Swift observations of the supersoft X-ray transient XRF 111111A / MAXI J0158-744

ATel #3759; K. L. Li, A. K.H. Kong, P. H.T. Tam, J. H.K. Wu (NTHU, Taiwan)
on 12 Nov 2011; 02:36 UT
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Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

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We report Swift follow-up observations of the soft X-ray transient MAXI J0158-744 (ATel #3756). The observations was performed with the XRT on 2011-11-11 15:33:38 UTC for a duration of 320s in the Photon Counting (PC) mode.

We found a bright X-ray source within the refined error region (including the 0.1 deg systematic uncertainty, GCN #12555) of MAXI J0158-744 in the XRT observations. Using the ``Build Swift-XRT products'' tool (Evans et al. 2009, MNRAS 397, 1177) in the UK Swift Science Data Centre at the University of Leicester, we got an UVOT enhanced position of

(R.A., Dec) = (29.85647 deg, -74.25795 deg) = (01 59 25.55 -74 15 28.6) (J2000)

with 2.3 arcsec uncertainty (90% error radius). We analyzed the energy spectrum of the observation and fitted with an absorbed blackbody model. Using a fixed column density nH=1.36e21 cm^-2 obtained from the weighted LAB map (Kalberla et al. 2005, A&A, 440, 775), we get a best fit temperature of 106+/-10 eV which is corresponding to an unabsorbed flux of 7.7e-11 (+0.4/-0.8) ergs/cm^2/s in the 0.2-1.0 keV band. Assuming a distance of 61.3kpc (distance of the SMC), we estimated the X-ray luminosity to be 3.5e37 ergs/s. It is worth noting the X-ray source is currently in a decaying phase from a peak count rate of 2.4 cts/s to < 0.2 cts/s in about 7 minutes.

Swift UVOT has also detected a bright UV source with U=13.08 +/- 0.04 (stat) +/- 0.02(sys) (in Vega system) at the X-ray position. In addition, we found a 16.16 mag B-band counterpart of this UV source in the USNO B1.0 catalog in the epoch 1989 with a 1.3 arcsec offset which suggested that the optical source could be a variable in the optical band. The bright UV source is also detected with the 2MASS with J=14.8, H=14.8, and K=14.4.

All the results are consistent with Kennea et al. (ATel #3758).

From the positional coincidence, similar hardness and luminosity, and the X-ray and optical variability, we believed that this X-ray source is indeed the soft X-ray transient MAXI J0158-744. By comparing it with the four other supersoft sources (SSSs) in the SMC, all five sources have similar features (i.e. low temperatures of tens eV and ultra high X-ray luminosity ~10^37-38 erg/s), which indicated that MAXI J0158-744 is a SSS.

We would like to thank the Swift team for their prompt response of our ToO request.