BeppoSAX and Chandra Observations of SAXJ0103.2-7209=2E0101.5-7225: a new Persistent 345s X-ray Pulsar in the SMC

4 Pages·0.59 MB·English

by  G. L. Israel

RECEIVED: 1999DECEMBER6;ACCEPTED: 2000JANUARY24
PreprinttypesetusingLATEXstyleemulateapj
BeppoSAXANDChandraOBSERVATIONSOFSAXJ0103.2–7209=2E0101.5–7225:ANEWPERSISTENT
345SX–RAYPULSARINTHESMC†
G.L.ISRAEL1,∗,S.CAMPANA2,∗,S.COVINO2,D.DALFIUME3,T.J.GAETZ4,S.MEREGHETTI5,T.
OOSTERBROEK6,M.ORLANDINI3,A.N.PARMAR6,D.RICCI7ANDL.STELLA1,∗
1.OsservatorioAstronomicodiRoma,ViaFrascati33,I–00040MonteporzioCatone(Roma),Italy
2.OsservatorioAstronomicodiBrera,ViaBianchi46,I–23807Merate(Lc),Italy
3.TeSRE,C.N.R.,ViaGobetti101,I–40129Bologna,Italy
4.Harvard–SmithsonianCenterforAstrophysics,MS–70,60GardenStreet,Cambridge,MA02138,USA
5.IFCTR,C.N.R.,ViaBassini15,I–20133Milano,Italy
0 6.AstrophysicsDivision,SpaceScienceDepartmentofESA,ESTEC,P.O.Box299,2200AGNoordwijk,TheNetherlands
0 7.SAXScienceDataCenter,ASI,VialeReginaMargherita202,I–00198Roma,Italy
0
Received:1999December6;Accepted:2000January24
2

n ABSTRACT
a
We reportthe resultsofa 1998JulyBeppoSAXobservationofa fieldin the SmallMagellanicCloud (SMC)
J
which led to the discovery of ∼345s pulsations in the X–ray flux of SAXJ0103.2–7209. The BeppoSAX X–
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ray spectrumis well fit by an absorbedpower–lawwith photonindex∼1.0 plusa black body componentwith
2 kT=0.11keV.Theunabsorbedluminosityinthe2–10keVenergyrangeis∼1.2×1036ergs- 1. Inaveryrecent

Chandra observationthe 345s pulsationsare also detected. The available periodmeasurementsprovidea con-
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v stantperiodderivativeof–1.7syr- 1 overthelastthreeyearsmakingSAXJ0103.2–7209oneofthemostrapidly
3 spinning–upX–ray pulsars known. The BeppoSAX position (30′′ uncertaintyradius) is consistent with that of
2 theEinsteinsource2E0101.5–7225andtheROSATsourceRXJ0103.2–7209. Thissourcewasdetectedatalu-
4 minosity levelof few 1035–1036ergs- 1 in alldatasets of pastX–raymissionssince 1979. The ROSATHRI and
1 Chandra positions are consistent with that of a m =14.8 Be spectral type star already proposed as the likely
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0 opticalcounterpartof2E0101.5–7225.Webrieflyreportanddiscussphotometricandspectroscopicdatacarried
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outattheESOtelescopestwodaysbeforetheBeppoSAXobservation.WeconcludethatSAXJ0103.2–7209and
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2E0101.5–7225arethesamesource,arelativelyyoungandpersistentX–raypulsarintheSMC.
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h Subjectheadings:binaries:general—pulsars:individual(SAXJ0103.2–7209;2E0101.5–7225)—stars:
p
emission–line,Be—stars: rotation—X–ray:stars
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o
r 1. INTRODUCTION In this Letter we report the discovery of 345s pulsations
t
s from the source SAXJ0103.2–7209 during a BeppoSAX ob-
a During1997–1998thenumberofX–raypulsarsfoundinthe
servationoftheSMC.ThecomparisonwiththedataofpastX–
: Small Magellanic Cloud (SMC) rapidly increased from three
v raymissionsallowsustoconcludethatSAXJ0103.2–7209and
(SMCX–1, RXJ0053.8–7226,and 2E0050.1–7247;Lucke et
i 2E0101.5–7225are the same object, a persistent source with
X al 1976; Hughes1994; Israelet al. 1997)up 14 (fora review
moderatevariability(within a factor of 5–10). We also report
see Yokogawaet al. 1998)thanksto sensitive observationsof
r theresultsofthetiminganalysisofarecentpublicChandraob-
a the large area detectors on board the RXTE and ASCA satel-
servationanddiscussopticalobservationscarriedoutatESO.
