RXJ0123.4-7321, a Be/X-ray binary in the wing of the SMC

0.84 MB·English

by  R. Sturm

Astronomy&Astrophysicsmanuscriptno.aa˙RXJ0123˙v3 (cid:13)c ESO2013
January16,2013
RXJ0123.4-7321, a Be/X-ray binary in the wing of the SMC
R.Sturm1,F.Haberl1,W.Pietsch1,andA.Udalski2
1 Max-Planck-Institutfu¨rextraterrestrischePhysik,Giessenbachstraße,85748Garching,Germany
2 WarsawUniversityObservatory,AlejeUjazdowskie4,00-478Warsaw,Poland
Received8November2012/Accepted13January2013
ABSTRACT
3
Aims.ToconfirmfaintBe/X-raybinarycandidatesfromtheXMM-NewtonsurveyoftheSmallMagellanicCloud,wesearchedfor
1
X-rayoutburstsinarchival ROSATobservations. Wefound that RXJ0123.4-7321 wasmuch brighterwhen detectedwithROSAT
0
thanseen16yearslaterbyXMM-Newton.
2
Methods.WeanalysedtheROSATobservationsandtheOGLEI-bandlightcurveoftheopticalcounterparttoinvestigatethenature

n ofthesystem.
a Results.Highlong-termvariabilityintheX-rayfluxofafactorof∼150wasfoundbetweentheROSATandXMM-Newtondetections,
J indicatingstrongoutburstactivityduringtheROSATobservations.TheI-bandlightcurverevealslong-termvariabilityandregular
outburstswithaperiodof(119.9±2.5)daysindicatingtheorbitalperiodofthebinarysystem.
5
1 Conclusions.ThelargeX-rayfluxvariationsandthepropertiesoftheopticalcounterpartconfirmRXJ0123.4-7321asanewBe/X-ray
binaryinthewingoftheSmallMagellanicCloud.

] Key words. galaxies: individual: Small Magellanic Cloud – galaxies: stellar content – stars: emission-line, Be – stars: neutron –
E
X-rays:binaries
H
.
h
p 1. Introduction nantsuggests(He´nault-Brunetetal.2012;Haberletal.2012b).
- Thismightcausedifferentpropertiesofthewingsample.E.g.a
o
Be/X-ray binaries (BeXRBs, e.g. Reig 2011) are binary sys- literature search reveals in the wing area three known BeXRB
r
st tems composed of a compact object and a Be star of spectral pulsars with Pspin < 40 s (Clarketal. 1997; Chakrabartyetal.
typeO9−B5andluminosityclassIII−V.Bestarsejectmatterin 1998;Corbetetal.2003)comparedtoonlyone(SXP1062)pul-
a
[ the equatorialplane leading to the build up of a decretion disc sar with Pspin > 40 s. In contrast to that, for the whole SMC
(Okazaki2001;Ziolkowski2002)aroundthestarthatproduces Kniggeetal.(2011) finda bimodalspindistributionwith more
1
emission lines and a near-infrared (NIR) excess. Both compo- systemsaboveP >40s,whichislikelycausedbyabimodal
v spin
nents show variability due to disc instabilities and sometimes distributionoforbitalperiods.Thesearchforadditionalsystems
2
duetointeractionwiththecompactobject.Thecompactobject in thewingof theSMC isessentialto enablea statistical com-
6
is in most cases a neutron star (NS). Systems with black holes parisonof the wing and barpopulationand excludeany obser-
3
3 arenotknownuptonow.However,systemsmaycontainwhite vationalbias.
. dwarfs (Lietal. 2012; Sturmetal. 2012a) that are observable TheXMM-NewtonsurveyoftheSMC(Haberletal.2012a)
1
atSMCdistanceassuper-softX-raysourceinthe caseofther- revealed new BeXRBs in X-ray-bright state (Townsendetal.
