The XMM-Newton survey of the Small Magellanic Cloud: Discovery of the 11.866 s Be/X-ray binary pulsar XMMUJ004814.0-732204 (SXP11.87)
R. Sturm, F. Haberl, M.J. Coe, E.S. Bartlett, D.A.H. Buckley, R.H.D. Corbet, M. Ehle, M.D. Filipović, D. Hatzidimitriou, S. Mereghetti, N. La Palombara, W. Pietsch, A. Tiengo, L.J. Townsend, A. Udalski
aa r X i v : . [ a s t r o - ph . H E ] N ov Astronomy&Astrophysicsmanuscript no. aa˙xmm˙XMMUJ004814.0-732204 c (cid:13)
ESO 2018September 6, 2018
The XMM-Newton survey of the Small Magellanic Cloud:Discovery of the 11.866 s Be/X-ray binary pulsarXMMU J004814.0-732204 (SXP11.87)
R. Sturm , F. Haberl , M.J. Coe , E.S. Bartlett , D.A.H. Buckley , R.H.D. Corbet , M. Ehle , M.D. Filipovi´c , D.Hatzidimitriou , , S. Mereghetti , N. La Palombara , W. Pietsch , A. Tiengo , L.J. Townsend , and A. Udalski Max-Planck-Institut f¨ur extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa University of Maryland, Baltimore County, Mail Code 662, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA XMM-Newton Science Operations Centre, ESAC, ESA, PO Box 78, 28691 Villanueva de la Ca˜nada, Madrid, Spain University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW1797, Australia Department of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis, GR15784Zografos, Athens, Greece Foundation for Research and Technology Hellas, IESL, Greece INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, PolandReceived 21 September 2010 / Accepted 6 November 2010
ABSTRACT
Aims.
One of the goals of the XMM-Newton survey of the Small Magellanic Cloud is the study of the Be / X-ray binary population.During one of our first survey observations a bright new transient − XMMU J004814.0-732204 − was discovered. Methods.
We present the analysis of the EPIC X-ray data together with optical observations, to investigate the spectral and temporalcharacteristics of XMMU J004814.0-732204.
Results.
We found coherent X-ray pulsations in the EPIC data with a period of (11 . ± . = ∼ α emission with an equivalent width of 3.5 ± Conclusions.
The X-ray spectrum and the detection of pulsations suggest that XMMU J004814.0-732204 is a new high mass X-raybinary pulsar in the SMC. The long term variability and the H α emission line in the spectrum of the optical counterpart identify it asa Be / X-ray binary system.
Key words. galaxies: individual: Small Magellanic Cloud – galaxies: stellar content – stars: emission-line, Be – stars: neutron –X-rays: binaries
1. Introduction
The Small Magellanic Cloud (SMC) hosts an extraordinary highnumber of about 80 known Be / X-ray binary systems, comparedto the ∼
70 known in the Galaxy (as of 2006, Liu et al. 2006)which is a factor of ∼
100 more massive than the SMC. Be / X-ray binaries are a subclass of high mass X-ray binaries contain-ing an early type Be donor star with equatorial mass ejection,and an accreting neutron star (NS). Due to the non-spherical andtime variable mass ejection these systems show up as X-ray tran-sients, when the NS crosses the disk during the periastron pas-sage, leading to enhanced matter accretion for a few days (typeI outbursts). Longer outbursts lasting several weeks (type II) arethought to be caused by expansion of the circumstellar disk (seee.g. Okazaki & Negueruela 2001).One of the aims of the XMM-Newton (Jansen et al. 2001)large program SMC survey (Haberl & Pietsch 2008a) is the on-going study of the Be / X-ray binary population of the SMC, which can be used as a star formation tracer for ∼
50 (30-70)Myr old populations (Antoniou et al. 2010). In this paper wepresent the analysis of X-ray and optical data from the newlydiscovered X-ray pulsar XMMU J004814.0-732204.
