Swift reports the detection of a new transient source Swift J1112.2-8238

ATel #3463; H. A. Krimm (CRESST/GSFC/USRA), J. A. Kennea (PSU), S. T. Holland (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), C. B. Markwardt (GSFC), D. Palmer (LANL), T. Sakamoto (CRESTT/GSFC/UMBC), G. Skinner (CRESST/GSFC/UMD), M. Stamatikos (OSU/GSFC), J. Tueller (GSFC), T. Ukwatta (MSU))
on 30 Jun 2011; 20:33 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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The hard X-ray transient monitor of the Swift Burst Alert Telescope (BAT) has detected a previously unknown transient source. Swift X-Ray Telescope (XRT) observations have confirmed the detection and produced a refined position.

The new source, Swift J1112.2-8238, was first detected by the BAT in a four-day integration covering the days 2011 June 16-19 (MJD 55728 - 55731) at an average rate of 0.0029 +/- 0.0007 counts/sec/cm2 (13 mCrab) in the 15-50 keV band. It remained at roughly the same brightness until 2011 June 25 and has faded since then, with an upper limit of 0.0016 counts/sec/cm2 on 2011 June 30. Examination of archival data shows that the source peaked on 2011 June 16 at a rate of 0.019 +/- 0.005 counts/sec/cm2, nearly 85 mCrab. Although a high count rate, the significance of this detection (4.0-sigma) was below the 5-sigma threshold for an alert from a previously unknown source.

A 3000-second Swift target of opportunity observation was performed starting at 2011-06-29 17:04 UT on 2011 June 29. The XRT observed in Photon Counting mode and a UVOT-enhanced position was determined at:

RA (J2000): 11 11 47.32 (167.94718 deg)
Dec (J2000): -82 38 44.2 (-82.64561 deg)
90% Error radius: 1.4"

There are no sources in Simbad, Vizier or the 2MASS catalog within the XRT error circle. There is also no detection in the Swift UVOT B band. The 3-sigma UVOT upper limit for detecting a source at the location of the XRT error circle is b > 22.0 mag. This is not corrected for the expected Galactic extinction in this direction corresponding to a reddening of E_{B-V} = 0.29 mag. The upper limit is being biased by the presence of a nearby star, which is clearly detected in the Swift UVOT B band observation. This star was found 4.9" away from Swift J1112.2-8238 in the USNO-B1 and 2MASS catalogs at RA=167.949007 and dec=-82.644241 (2MASS J121114776-8238392), with catalog magnitudes of R1=16.77, B2=18.43, I=14.96. The catalogued R-I colour of the USNO source suggests that it may be an M5V dwarf or cooler. Alternately, its red color could be telling us that we are underestimating the extinction in this direction. It could also be an uncatalogued galaxy. However, its location outside the XRT error circle means that it cannot be the counterpart to Swift J1112.2-8238

The XRT data are well fitted by an absorbed power-law model (Cstat = 298 for 334 dof) with the following parameters:

N_H = 1.5 +/- 0.5 x 1021 cm-2
Gamma = 1.57 +/- 0.16
Flux (0.3-10 keV) = 1.3 +/- 0.17 x 10-11 erg/s/cm-2
There is no evidence of any lines or other deviations from a smooth spectrum. The XRT flux is at the same level (0.2 ct/s) over the four successive pointings. The spectral fitting was carried out using data and analysis based on Evans et al. (2009, MNRAS, 397, 1177). The enhanced position used the method of Goad et al. (2007, A&A, 476, 1401).

The current results do not allow us to determine the nature of the source. The spectrum is consistent with a low-mass X-ray binary in a hard state. A high-mass X-ray binary interpretation is much less likely given the absence of an optical counterpart and the high galactic latitude (-20.4 degrees). Another speculation is that Swift J1112.2-8238 could be a similar tidal disruption event to Swift J164449.3+573451 (Burrows et al, 2011, arXiv 1104.4787) or Swift J2058+0516 (Cenko et al, ATel #3426), given its high galactic latitude, a BAT light curve which includes a possible flare at the start of the outburst and the absence of a UVOT and 2MASS counterpart even when there is relatively low absorption in the direction toward the source. However, the lack of variability in the XRT is an argument against this interpretation.

BAT hard X-ray transient monitor light curve for Swift J1112.2-8238