0
monuclearsurfaceburningonthewhitedwarf.Causedbyakick 2011;Sturmetal.2011;Coeetal.2011,2012).Inaddition,sev-
3
experiencedbythe NSduringthe supernovaexplosion,theNS eral candidates for BeXRBs were found in the XMM-Newton
1
: canhaveaneccentricorbitaroundtheBestar.Accretionispos- SMC point-source catalogue (Sturmetal. 2013). One of these
v
sible, when the NS is close to the decretiondisc of the Be star candidatesis RXJ0123.4-7321(Haberletal. 2000), whichwas
i
X duringperiastronpassage.Thiscausesso-calledtype-Ioutbursts detected with ROSAT (Tru¨mper 1982) previously at much
thatlastforsomedays.Longer(severalweeks)type-IIoutbursts higherX-ray luminosity,butnotrecognisedas BeXRB. In this
r
a arelikelyconnectedwithdecretiondiscinstabilities(Ziolkowski paper,we presentouranalysis of the ROSAT observationsand
2002). the OGLE (Udalskietal. 2008) data of the opticalcounterpart,
allowingustoconfirmRXJ0123.4-7321asanewBeXRBinthe
TheSmallMagellanicCloud(SMC)hostsalargepopulation
wingoftheSMCwithadeterminedorbitalperiod.
of BeXRBs. A literature search (e.g. Haberl&Pietsch 2004;
Coeetal. 2005; Liuetal. 2005; Laycocketal. 2010) reveals
about 60 pulsars in more than 100 BeXRB systems (including
2. AnalysesandresultsofX-rayobservations
candidates),whichallowsacomparisonoftheirstatisticalprop-
ertieswiththoseoftheGalacticsample.MostoftheBeXRBsin RXJ0123.4-7321 was detected four times in X-ray observa-
theSMCarefoundinthebarwhereincreasedstarformation∼40 tions as listed in Table 1. The source was found in three
Myr ago created them in large number (Antoniouetal. 2010). ROSAT observationsof SMCX-1 performed between October
However, in the eastern wing of the SMC only a few systems 1991 and June 1993 at an off-axis angle of ∼27.7′. The
are known, probably due to a younger stellar population (∼10 ROSAT position from Haberletal. (2000, their source 460)
Myr, Harris&Zaritsky 2004). Also the BeXRBs in the wing is RA (J2000)=01h23m27s.2 and Dec (J2000)=−73◦21′25′′with
mightbeyounger,ase.g.SXP1062foundin a supernovarem- a 90% uncertainty of 4.6′′. This is consistent with the XMM-
1
Sturmetal.:TheBe/X-raybinaryRXJ0123.4-7321
Table1.X-rayobservationsofRXJ0123.4-7321.
Satellite ObsID MJDa Durb Orbitc Exp Cts F F Ld Ld
observed model
Instrument (d) Phase (ks) (ergcm−2s−1) (ergcm−2s−1) (ergs−1) (ergs−1)
0.1−2.4keV 0.2−4.5keV 0.1−2.4keV 0.2−4.5keV
ROSAT/PSPC rp400022n00 48536.2 0.96 0.65 16.6 936 8.84×10−13 1.96×10−12 6.43×1035 1.07×1036
ROSAT/PSPC rp400022a01 48895.7 1.87 0.66 9.0 584 1.02×10−12 2.27×10−12 7.43×1035 1.24×1036
ROSAT/PSPC rp400022a02 49141.0 1.20 0.70 11.9 147 1.95×10−13 4.32×10−13 1.42×1035 2.36×1035
XMM/EPIC 0601212401 55011.6 0.41 0.71 25.4 58 – 1.55×10−14 – 7.86×1033
Notes. (a) ModifiedJuliandateofthebeginningoftheobservation.(b) Durationoftheobservation. (c) Phase=0.5correspondstomaximumof
opticaloutbursts.(d)Unabsorbedluminosity.Asourcedistanceof60kpcisassumed.