2. Observations and data reduction
The new transient was discovered on 2009 Oct. 03, during ob-servation 13 (observation ID 0601211301) of the XMM-Newtonlarge program SMC survey. The source was located near theborder of CCD 1 (partly spread onto CCD 4) of the EPIC-pninstrument (Str¨uder et al. 2001) and on CCD 2 of EPIC-MOS2(Turner et al. 2001). There are no MOS1 data for this sourcebecause it was located on CCD 6, which is switched o ff sinceXMM-Newton revolution 961. The soft proton background wasat a very low level during the whole observation. Therefore, nobackground screening was necessary, resulting in net exposure Sturm et al.: A 11.87 s Be / X-ray binary pulsar in the SMC
Right ascension D ec li n a t i on SC MOS2SC pnBackground
Fig. 1.
EPIC colour image of XMMU J004814.0-732204 com-bining pn and MOS data. The red, green and blue colours rep-resent the X-ray intensities in the 0.2 − − − ′′ and 57 ′′ for pn and MOS2 source regions and 45 ′′ for the background).times of 30779 s and 32368 s for EPIC-pn and EPIC-MOS2,respectively.We used XMM-Newton SAS 10.0.0 to process the data.We identified sources in the field of view (FoV) for astrometricbore-sight correction by comparison with the Magellanic CloudsPhotometric Survey of Zaritsky et al. (2002), obtaining a shift of ∆ RA = -0.15 ′′ and ∆ Dec = -1.23 ′′ . The corrected position of thetransient as found by emldetect is R.A. = h m . s
07 andDec. = –73 ◦ ′ . ′′ ′′ and a systematic uncertainty of ∼ ′′ (1 σ confidence for bothcases).For the extraction of EPIC spectra, we selected single-pixelevents from the EPIC-pn data ( PATTERN = 0 ) and single toquadruple events with
PATTERN ≤ from EPIC-MOS2 data,both with FLAG = 0 . The SAS task eregionanalyse was usedto determine circular source extraction regions by optimizing thesignal to noise ratio as shown in Fig. 1. We ensured that thesource extraction region has a distance of > ′′ to other detectedsources. For the background extraction region, we chose a cir-cle in an area free of point sources and on the same CCD asthe source for both instruments. The EPIC-pn and EPIC-MOS2spectra contain 9286 and 8054 background subtracted counts, re-spectively, and were binned to a minimum signal-to-noise ratioof 5 for each bin. For the timing analysis, we used also double-pixel events for EPIC-pn. To increase the statistics for the timinganalysis we also generated a merged event list from both instru-ments, containing 25945 cts (source + background). Science Analysis Software (SAS), http: // xmm.esac.esa.int / sas /
3. X-ray data analysis and results
We used
XSPEC (Arnaud 1996) version 12.5.0x for spectral fit-ting. The two EPIC spectra were fitted simultaneously with acommon set of spectral model parameters, only a relative nor-malisation factor was allowed to vary to account for instrumen-tal di ff erences. The spectrum (Fig. 2) was modelled first withan absorbed power-law. We fixed the Galactic photo-electric ab-sorption at a column density of N H , GAL = × cm − withabundances according to Wilms et al. (2000), whereas the SMCcolumn density was a free parameter with abundances for ele-ments heavier than Helium fixed at 0.2. The best-fit parametersare summarised in Table 1 where errors denote 90% confidenceranges.The extraction for the EPIC-pn spectrum is hampered by theCCD gap cutting the extraction region. The missing area is takeninto account in the calculation of the e ff ective area by arfgen .However, we noticed that when using the default spatial resolu-tion (parameter badpixelresolution = ′′ ) the flux derived fromthe EPIC-pn spectrum is higher by (21 ± = ′′ reduces the flux discrep-ancy to 7%, which is within the expected systematic uncertain-ties in the presence of gaps. Extracting the EPIC-pn spectrumfrom a smaller source region with radius 6 ′′ , so that the com-plete source region is placed on CCD 1, yields a flux that onlydi ff ers by ∼
