New SWIFT X-ray transient in the SMC: A new Be/X-ray binary?

ATel #3575; R. Sturm, F. Haberl, W. Pietsch (all Max-Planck-Institut fuer extraterrestrische Physik), S. Immler (NASA/CRESST/GSFC), A. Udalski (Warsaw University Observatory)
on 22 Aug 2011; 16:59 UT
Credential Certification: Richard Sturm (rsturm@mpe.mpg.de)

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We report the detection of a transient source with SWIFT during the SMC survey (PI Immler). The source was detected in two observations on 2011-08-20 from 02:19 to 18:28 (UT) and 2011-08-21 from 00:45 to 04:06 with 1909 s and 1337 s exposure, respectively. The count rate increased from (0.027 ± 0.004) cts/s to (0.036 ± 0.006) cts/s. The positions (J2000) are RA = 00 52 00 DEC = -73 29 25.3 and RA = 00 52 01 DEC = -73 29 20, respectively, with relatively large statistical errors of ~4.8 arcsec due to large off-axis angles. Both positions are in agreement with that of an optical source from the Magellanic Cloud Photometric Survey (Zaritsky et al. 2002, AJ, 123, 855) at RA = 00 52 00.68 DEC = -73 29 25.3 with magnitudes of U=14.407, B=15.229, V=15.184, and I=14.979 and with a 2MASS source at RA = 00 52 00.593 DEC = -73 29 25.58 with magnitudes of J=15.065, H=15.030, and K=14.750. The colours are in agreement with a main sequence B star in the SMC. The I band light curve from OGLE III (SMC103.4 33693) shows variability of ~0.5 mag on a 500-1000 day time scale, in addition to shorter flares of ~0.1 mag on a 100 day scale. By assuming an X-ray spectrum typical for Be/X-ray binaries of an absorbed power-law (photon index = 1.0, Galactic foreground absorption = 6x10²⁰ cm^-2), we derive fluxes of 1.9x10^-12 erg s^-1 cm^-2 and 2.5x10^-12 erg s^-1 cm^-2. This corresponds to 0.8 and 1.1 x 10³⁶ erg s^-1 for a SMC distance of 60 kpc. From an XMM-Newton observation on 2009-09-13 from 00:52 to 12:51 we derived an upper limit in the (0.2-10.0) keV band of 6.2-15 erg s^-1 cm^-2. This results in a variability of more than a factor of 400. The source might previously be detected by ROSAT as source 496 in the SMC X-ray sources ROSAT PSPC catalogue (Haberl et al. 2000, A&A, 142, 41) and source 42 in the ROSAT HRI catalogue of SMC X-ray sources (Sasaki et al. 2000, A&A, 147, 75). The source position is also compatible with a previous detection by the Einstein observatory (source 42 in Wang & Wu 1992, ApJ, 78, 391). The high X-ray variability and the likely blue optical counterpart point to the discovery of a new Be/X-ray binary in the SMC.