20:40
V−1 0.01 50
e
unts s k−1 0)-73:21:0100
o 0
C 10−3 20 20
c (J XMM-Newton
De 30 ROSAT
40
1
50
χ 0
22:00
−1
10
0.2 0.5 1 2 40 35 1:23:30 25 20 15
Channel Energy (keV) RA (J2000)
Fig.1. Merged ROSAT/PSPC spectrum of RXJ0123.4-7321 Fig.2. OGLEIII I-band finding chart of RXJ0123.4-7321,
fromobservationsrp400022n00andrp400022a01togetherwith marked with a white cross. Circles indicate the 90% position
thebest-fitpower-lawmodel.Thelowerpanelgivestheresiduals uncertaintiesoftheROSATandXMM-Newtondetections.
inunitsofσ.
find a high long-term variability of a factor of 146±20, that
wouldbeatypicalforanAGN.
Newton position of the SMC-survey catalogue (Sturmetal. We used a Bayesian odds ratio (Gregory&Loredo 1996;
2013, source 3003) of RA (J2000)=01h23m27s.46 and Dec Haberletal. 2008) and a Rayleigh Z2 (Buccherietal. 1983;
1
(J2000)=−73◦21′23′.′4witha1σuncertaintyof1.1′′. Haberl&Zavlin 2002) test to search for the spin period of the
ROSATspectrawereextractedusingtheftools(Blackburn NS. The search for pulsations in the ROSAT data with best
1995)taskxselect.Forspectralanalysis,wemergedtheobser- statistics (rp400022n00) in the (1−200) s band did not reveal
vationsrp400022n00andrp400022a01toincreasethestatistics convincingperiodicities.NotethattheROSATobservationwas
and used xspec(Arnaud1996) version 12.7.0uto fit the spec- performed with individual exposures between 200 and 1300 s,
trumwithapower-lawmodelandphotoelectricabsorptionby a preventingthedetectionoflongperiods.Thebestcandidatefor
GalacticcolumndensitysettoN =3.3×1020cm−2(afore- the spin period with 1σ uncertainty is found at 17.34529(55)
H,gal
ground Hi map was kindly provided by Erik Muller, see also s.Sincethisperiodhasasignificanceof3.3σandcouldnotbe
Mulleretal. 2003) and an additional column density, account- confirmedintheotherobservations,probablyowingtothelower
ingfortheinterstellarmediumoftheSMCandsourceintrinsic numberofdetectedcounts,thespinperiodneedsfurtherconfir-
absorption.Wefindagooddescriptionofthedatabythismodel mation.
withχ2/dof=14.8/17.FortheSMCandsourceintrinsicabsorp-
tionweobtainN =4.32+0.36×1021cm−2.Thetotalline-of-
H,smc −0.28 3. Analysesandresultsofopticalobservations
sightN columndensityatthepositionofRXJ0123.4-7321
H,smc
is3.02×1021cm−2(Stanimirovicetal.1999),pointingtosome An I-band finding chart from OGLEIII is presented in Fig. 2.
intrinsic absorption in the binary system. The photon index is An optical counterpart, GSC09142-02001, is found with an
determined to be Γ = 1.18+0.63, which suggests a neutron-star angular separation to the XMM-Newton position of 1.3′′ for
−0.54
BeXRB(Γ≈ 1, Haberl&Pietsch2004)butwecannotexclude the entry in the photometric catalogue of Massey (2002), giv-
an AGN (Γ ≈ 1.75, Tozzietal. 2006). The ROSAT spectrum ing U = 14.48 mag, B = 15.39 mag, V = 15.45 mag, and
togetherwiththebest-fitmodelispresentedinFig.1. R = 15.42 mag as measured on MJD 51186. The NIR coun-
The XMM-Newton detection is based on only 58 counts. terpart, 2MASS01232743-7321223 (Skrutskieetal. 2006), is
However,hardnessratiosareconsistentwithaBeXRBnatureof foundwith an angularseparationof 1.1′′ with J = 15.32mag,
the source. Assuming the same spectral shape for the ROSAT H =14.82mag,andK =15.06magmeasuredonMJD51034.