1% from the MOS2 value. The spectral shape is nota ff ected by the CCD gap, but the number of source counts forthe smaller extraction region is a factor of two lower.In principle, this fit is formally acceptable and additionalcomponents are not required. However, soft excesses and flu-orescent emission from iron are known to contribute to the X-ray emission of some Be X-ray binaries (e.g. Eger & Haberl2008; La Palombara et al. 2009; Hickox et al. 2004). To inves-tigate these possibilities we first added a black-body emissioncomponent to the model (Table 1). This component contributes ∼
2% to the observed flux and ∼
3% to the absorption cor-rected luminosity. For the bolometric luminosity we obtained(1 . ± . × erg s − . Compared to the single power-law thereduced χ improved from 1.09 to 1.01, which corresponds to aF-test chance probability of 2 . × − and formally proves thesignificance of this component (but see Protassov et al. 2002, forlimitations of the F-test). An additional emission line with fixedenergy at 6.4 keV and unresolved line width (fixed at 0), yieldeda line flux of 4 . ± . × − photons cm − s − correspondingto an equivalent width given in Table 1. Substituting the 6.4 keVline by a 6.7 keV line for ionized Fe XXV resulted in an upperlimit for the equivalent width of 41 eV.If we replace the black-body component by a multi-temperature disk black-body model (diskbb in XSPEC ), we derivea lower limit for the inner disk radius of R in = . + . − . km (fora disk inclination of Θ = R in ∝ / √ cos Θ ). FollowingHickox et al. (2004) to estimate the inner disk radius we infer R in = p L X / (4 πσ T ) =
39 km.
We corrected the event arrival times to the solar system barycen-tre using the SAS task barycen and searched for periodicitiesin the X-ray light curves using fast Fourier transform (FFT)and light curve folding techniques. The power density spectraderived from light curves in various energy bands from bothEPIC instruments showed a periodic signal at 0.084 Hz. To in- turm et al.: A 11.87 s Be / X-ray binary pulsar in the SMC 3
Table 1.
Spectral fit results.
Model (1)
SMC N H γ kT R (2) EW Fe Flux (3) L (4)x χ / dof[10 cm − ] [eV] [km] [eV] [erg cm − s − ] [erg s − ]PL 1.72 ± ± ± × − × / + BB 2.32 ± ± ±
43 12.2 ± ± × − × / + BB + Fe-line 2.34 ± ± ±
40 12.2 ± ±
30 (9.6 ± × − × / + DiskBB 2.99 ± ± ± > ± ± × − × / (1) For definition of spectral models see text. (2)
Radius of the emitting area (for BB) or inner disk radius (DiskBB, for the definition see text). (3)
Observed 0.2-10.0 keV flux. (4)
Source intrinsic X-ray luminosity in the 0.2-10.0 keV band (corrected for absorption) for a distance to the SMC of60 kpc (Hilditch et al. 2005). −3 C oun t s s − k e V − −4−202 χ χ Channel energy (keV)
Fig. 2.
EPIC spectra of XMMU J004814.0-732204. The toppanel shows the EPIC-pn (black) and EPIC-MOS2 (red) spec-tra together with the best-fit model (solid line) of an absorbedpower-law (dashed line) plus black-body (dotted line) and ironfluorescent line (dash-dotted line). The residuals (for better com-parison they are re-binned by an additional factor of three) areplotted for this model (bottom panel) and for the best-fit singlepower-law model (middle panel).crease the signal to noise ratio, we then created light curves fromthe merged event list of EPIC-pn and EPIC-MOS2 (delimitedto common time intervals). Figure 3 shows the inferred powerdensity spectrum from the 0.2-10.0 keV energy band with theclear peak at a frequency of 0.084 Hz. Following Haberl et al.(2008) we used a Bayesian periodic signal detection method(Gregory & Loredo 1996) to determine the pulse period with 1 σ error to (11 . ± . i = (R i + − R i ) / (R i + + R i ) with R i denoting the background-subtracted count ratein energy band i (with i from 1 to 4). Assuming a sinusoidalpulse profile, we determined a pulsed fraction of (7 . ± . − P o w e r Frequency (Hz)
Fig. 3.