lites. The majority were found to be associated with massive
BespectraltypestarsshowingintenseHαemissionlines. Only
SMCX–1, which is associated with a super–giantB0 spectral
type star in a 3.9d orbital period binary system, is a persis-
2. OBSERVATIONSANDDATAANALYSIS
tent (although moderately variable) X–ray pulsar. For all the
remainingX–raypulsarspronouncedvariability(afactor≥50) 2.1. BeppoSAXobservation
or,moreoften,transientbehaviourhasbeendefinitivelyproven.
The SMC field including the position of the 2E0101.5–
Thesource2E0101.5–7225wasdetectedatanearlyconstant
7225 was observed by the Narrow Field Instruments (NFIs)
fluxlevelinalltheEinstein,ROSATandASCApointingswhich
on board the BeppoSAX satellite (Boella et al. 1997a) on
surveyedtherelevantregionoftheSMC(seeHughes&Smith
1998 July 26–27 (effective exposure time of 40320s). We
1994), but pulsations were not found due to poor statistics.
used data from the Medium Energy (MECS; Boella et al.
BasedontheaccuratepositionobtainedwiththeROSATHRI,
1997b) and Low Energy (LECS; Parmar et al. 1997) instru-
theseauthorsfoundthat2E0101.5–7225isverylikelyassoci- ments. A bright X–ray source (∼3.7×10- 2cts- 1, 1–10keV)
atedwithaBespectral–typestar(R.A.=01h03m13.s86,Dec.=– was detected on–axis in the MECS, at R.A.=01h03m13s,
72◦09′14.′′1; equinoxJ2000). 2E0101.5–7225is located near Dec.=–72◦09′16′′ (J2000; 90% confidence uncertainty ra-
the optical limb of the supernova remnant SNR0101–72.4.
dius of 30′′ ). The MECS event list and spectrum were ex-
Hughes & Smith (1994) present several arguments that make
tracted from a circular region of 4′ radius (corresponding to
theBe/X–raybinary–SNRassociationunlikely.
an encircled energy of ∼90%) around the X–ray position.
†TheresultsreportedinthisLetterarepartiallybasedonobservationscarriedoutatESO,LaSilla,Chile(61.D–0513).
∗AffiliatedtotheInternationalCenterforRelativisticAstrophysics(I.C.R.A.)
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2 2E0101.5–7225:A345sPERSISTENTX–RAYPULSARINTHESMC
FIG. 1.— 1998July (upperleft panel)and 1999August(lower leftpanel) powerspectra of the 1–10 keV BeppoSAXMECS and
Chandra ACIS light curves of 2E0101.5–7225. The corresponding folded light curves at the best periods are also shown in the
centralpanels.TheBeppoSAXfoldedlightcurveisgivenseparatelyinthe1–4keV(S)and4–10keV(H)ranges.Theperiodhistory
of2E0101.5–7225isshownintherightpanel.
(panel H) folded light curves provides marginal evidence for
anenergydependentpulseprofile: nearlysinusoidalatlowen-
ergies,whiledouble–hornedabove4keV. Howeverthe pulsed
fraction (semi–amplitude of modulation divided by the mean
source count rate) of ∼45% is constant over the two energy
intervalsconsidered.
A spectral analysis was performed in the 0.7–6.5 and 1.6–
10keV energy ranges for the LECS and MECS, respectively.
The spectra were rebinned to have at least 20 counts in each
energychannel, such thatχ2 fitting techniquescould be used.
No single componentmodel was found to fit the data well (a
power–law gave a χ2/dof =41/15; where dof is the degree
of freedom). Among double componentmodels, an absorbed
power–lawplusablackbodygavethebestfit(χ2/dof=15/15)
for a photonindexof 1.0±0.2, N ≤3.8±7.5×1021cm- 2, and
0.1 H 3.8
ablackbodytemperatureof0.11±0.03keV(uncertaintiesre-
FIG. 2.— BeppoSAX (crosses) and ROSAT PSPC (triangles) fer to 1σ). The observed flux in the 2–10 keV energy band
spectra of 2E0101.5–7225. The best fit model is shown, to- was 2.7×10- 12ergs- 1 cm- 2 (the soft componentaccountsfor
getherwiththecorrespondingresiduals. ∼15%ofthetotal)correspondingtoanunabsorbed2–10keV
X–ray luminosity of 1.2×1036ergs- 1 assuming a distance of
A4′extractionradius(∼85%encircledenergy)wasalsoused
62kpc(Laney&Stobie1994).