S
and XMM-Newton detections and comparing their fluxes, we The corresponding OGLE sources are OGLEIIISMC121.2.50
2
Sturmetal.:TheBe/X-raybinaryRXJ0123.4-7321
100
U
10−14 B
) V 80
−1
Å R er
−2−1m s 10−15 I gle pow 60
Flux (erg c 10−16 J H K Lomb-Scar 2400
10−17 0
5000 10000 15000 20000 25000 10 100 1000
Wavelength (Å) Period (d)
Fig.3. OpticalandNIRphotometryofRXJ0123.4-7321dered- Fig.5. Lomb-Scargleperiodogramof the I-band light curve of
dened and compared with normalised Kurucz model atmo- RXJ0123.4-7321fromOGLEIIIdatabeforeMJD55000.
spheresforaB1IIIandB3IIIstar.Labelsgivefilters.
15.1 15.30
15.32
15.2
15.34
ag) 15.3 mag) 15.36
I (m 15.4 I ( 15.38
15.40
15.5
15.42
15.6
0.0 0.5 1.0 1.5 2.0
52000 53000 54000 55000 56000
Phase (119.78 d)
MJD
Fig.6. OGLEIII light curve of RXJ0123.4-7321 for MJD <
Fig.4. Optical I-band light curve of RXJ0123.4-7321 from
55000foldedwiththe119.87dperiod.Phase=0.5corresponds
OGLEIII (before MJD 55000) and OGLEIV (after MJD
toMJD54387.14.ThephasesofthedatesoftheX-rayobserva-
55000).Predicted outburst maxima are indicated by the dotted
tionsareindicatedbydashedlines(seeTable1).
bluelines.ThetimeoftheXMM-Newtonobservationismarked
bythedashedredline.
MJD = 52181.1 MJD = 53259.1 MJD = 53618.5
0 0 0
15.10
g) 15.20
a
andOGLEIVSMC733.26.24withaseparationof1.2′′totheX- m
raysource.TheOGLEI-bandvalueofI ≈15.4magisatypical I ( 15.30
magnitude of the variable light curve (see below). The magni- 15.40
tudesandcoloursarecharacteristicofaBstarintheSMC.From
optical spectroscopy, Evansetal. (2004) classified the source 15.10 MJ D 0 = 539 77.8 MJ D 0 = 543 37.1 MJ D 0 = 546 96.5
(2dFS3624)asB1-3IIIstar.Sincethedensityofearly-typestars
isrelativelylowinthewingoftheSMC,acorrelationwiththe ag) 15.20
m
X-raysourcebychanceisunlikely. I ( 15.30
We dereddened these magnitudes to correct for a Galactic
15.40
foregroundof E = 0.07mag(Schwering&Israel1991)and
B−V
compare them to a Kurucz model atmosphere (Kurucz 1979;
0 20 40 60 80 0 20 40 60 80 0 20 40 60 80 100
Howarth2011)ofaB1IIIandB3IIIstar(Teff = 25000,18000 MJD-MJD0 MJD-MJD0 MJD-MJD0
Kandlog(g)=3.0)inFig.3.Themodelshavebeennormalised
to the B-band measurement. To account for possible variabil- Fig.7. OGLEIIIlightcurvesofRXJ0123.4-7321forsix indi-
ity of the source and the non-simultaneous observations, we vidualoutbursts.Predictedoutburstmaximaareindicatedbythe
included an uncertainty of 0.2 mag for the I and NIR magni- dottedbluelines.
tudes.ThereisevidenceforanNIRexcess,likelycausedbythe
circum-stellardecretiondiscandsupportingtheBeclassification
ofthestar.
