsduringthedecayphase,spectralevolution the interveninggasiscreatedduringrisingphaseandgrad- suggestion ofhardeningduringtherisingphase(seeFig.2).If ening duringtherisingphasein1983outburst,and absorption columndensity.Inourobservations,thereisalsoa hardening isinterpretedasanincreaseoftheintervening moto etal.(1984)reportedthat4U1543—47showedahard- decrease oftheinterveningabsorptioncolumndensity.Kita- decrease ofthetemperatureX-rayemittingregionora al. 1988). though someproblemsstillremaintobesolved(Miyamotoet Comptonization byahotplasma(LiangandNolan1984), The originofthehardtailisrecognizedtoberesult of blackholecandidates(Makishimaetal1986;Oda1977). the surfaceofaneutronstar. source isablackhole,becauseoftheabsenceX-raysfrom plus apower-lawhardtailindicatesthatthecentralcompact are well-describedbyadiskblackbody(Mitsudaetal.1984) zl, anafromJune15to25).Thecontourscorrespond68%confidencelevels. 21 andy^|Wackbodytothespectrainthreeintervals(fromFebruary25March20,from24April to 594 L45. Cruise, A.M.,andOsborne,J.P.1981,SpaceSei.Rev.,30,373. The e-foldingdecaytimeofabout2monthsweobserved is An anticorrelationbetweenhardtailandsoftX-raycom- The spectralfittinganalysesforthedecayphaseindicatea The hardtailisalsoacharacteristicfeatureofthespectrum © American Astronomical Society • Provided by the NASA Astrophysics Data System KITAMOTO ETAL. REFERENCES Koyama, K.,etal.1981,Ap.J.(Letters), 247,L27. Kitamoto, S.,Miyamoto,Tsunemi, H.,Makishima,K.,andNakagawa,M Kaluzienski, L.J.,etal.1975,Ap.J.(Letters), 201,L121. Kaluzienski, L.J.,Holt,S.S.,Boldt, E.A.,andSerlemitsos,P.J.1977,Ap.J Ilovaisky, S.A.,etal.1990,inpreparation. Ilovaisky, S.A.,Chevalier,C,Motch,andChiappetti,L.1986,Astr. Ap., Guibert, J.1990,PrivateCommunication. des Sciencesdel’Univers(INSU/CNRS). The MAMAscanningmachine,locatedatObservatoire de ported bythegrant-in-aidforscientificresearchNo.63540197. Paris, hasbeenfundedandisoperatedbytheInstitutNational the Gingateam,aswelllaunchingstaffinInstitute of Space andAstronauticalScience.Thisworkwaspartly sup- clarify this. the caseofA0620-00(McClintockandRemillard1986),may yet nodecisiveevidencethatthesystemcontainsablackhole. Optical observationsduringtheX-rayinactivephase,justasin they arethesamesource,ithasatleastfourstates.Thereisas sources showeddifferentcharacteristicsthanGS1354-64.If region: MX1353—63andCenX-2.However,bothofthese and Pringle1981). (Kaluzienski etal.1977),andGS2000+25(Tsunemi studied sofar.TherearetwoX-rayoutburstsreportedinthis This isoneofthecharacteristicsblackholecandidates keV andapower-lawcomponentwithphotonindexof2.1. tion ortheviscosity(a-value)inaccretiondisk(e.g.,Bath F ponents: adiskblackbodyspectrumwithtemperatureof0.7 GS 1354—64.TheX-rayspectrumconsistsoftwocom- optical identificationoftheultrasoftX-raytransientsource, 3U 1543—47(=4U1543—47)(Matilskyetal.1972;Li 1989). Thesetimescalesmayrelatetotheaccretiondiskforma- 1976), A1524-62(Kaluzienskietal.1975),A0620-00 212,203. 1984, Pub.Astr.Soc.Japan,36,799. 164,67. The authorsexpresstheirthankstotheothermembers of We reportedthediscovery,X-raylightcurve,and IV. CONCLUSION Vol. 361 1990ApJ. . .361. .590K .1988,1AUCire.,No.4571. Matilsky, T.A.,Giacconi,R.,Gursky,H.,Kellogg,E.M.,andTananbaum, No. 2,1990 Makino, F.,andGingaTeam1987,1AUCire.,No.4342. Makino, F.,andAstro-CTeam1987,Ap.Letters,25,223. Liang, E.R,andNolan,P.L.1984,SpaceSei.Rev.,38,353. Li, F.K.,Sprott,G.F.,andClark,W.1976,Ap.J.,203,187. Landolt, A.U.1983,Astr.J.,88,439. Holger Pedersen:CopenhagenUniversityObservatory,OesterVoldgade3,DK1350,K,Denmark Markert, T.H.,etal.1979,Ap.J.Suppl,39,573. Marker!, T.H.,Cañizares,C.R.,Clark,G.W.,Hearn,D.Li,F.K.,Sprott, Makishima, K.,etal.1986,Ap.J.,308,635. Miyamoto, S.,Kitamoto,Mitsuda,K.,andDotani,Y.1988,Nature,336, Mitsuda, K.,etal.1984,Pub.Astr.Soc.Japan,36,741. McClintock, J.E.,andRemillard,R.A.1986,Ap.J.,308,110. Matsuoka, M.,etal.1980,Ap.J.(Letters),240,LI37. Toyonaka, Osaka,560,Japan Murakami, T.,etal.1980,Ap.J.(Letters),240,LI37. Astrophysics, Roetersstraat15,1018WBAmsterdam,TheNetherlands Shunji KitamotoandHiroshiTsunemi:DepartmentofPhysics,FacultyScience,OsakaUniversity,1-1,Machikaneyama-cho, Sergio A.Ilovaisky:ObservatoiredeHaute-Provence(CNRS),F04870St.Michell’Observatoire,France Michiel vanderKlis:AstronomicalInstitute“AntonPannekoek,”UniversityofAmsterdam,andCenterfor High-Energy H. D.1972,Ap.J.(Letters),174,L53. G. F.,andWinkler,P.F.1977,Ap.J.,218,801. 450. © American Astronomical Society X-RAY LIGHTCURVEOFGS1354-64 Provided bythe NASA Astrophysics Data System Wood, K.S.,etal.1984,Ap.J.Suppl,56,507. White, N.E.,andMarshall,F.E.1984,Ap.J.,281,354. Tsunemi, H.,Kitamoto,S.,Okamura,andRoussel-Dupre,D.1989,Ap.J. Tsunemi, H.,Kitamoto,S.,Manabe,M.,Miyamoto,Yamashita,K.,and van Paradijs,J.1983,inAccretion-drivenStellarX-RaySources,ed.W.H.G. Seward, F.D.,Page,C.G.,Turner,M.J.L.,andPounds,K.1976,M.N.R.A.S., Ricketts, M.J.,Pounds,K.A.,andTurner,J.L.1975,Nature,257,657. Ricketts, M.J.1983,Astr.Ap.,118,L3. Remillard, R.1990,privatecommunication. Priedhorsky, W.C,andHolt,S.1987,SpaceSei.Rev.,45,291. Pedersen, H.,Ilovaisky,S.,andvanderKlis,M.1987,1AUCire.,No.4357. Parmar, A.N.,Stella,L.,andWhite,N.E.1986,Ap.J.,304,664. Oda, M.1977,SpaceSei.Rev.,20,757. Press), p.207. (Letters), 331,L81. Nakagawa, M.1989,Pub.Astr.Soc.Japan,41,389. Le winandE.P.J.vandenHeuvel(Cambridge:CambridgeUniversity 111, 13. 595