fortheLECSinordertominimizethecontaminationfromthe
2.2. ArchivalX–rayObservations
soft X–ray emission of the bright nearby source 2E0102.3–
7217. The local backgroundwas measured in a region of the After the discovery of 345s pulsations in 2E0101.5–7225
MECSandLECSimagesfarfromanydetectedfieldsources. (Israeletal. 1998),Yokogawa&Koyama(1998)founda sig-
Thearrivaltimesofthe∼1500photonswerecorrectedtothe nalataperiodof348.9sduringa1996MayASCAobservation
barycenterofthesolarsystemandbackgroundsubtractedlight (absorbed luminosity of 5×1035ergs- 1; 2–10keV) implying
curveswereaccumulatedin0.5sbins.Asinglepowerspectrum a periodderivative(with respectto BeppoSAX)of –1.7syr- 1.
wascalculatedovertheentiretimespancoveredbytheobserva- The source was also detected by ASCA on 1993 May 12 and
tioninordertomaximizethesensitivitytocoherentpulsations. 1997 November 14 at an absorbed luminosity level of 6 and
Significantpowerspectrumpeakswere searchedforusing the 5×1035ergs- 1,respectively. Are–analysisofthelatterASCA
algorithmdescribedinIsrael&Stella(1996). Ahighlysignif- datasetsallowedustoinferanupperlimit(3σconfidence)inthe
icant peak (∼9.4σ based on the fundamentalonly; see upper 60–80%rangeonthepulsedfractionforaperiodof∼345s.
left panelof Fig.1) wasfoundata frequencyof0.002889Hz, To better constrain the absorption we used archival data
correspondingtoaperiodof345.2s. Anaccuratedetermination from the ROSAT satellite (PSPC; 0.1–2keV band) which ob-
oftheperiodwasobtainedbyfittingthephasesofthemodula- served the field of 2E0101.5–7225 between 1991 October
tionover4differentintervalsof∼16000seach. Thescatterof 8 and 1992 April 28 (sequence 600195; effective exposure
thephaseresidualswasconsistentwithastrictlyperiodicmod- time 26630s). Assuming that the source showed the same
ulationatthebestperiodof345.2±0.3s(90%confidence). A flux and spectrum during the two observations, we fitted the
comparison of the 1–4keV (panel S in Fig.1) and 4–10keV ROSAT and BeppoSAX data together. We found that the two
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FIG.3.—45′′×45′′ChandraACISimageoftheregionaround2E0101.5–7225(leftpanel). TheimagewassmoothedwithaGaus-
sianofσ=2′′. ThecrossmarkstheopticalpositionoftheBestaridentifiedbyHughes&Smith(1994).Thedouble–peakedshapeof
thesourceisartificial(seetextfordetails). 2′×2′HαimagetogetherwiththeROSATandBeppoSAXpositioncircles(rightpanel).
Theproposedopticalcounterpartisalsomarked.
component model derived above again gave the best fit; a tectedatasignificancelevelof7.5σ. Arefinedperiodwasde-
χ2/dof=23/25 was obtained for a photon index of 1.1±0.1 termined by means of the phase fitting technique; this gave a
with N of3.9±4.0×1021cm- 2 anda blackbodytemperature value of 343.5±0.5s (see Table1). We note that the Chan-
H 2.4
of0.12±0.04keVcorrespondingtoanequivalentblackbodyra- dradata were notcorrectedto thebarycenterof the solar sys-
0.03
dius R in the 2–60km range (see Fig.2; 1σ uncertainties). tem. However, for a relatively long period pulsar in the di-
bb
The corresponding power spectrum shows no significant (3σ rection of the SMC, the effect of the spacecraft and of the
confidence)peakinthe339–359speriodintervalwitha∼60% Earth motion, would cause a maximum correction of a factor
upperlimitonthepulsedfraction. of10smallerthanthestatisticaluncertaintyoftheperiodgiven
2.3. Chandraobservation above. The pulse profile is sinusoidal (over the whole Chan-
draenergyband)andthepulsedfractionis∼45%(seeFig.1).