Power density spectrum created from the merged EPIC-pn and EPIC-MOS2 data in the 0.2-10.0 keV energy band. Thetime binning of the input light curve is 1.882 s.(e.g. Wilson et al. 2003; Haberl et al. 2008) and luminosity (e.g.Bildsten et al. 1997) is seen from a number of high mass X-raybinaries.
The position of XMMU J004814.0-732204 was covered in twoprevious XMM-Newton observations on 2000 Oct. 15 (ObsID:0110000101) and 2007 Apr. 11 (ObsID: 0404680301) with abackground-screened net exposure of 21.6 ks and 17.6 ks, re-spectively. In the latter observation the source position was onlycovered by the MOS2 FoV. In both observations, no sourcewas detected above a likelihood threshold of 6. Using sensi-tivity maps we derived 3 σ upper limits of 2.5 × − cts s − and 2.7 × − cts s − , respectively. Assuming the same spec-trum as during the outburst, this corresponds to a flux limit of1.7 × − erg cm − s − (from Oct. 2000, measured by EPIC-pn) and 6.1 × − erg cm − s − (Apr. 2007, EPIC-MOS2) in the0 . − . × erg s − and 2.7 × erg s − , respectively.Also in a Chandra (Weisskopf et al. 2000) ACIS-I observa-tion (Observation ID 2945) on 2002 Oct. 2 this position wascovered with a 11.8 ks exposure, and no source was detected.We used the CIAO (Version 4.2) task aprates to estimate a 3 σ upper limit of 5.1 × − cts s − . Assuming the same spectrum asabove, this corresponds to a flux limit of 1.6 × − erg cm − s − in the 0 . − . × erg s − ). Sturm et al.: A 11.87 s Be / X-ray binary pulsar in the SMC . - . k e V . - . k e V . - . k e V . - . k e V . - . k e V H R H R H R H R Fig. 4.
Left: Pulse profiles obtained from the merged EPIC datain di ff erent energy bands (for better statistics the first two stan-dard energy bands were combined in the top panel, the bottompanel shows all five energy bands combined). The profiles arebackground-subtracted and normalized to the average count rate(0.116, 0.228, 0.249, 0.207 and 0.801 cts s − , from top to bottom.Right: Hardness ratios as a function of pulse phase derived fromthe pulse profiles in two neighbouring standard energy bandsThe upper limits from the XMM-Newton and Chandra ob-servations show that XMMU J004814.0-732204 increased inbrightness at least by a factor of 560 during its outburst.RXTE monitoring of the SMC has been carried out for nearlya decade (Galache et al. 2008) and XMMU J004814.0-732204has frequently fallen within the pointing direction of the tele-scope, often at a collimator response of ≥ / s / PCU.This approximately translates into ∼ / s / PCU - depend-ing on the collimator response and pulsed fraction (which is notvery high for XMMU J004814.0-732204) - or a luminosity limitof ∼ × erg s − for a source in the SMC.
