2XMM J010247.4-720449 is a Be/X-ray binary, detected in outburst with Swift

ATel #3761; R. Sturm, F. Haberl, W. Pietsch (all Max-Planck-Institut fuer extraterrestrische Physik), A. Udalski (Warsaw University Observatory)
on 14 Nov 2011; 12:42 UT
Credential Certification: Richard Sturm (rsturm@mpe.mpg.de)

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2XMM J010247.4-720449 was classified as candidate for a Be/X-ray binary in the Small Magellanic Cloud due to X-ray hardness-ratio criteria (HR2 = 0.31 and HR3 = 0.25) and correlation with an early-type star (Eger 2008, diploma thesis TU Munich). In our XMM-Newton SMC-survey data analysis the source was detected six times. We obtain a boresight-corrected best-fit position of R.A.= 01:02:47.5 and Dec. =-72:04:50.8 (J2000, uncertainty 0.84 arcsec). Searching the Swift/XRT archive, we found an X-ray transient consistent with this position, proofing the Be/X-ray binary nature of this source.

Swift detected the source serendipitously in outburst on 2010 Mar. 27 from 05:03 to 19:34 (UT) in a 7586 s exposure (ObsID 00050050072) at a count rate of 0.0059 cts s^-1. The best-fit position is R.A.= 01:02:46.8 and Dec. =-72:04:55.7 (J2000, uncertainty 4.6 arcsec). Fitting an absorbed power law with C-statistics yields a photon index of Γ=0.9±0.5, typical for neutron-star high-mass X-ray binaries. For the SMC column density, we obtain an upper limit of N_H< 1.5 × 10²² cm^-2. The SMC absorption was modelled with abundances set to 0.2 for elements heavier than helium. An additional Galactic foreground absorption was set to 6 × 10²⁰ cm^-2. The detected flux is (6.0±2.6) × 10^-13 erg cm^-2 s^-1 in the (0.2-10.0) keV band, which translates into an unabsorbed luminosity of 2.8 × 10³⁵ erg s^-1 by assuming a distance of 60 kpc. Using an XMM-Newton observation on 2005 Apr. 17 from 22:44 to Apr. 18 08:33 (ObsID 0135722501) where the source was not detected, we derive an upper limit of 1.2 × 10^-14 erg cm^-2 s^-1 in the (0.2-10.0) keV band for the spectral model from above. This yields an X-ray variability of the source of at least a factor of 30. The XMM-Newton detections have fluxes between 2 and 3 × 10^-14 erg cm^-2 s^-1.

The optical counterpart at R.A.= 01:02:47.61 and Dec. =-72:04:51.2 has U = 14.69 mag, B = 15.75 mag, V = 16.0 mag, and I = 16.3 mag (Zaritsky et al. 2002, AJ 123, 855). The near infrared magnitudes are J = 16.58 mag, H = 16.52, and K_s = 16.61 mag (Kato et al. 2007, PASJ 59, 615). The I-band light curve from OGLE II and III shows a constant behaviour till the end of 2008 at I ≈ 16.25 mag followed by a brightening observed till 2009 May 2 to I = 15.7 mag. OGLE IV data will reveal if this is an transition to a high state or an outburst. The brightness, colours, and variability are typical for a donor in a Be/X-ray binary in the SMC.
In a deep merged Chandra image of this region we found a clear source at a position consistent with the optical coordinates from above. Therefore, we reject a second more distant candidate optical counterpart at R.A.= 01:02:47.94 and Dec. = -72:04:49.8 with similar colours and constant OGLE light curve.

We acknowledge the use of public data from the Swift data archive.