The field including 2E0101.5–7225 was observed by the
Thisresultimpliesthatthepulsariscontinuingtospin–upata
NASA Advanced X–ray Astrophysics Facility satellite Chan- constantrateof–1.7syr- 1since1996(seeFig.1;rightpanel).
draon1999August23withtheImagingSpectrometer(ACIS;
In order to further address the issue of the double peaked
Garmireetal. 1992,Bautzetal. 1998andreferencestherein)
Chandra image (see Fig.3), we extracted two separate light
inthehigh–resolutionimagingmodeforaneffectiveexposure
curvesfor each of the two peaks. We foundthat the signal at
time of19551s. Thesourcewasdetectedatanoff–axisangle
343.5swaspresentwithasimilarpulsedfractioninbothlight
of about 9′ in the S4 (front illuminated) CCD of the ACIS–
curves;thisresultisconsistentwiththehypothesisofapoint–
S detector with a count rate of ∼9×10- 2 ct s- 1 (in the 0.5–
like source. An independent confirmation was also obtained
10keV energyrange; see Fig.3 leftpanel). Thesourceshows
through a raytracing simulation, encompassing a monochro-
twoemissionpeaksseparatedby∼5′′. Howeverthesizeofthe
matic source (1.49keV) and models for the mirror assembly
emission regionis comparablewith the 90% encircled energy
andACISdetector.TheresultsindicatethatexpectedPSFatan
regionofasourceat9′off–axisangle.
off–axisangleof9′′isartificiallyelongatedinthedirectioncon-
We extracted a 3.24s binned light curve of the source
nectingthetwopeaksdetectedintheChandraimagearoundthe
from a region within 8′′ from the center of the emission:
position of 2E0101.5–7225; the two peaks are probably arte-
R.A.=01h03m14.s06, Dec.=–72◦09′15.′′25 (equinox J2000).
factsresultingfromthe smallnumberofX–raysinthe image.
Thestatisticaluncertaintyradiusisonly∼0.5′′butatthisstage
Weconcludethatthesourceisconsistentwithbeingpoint–like.
the uncertaintyin the absolute positioningat such an off–axis
angle might be as large as 90′′. However the detection of the 2.4. OpticalObservations
pulsations clearly associates the Chandra source with that of
Optical images (Hα and Hα red–continuum; 200s each)
BeppoSAX. We also note that the Chandra coordinates are
of the BeppoSAXerrorcircle of SAXJ0103.2–7209were ob-
consistent with the ROSAT HRI uncertainty circle and differ
tained on 1998 July 24 at the Danish 1.5m telescope with
by less than 2′′ from those of the proposed optical counter-
theDanishFaintObjectSpectrometerCamera(DFOSC)atLa
part(seebelow)makingtheassociationverylikely. According
Silla (Chile) in order to search for emission–line stars. The
to the Chandrasource namingconventionwe designatedit as
datawerereducedusingstandardESO–MIDASproceduresfor
CXOJ010314.1–720915.
biassubtraction,flat–fieldcorrection,aperturephotometryand
A single powerspectrumwascalculatedoverthe entireob-
onedimensionalstellar andskyspectraextraction. Withinthe
servation. The search was performed over a period interval
BeppoSAX position circle we found only one Hα active ob-
aroundthatdetectedbyBeppoSAXandassumingamaximum
|P˙| of ∼3 s yr- 1 which translates into a search over only two ject: this is the O9–B1III–Ve mV=14.8 star originally sug-
gestedastheopticalcounterpartof2E0101.5–7225(Hughes&
Fourierfrequencies(see Fig.1 lower panels). A peakwas de-
Smith 1994). A 10A˙ resolution 3800–8500A˙ spectrum (2.′′0
4 2E0101.5–7225:A345sPERSISTENTX–RAYPULSARINTHESMC
TABLE1
PERIODHISTORYFOR2E0101.5–7225.
Mission Instrument Period Date Exposure 2–10keVL Reference
X
(s) (s) (1035ergs- 1)
ASCA GIS 348.9±0.3 1996May21–23 32912 5 1
BeppoSAX MECS 345.2±0.2 1998Jul26–27 40320 11 2;thiswork
Chandra ACIS 343.5±0.5 1999Aug23 19551 — thiswork
Note:Luminositiesrefertoabsorbedfluxes.90%uncertaintiesarereported.