4. Optical data
Searching optical catalogues of Zaritsky et al. (2002), MACHOand OGLE, we found three stars which are located within the 3 σ error radius around the XMM-Newton position. Their positionsand magnitudes from Zaritsky et al. (2002) and their OGLE-IIand MACHO entries are listed in Table 2. A finding chart pro-duced from OGLE-III data is shown in Fig. 5.The star closest to the X-ray position (OGLE-III 14642) hascolours and brightness consistent with an early B star. Its po-sition on the U-B vs. B-V diagram of Be stars (e.g. Fig. 1 ofFeinstein & Marraco 1979) is also entirely consistent with it be- ing a Be star. The same holds for the reddening-free Q-index of-0.85 (Johnson & Morgan 1955; Massey et al. 2007). This can-didate also appears as number 10287 in the survey list of Massey(2002). The (B-V) colour index from that catalogue is (B-V) = -0.12 ± = = -0.21 ± / X-ray binaries in the SMC (McBride et al. 2008). However,care must always be taken when interpreting colour informationas a spectral type in systems that clearly have circumstellar diskscontributing some signal to the B and V bands.Optical photometry was performed at the Faulkes TelescopeSouth (FTS) on 25 November 2009 (MJD 55160). The telescopeis located at Siding Spring, Australia and is a 2m, fully au-tonomous, robotic Ritchey-Chr`etien reflector on an alt-azimuthmount. The telescope employs a Robotic Control System (RCS).The telescope was used in Real Time Interface mode for theobservation of XMMU J004814.0-732204. All the observationswere pipeline-processed (flat-fielding and de-biasing of the im-ages). The I-band magnitude of the optical counterpart was de-termined to be 15.30 ± ff erent epochs - see Table 3 for the actualvalues. The earliest optical data come from Massey (2002) andwere recorded on 1999 Jan. 8. These data are combined with IRmeasurements taken on 2002 Aug. 31 with the Sirius camera onthe 1.5m IRSF telescope in South Africa (Kato et al. 2007). Alsoincluded is the OGLE I band measurement taken simultaneouslywith the Sirius IR data set. These early data are compared to aB, V, R & I photometric data set recorded on 2009 Nov. 25 fromthe FTS.For comparison, a stellar atmosphere model (Kurucz 1979)representing a B2V star (T e ff = = α emission (see Section 4.3 below).The star OGLE-III 14688 is probably a red (K to M) giantfrom its OGLE colours. The light curve shows small variationsof the order of 0.1 mag in the I-band, but no evidence for anycoherent fluctuations. There is an object from the Two MicronAll Sky Survey (Skrutskie et al. 2006) − − which is closest to the position of this star, withJ = = = = = turm et al.: A 11.87 s Be / X-ray binary pulsar in the SMC 5
Table 2.
Possible optical counterparts of XMMU J004814.0-732204.
RA(2000) ( a ) DEC(2000) ( a ) dist. ( b ) U (mag) ( a ) B (mag) ( a ) V (mag) ( a ) I (mag) ( a ) Q (mag) ( c ) OGLE-III MACHO00 48 14.10 -73 22 03.6 0 . ′′
76 13.96 ± ± ± ± ± . ′′ − ± ± ± − . ′′
07 15.20 ± ± ± ± ± ( a ) according to Zaritsky et al. (2002). ( b ) distance of the Zaritsky et al. (2002) positions to the bore-sight corrected XMM-Newton position. ( c ) Reddening-free Q-index = (U-B) − × (B-V). Table 3.
Optical and IR photometry of OGLE-III 14642
M2002 ( a ) K2007 ( b ) FTS ( c ) Sirius ( c ) ± − ± − V 14.66 ± − ± − R 14.66 ± − ± − I − ± ± − J − ± − ± − ± − ± − ± − ± ( a ) Massey (2002). ( b ) Kato et al. (2007). ( c ) This work.
Fig. 5.
Finding chart of SXP11.87. The I band image fromOGLE-II shows the 3 close objects near the X-ray positionmarked with their OGLE-III identification (arrows). The twolines further mark the likely counter part. The image size is 1 . ′ . ′ Fig. 6.
Combined optical-IR flux for our counterpart OGLE-III14642 at two epochs. (a) a historical data set (1999-2002) - solidsymbols; (b) data set from the time of outburst (Nov-Dec 2009) -open symbols. See text for details of the observations. Both datasets are compared to a Kurucz model atmosphere for a B2V starin which this stellar model has been normalised to the outburstB band point.
The identification of XMMU J004814.0-732204 with OGLE-III14642 is supported by the MACHO and OGLE light curves.This star shows strong outbursts repeating on time scales of ∼ Sturm et al.: A 11.87 s Be / X-ray binary pulsar in the SMC
Fig. 7.