3
Sturmetal.:TheBe/X-raybinaryRXJ0123.4-7321
Thelong-termI-bandlightcurvefromOGLEphaseIIIand
-73:20:00
IV ispresentedin Fig. 4. Besides a long-termvariability,regu-
laroutburstsareseen.ThelightcurveistypicalforBe stars.A
Lomb-Scargle periodogram(Lomb 1976; Scargle 1982) of the
21:00
OGLEIII data confirmsthe variability with a period of 119.87
days (Fig. 5). We derive an ephemeris for the I-band outburst
maximumof 0) 22:00
0
0
2
J
MJD=54387.14±0.10+(119.9±2.5)×N, c (
e
D 23:00
withN aninteger.ThefoldedI-bandlightcurveispresentedin
Fig.6.Theaverageoutburstduration(FWHM)is30daysasde-
rivedfromthefoldedlightcurve.However,theevolutioncanbe 24:00
differentforindividualoutbursts.Forseveraloutburstswefinda
double-peakedshape,asalsoindicatedinthefoldedlightcurve.
Otheroutburstsonlyshowonepeak,wherethemaximumcanbe 25:00
24:00 50 40 30 20 10 1:23:00 50 40 22:30
∼15daysbeforeoraftertheexpectedtimeoftheoutburstmax-
RA (J2000)
imum.ExamplesforindividualoutburstarepresentedinFig.7.
InotherBeXRBs, typicallyafastriseandexponentialdecayis Fig.8. Continuum-subtracted emission-line image of
observedduringopticaloutbursts(Birdetal.2012).Ingeneral, DEMS160fromMCELS(Winkleretal.2005).Red/green/blue
individualoutburstprofileswilldependonNS-orbitparameters give logarithmically scaled intensities for Hα/[Sii]/[Oiii].
andperturbationsofthedecretiondisc(Okazaki1991). RXJ0123.4-7321ismarkedwithawhitecross.
All X-rayobservationsweredoneshortlyafterthetimesof
possible optical outbursts, as indicated by the vertical lines in
neutron-starnatureofthecompactobject.Ifthecurrentlow-state
Fig. 6. If both optical and X-ray outburst appear during peri-
oftheI-bandemissioniscausedbyalossofthedecretiondisc,
astron passage, we would expect quasi-simultaneous outbursts
noX-rayactivityisexpected.Follow-upX-rayobservationsare
ase.g.observedinAXJ0058-720(Haberl&Pietsch 2007)and
most likely successful, during new I-band outbursts, which in-
IGRJ05414-6858 (Sturmetal. 2012b). However, we note that
dicateinteractionoftheNSandthedecretiondisc.Opticalout-
theX-rayobservationshavebeenperformedduringtimeswith-
burstsmightreappearwithbrighteningI-bandemissionandwill
out OGLE coverage where the occurrence of optical outbursts
befoundinfutureOGLEmonitoring.Thesystemisincludedin
isuncertain.Therecentlyobserveddecreaseinthe I-bandlight
theX-RayvariablesOGLEMonitoring(XROM,Udalski2008)
curveandthevanishingoftheoutburstscanbecausedbyaloss
system.
of the decretion disc around the Be star. Interestingly, first the
outburstsweremissingafterMJD55400,althoughthequiescent
Acknowledgements. TheOGLEprojecthasreceivedfundingfromtheEuropean
I-bandfluxwasatasimilarlevelwhenoutburstshavebeenob- Research Council under the European Community’s Seventh Framework
servedaroundMJD 53000.Between MJD 55700and 56000,a Programme(FP7/2007-2013)/ERCgrantagreementno.246678toAU.RSac-
dropoftheI-bandfluxwasobserved,followedbyabrightening, knowledgessupportfromtheBMWI/DLRgrantFKZ50OR0907.
probablyduetoarecoveringofthedecretiondisc.
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