Swift reports the discovery of the galactic transient Swift J1943.4+0228

ATel #4049; H. A. Krimm (CRESST/GSFC/USRA), S. T. Holland (STScI), J. A. Kennea (PSU), 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 17 Apr 2012; 21:18 UT
Distributed as an Instant Email Notice Transients
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 discovered what is likely a new galactic transient source. Swift X-Ray Telescope (XRT) observations have confirmed the detection and produced a refined position.

The source, to which we give the name Swift J1943.4+0228, was first detected by the BAT in a 16-day integration covering the days 2012 March 29 through 2012 April 13 (MJD 56015 - 56020). Since March 26, it has remained at an average rate of 0.002 +/- 0.001 counts/sec/cm^2 (~10 mCrab) in the 15-50 keV band.

A 3000-second Swift target of opportunity observation was performed starting at 23:46:24 UT on 2012 April 16. The XRT observed in Photon Counting mode and a UVOT-enhanced position was determined at:

RA (J2000): 19 43 34.21 (295.89255 deg)
Dec (J2000): +02 27 54.8 (+2.46522 deg)
90% Error radius: 1.5"

The source is also detected in the UVOT in the B band, at b = 18.17 +/- 0.04, with a possible slight contamination from a second, fainter, nearby source. The best position for the source is

RA (J2000) 19:43:34.13 (295.89221 deg)
Dec (J2000) +02:27:55.0 (+2.46528 deg)
90% Error radius: 0.42"

The XRT and UVOT positions are consistent. An examination of the Vizier catalogs and Digitized Sky Survey images shows that there is no catalog source within the error radius. A rough estimate of the limiting magnitude from comparing to nearby USNO-A2.0 stars is B=19.4, R=18.7.

The XRT data are well fitted by an absorbed power-law model (Cstat = 451 for 486 dof) with the following parameters:

N_H = 1.9 +/- 0.4 x 10^21 cm^-2
Gamma = 1.17 +/- 0.12
Observed Flux (0.3-10 keV) = 8.3 +/- 0.64 x 10^-11 erg/s/cm^-2
Unabsorbed Flux (0.3-10 keV) = 9.0 +/- 0.57 x 10^-11 erg/s/cm^-2

The Galactic column density is 1.44 x 10^21 cm^-2 (Kalberla et al. 2005)

There is no evidence of any lines or other deviations from a smooth spectrum. The XRT flux shows a roughly constant level of 1.7 +/- 0.4 ct/s. 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 position of Swift J1943.4+0228 is near the galactic plane, Glon = 41.17551, Glat = -10.42249, suggesting that the source is galactic. The faintness of the optical counterpart suggests that the source is a low-mass X-ray binary. Further Swift observations have been requested

Swift/BAT Hard X-ray Transient Monitor Results for Swift J1943.4+0228