Swift J053041.9-665426: a new LMC X-ray transient

ATel #3747; R. Sturm, F. Haberl, W. Pietsch, (Max-Planck-Institut fuer extraterrestrische Physik)
on 8 Nov 2011; 14:06 UT
Credential Certification: Richard Sturm (rsturm@mpe.mpg.de)

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Swift detected a new X-ray transient in the Large Magellanic Cloud. The observation (00045769001) was performed with 407 s exposure on 2011 Nov. 6 from 03:57 to 04:03 UT. The source was serendipitously detected at large off-axis angle with a count rate of 0.45 cts s^-1. The position is R.A. = 05:30:41.9 and Dec. = -66:54:26 (J2000) with an uncertainty of 4.0 arcsec. The spectrum can be described by an absorbed power-law with photon index of 1.7 ± 0.5 and LMC column density of 2.1 (0.1-5.1) × 10²¹ cm^-2. The LMC absorption was modelled with abundances set to 0.5 for elements heavier than helium. An additional Galactic foreground absorption was set to 6×10²⁰ cm^-2. The detected flux in the (0.2-10.0) keV band is (2.5 ± 0.4)×10^-11 erg cm^-2 s^-1, corresponding to an unabsorbed luminosity of 9.7×10³⁶ erg s^-1 for a source distance of 50 kpc. We could not find a previous detection of this source in other X-ray catalogues. From its position, we name the source Swift J053041.9-665426. Using a ROSAT/PSPC observation (RP900553N00) from 1993 Nov. 9 to 1993 Dec. 1 with 1132 s exposure, we estimated an upper limit of 0.006 cts s^-1, yielding a variability of the source by a factor of at least 80. In the USNO-B1.0 catalogue, we find a possible optical counterpart at R.A. = 05:30:42.158 Dec. = -66:54:30.20 with B= 14.71 mag and R= 15.31 mag. A corresponding 2MASS near infrared detection of this source at R.A. = 05:30:42.151 and Dec. = -66:54:30.30 has J = 15.258 mag, H = 14.979 mag, and K = 14.908 mag. Due to the high X-ray variability, the source is unlikely an AGN. The high photon-index seems atypical for a neutron-star high-mass X-ray binary and may point at an black hole as the compact object. X-ray and optical follow up observations should clarify the optical identification and demonstrate the development of the X-ray intensity and spectrum of the source and allow to search for periods in detailed time variability studies. We acknowledge the use of public data from the Swift data archive.