Swift reports the detection of a new transient source Swift J2058.4+0516

ATel #3384; 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 (GSFC/GWU)
on 28 May 2011; 21:30 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 J2058.4+0516, was first detected by the BAT at a daily average rate of 0.0029 +/- 0.0007 counts/sec/cm2 (15 mCrab) in the 15-50 keV band on 2011 May 18 (MJD 55699). It has remained at roughly the same brightness for the past ten days, reading 0.0025 +/- 0.001 on May 27.

A Swift target of opportunity observation was performed starting at 21:56 UT on 2011 May 27. The XRT observed in Photon Counting mode on 2011 May 27 and a position was determined at:

RA (J2000): 20 58 19.85 (314.58272 deg)
Dec (J2000): +05 13 33.0 (5.22583 deg)
90% Error radius: 1.7"

There are no sources in Simbad, Vizier or the Sloane Digital Sky Survey within the XRT error circle. Also, no point source was found in the Swift UVOT B band observation.

The XRT data are well fitted by an absorbed power-law model (Reduced chi2 = 0.95, 456 dof) with the following parameters:

N_H = 1.9 +/- 0.4 x 10^21 cm-2
Gamma = 1.78 +/- 0.12
Flux (0.3-10 keV) = 5.1 +/- 0.3 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 (1.3 ct/s) in two successive pointings. The spectral fitting was carried out using data and analysis based on Evans et al. (2009, MNRAS, 397, 1177)

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 less likely given the absence of an optical counterpart and the high galactic latitude (-25.1 degrees). It is also possible that Swift J2058.4+0516 is an AGN.

Further Swift observations have been requested.

BAT hard X-ray transient monitor light curve for Swift J2058.4+0516