Multi-wavelength light curves of theXMMU J004814.0-732204 / OGLE-III 14642 system. Theupper two panels show the MACHO B- and R-band. In thethird panel the OGLE-III I-band light curve is plotted, withthe last data point indicating our own measurement using theFaulkes telescope (see text). Dashed lines indicate the times ofX-ray measurements, as shown in the bottom panel. Arrowsmark upper limits (XMM-Newton, Chandra and XMM-Newtonin chronological order, see Sect. 3.3), the cross indicates theXMM-Newton detection.
Spectroscopic observations of the H α region were made on 11Dec. 2009 (MJD 55176) using the 1.9 m telescope of the SouthAfrican Astronomical Observatory (SAAO). A 1200 lines permm reflection grating blazed at 6800 Å was used with the SITeCCD which is e ff ectively 266 × / pixel. The data were reduced using IRAFstandard routines and the resulting spectrum is shown in Fig. 8.The peak is at 6566 Å which is consistent with the correspond-ing rest wavelength of the H α line corrected for the motion ofthe SMC. In this mode the spectral resolution is ∼ ∼
10. We measured an H α line emissionwidth of EW = ±
5. Discussion and Conclusions
One of the first XMM-Newton observations of the SMCsurvey revealed the new high mass X-ray binary pulsarXMMU J004814.0-732204 with a pulse period of 11.866 s (fol-
Fig. 8. H α spectrum of the OGLE optical candidate α emission line in the optical spectrum clearly con-firm XMMU J004814.0-732204 as another Be / X-ray binary inthe SMC.The power-law photon index derived from the EPIC spectraof 0.53 − / X-ray binaries in the SMC, which shows a max-imum at ∼ − − −
728 (also using XMM-Newton data in the same en-ergy band; Haberl & Pietsch 2008b). It should be noted, that thelatter, also called SXP6.85, was detected at energies up to 35keV with RXTE and INTEGRAL during a long type II outburst(Townsend et al. 2010) showing that the hard spectrum extendsto energies beyond the sensitivity of XMM-Newton. SXP11.87and SXP6.85 also show similarities in their spectra at energiesbelow 2 keV, indicating a soft X-ray excess. However, the energyresolution of the CCD instruments is not su ffi cient to determinethe exact nature of this component. Some constraints can be in-ferred by using di ff erent models for the soft component. If oneassumes a black-body component, a temperature of ∼
280 eV andblack-body radius of ∼ turm et al.: A 11.87 s Be / X-ray binary pulsar in the SMC 7 by di ff use gas through collisional heating or photoionisation ispossible for both cases (Hickox et al. 2004).The MACHO and OGLE light curves of the optical coun-terpart of XMMU J004814.0-732204 show prominent outburstsrepeating on a time scale of about 1000 days. Very simi-lar behaviour was reported from the optical counterpart ofthe 18.37 s Be / X-ray binary pulsar XMMU J004911.4-724939which showed two outbursts separated by about 1300 daysin MACHO and OGLE-I data (Haberl et al. 2008). Such out-burst behaviour is also observed from other (single) Be stars(Mennickent et al. 2002). Because of this and the fact that theoutbursts do not repeat strictly periodically, it is unlikely thatthey are related to the orbital period of the binary system.Moreover, from the Corbet relation between neutron star spinperiod and the orbital period (Corbet 1984) a much shorter or-bital period of about 20 −
200 days is expected (see Laycock et al.2005; Corbet et al. 2009, for more recent versions of the P s / P orb diagram). Acknowledgements.
The XMM-Newton project is supported by theBundesministerium f¨ur Wirtschaft und Technologie / Deutsches Zentrumf¨ur Luft- und Raumfahrt (BMWI / DLR, FKZ 50 OX 0001) and the Max-PlanckSociety. R.S. acknowledges support from the BMWI / DLR grant FKZ 50 OR0907. S.M., N.L. and A.T. acknowledge the support of ASI through contractI / / /
0. L.J.T. is in receipt of a University of Southampton MayflowerScholarship. A.U. acknowledges support from the MNiSW / BST grant.