1–Yokogawa&Koyama(1998);2–Israeletal.(1998).
slit) of this star was obtained with the same instrument on tinuously supplies matter to the compact object. Assuming a
1998October20. StrongHαandHβ emission–lineswerede- main sequence star, an orbital period of ∼300d is expected
tected, with equivalentwidth –20±2A˙, and –1.8±0.2A˙, re- for2E0101.5–7225basedonthepulseperiod–orbitalperiod
spectively. Theseresultsareingoodagreementwiththoseob- correlationof Be star/X–ray pulsar binaries(Corbet1984). A
tained byHughes&Smith (1994)duringobservationscarried timing analysisof I band photometricmeasurementsfrom the
outonDecember1992. OpticalGravitationalLensingExperiment(OGLE)revealedno
evidence of any coherent signal in the 1–50d period interval
3. DISCUSSION (see Coe & Orosz 1999) suggesting that 2E0101.5–7225is a
Although2E0101.5–7225sharesseveralcharacteristicswith longperiodsystem.
other X–ray pulsatorsin the Magellanic Clouds(MCs) and in The characteristics of 2E0101.5–7225 have their closest
theGalacticplane(i.e.,thespinperiodandpulsedfraction,the analogy to those of the persistent low–luminosity (1035–
spectralshapeathighenergyandthepresenceofasoftthermal 1036ergs- 1; 2–10keV band) X–ray pulsar RXJ0146.9+6121,
component,theassociationwithahighmasscompanion,etc.), a1455s(spin–uptimescaleof∼250yr)spinningneutronstar
ithasalsosomepeculiardifferenceswhichmakeitunusual. inabinarysystemwithaB1Vecompanionstar(seeReigetal.
The absence of long–term variability greater than a factor 1997; Haberlet al. 1998; Mereghettiet al. 2000). Two other
of∼5–10overanintervalof∼20yearsisstronglysuggestive recentlydiscoveredlongperiodBe/X–raypulsarsintheSMC,
of a persistent X–ray pulsar, the second in the SMC since the namely AXJ0051–733 (Ps=323s; Imanishi et al. 1999) and
discovery of pulsations from SMCX–1. Moreover, the asso- AXJ0058–7203 (Ps=280s; Tsujimoto 1999), although highly
ciation of the companion with a Be spectral–type star makes variable(afactorof10–100),haveamaximumluminositylevel
2E0101.5–7225 the first example of a persistent (main se- closetothatof2E0101.5–7225.
quence)Be/X–raybinarysystemintheSMC.2E0101.5–7225 Inconclusion,wediscoveredanewX–raypulsarwithape-
is also a relatively low–luminosity (1035–1036ergs- 1) X–ray riodof∼345sintheSMCwhichispersistent,hasarelatively
pulsar. Finally,theinferredperiodderivativeof–1.7syr- 1cor- low–luminosity, is rapidly spinning–upand is associated with
respondsto a secularspin–uptime–scale of∼200yrwhich is aO9–B1III–Vestar. Therelativelyhighvalueinferredforthe
theshortestofanyknownX–raypulsarinaHighMassX–ray magneticfieldandperiodderivativepointtoayoungobject.All
binary. these findingsmake 2E0101.5–7225an unusualX–raypulsar
ByusingthePandP˙ measurementsandanX–rayluminosity whichdeservesmoredetailedinvestigationswithinstrumenta-
in the 0.5–2×1036 ergs- 1 range, we infer a magneticfield of tionrangingfromtheX–raytotheIRband.
4–12×1012Gaussand,correspondingly,amaximummagneto-
sphericradiusofr ∼4×109cm(seeLambetal.1973).Since This work was partially supportedthroughASI and CNAA
m
thederivedcorotationradiusr is∼8×109cm,ther iscon- grants. We thank the Chandra Data Archive (CDA) of the
co m
siderably smaller than r , and accretion on the surface of the ChandraX–RayObservatoryScienceCenter(CXC)atCfAfor
co
neutron star proceedsunaffectedby the magnetosphere’scen- apromptreleaseofthedata.WethankR.Ragazzoniforobtain-
trifugal drag, as long as the luminosity of the source is larger ingtheopticalspectrumofthecounterpartof2E0101.5–7225.
than ∼2×1035ergs- 1. Such a luminosity is not unusual for We thank H. Tananbaum, R.J. Edgar and L. Burderi for their
a bright X–ray pulsar in a nearly circular orbit around a gi- helpful comments. This work has been supported in part by
ant OB companion star where the intense stellar wind con- NASAcontractNAS8-39073.
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