Swift reports the detection of the transient source Swift J1836.6+0341/XTE J1837+037

ATel #3684; H. A. Krimm (CRESST/GSFC/USRA), 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 15 Oct 2011; 12:05 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 detected an outburst from a galactic transient source. Swift X-Ray Telescope (XRT) observations have confirmed the detection and produced a refined position.

The position of the source is consistent with that of XTE J1837+037, an unidentified source listed in the RXTE/All-Sky Monitor catalog. A literature search has revealed no previous reports on this object, save for a passing mention in Remillard et al, (2006, ApJ 646, 407).

The source, to which we give the additional name Swift J1836.6+0341, was first detected by the BAT in a 16-day integration covering the days 2011 Sep 25 through 2011 Oct 10 (MJD 55829 - 55844). Since Sep 25, it has remained at an average rate of 0.0016 +/- 0.001 counts/sec/cm^2 (~7 mCrab) in the 15-50 keV band, although recently it has appeared to fade somewhat. The source has also shown increased activity in the RXTE/ASM since roughly 2011 June 29, with an average rate of 2.8 +/- 1.4 ct/sec.

A 924-second Swift target of opportunity observation was performed starting at 14:01 UT on 2011 October 14. The XRT observed in Photon Counting mode and a UVOT-enhanced position was determined at:

RA (J2000): 18 36 39.46 (279.16441 deg)
Dec (J2000): +03 41 00.3 (+3.68343 deg)
90% Error radius: 2.2"

An examination of the Vizier catalogs shows that there is no catalog source within the error radius. There is also no detection in the UVOT U band.

The XRT data are well fitted by an absorbed power-law model (Cstat = 225 for 249 dof) with the following parameters:

N_H = 3.5 +/-0.5 x 10^21 cm^-2
Gamma = 1.80 +/- 0.25
Flux (0.3-10 keV) = 1.2 +/- 0.16 x 10^-10 erg/s/cm^-2

There is no evidence of any lines or other deviations from a smooth spectrum. The XRT flux shows a roughly constant level for the first 800 seconds of 2.6 +/- 0.4 ct/s and then drops to 1.3 +/- 0.6 at the end of the observation. 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. Further Swift observations have been requested.

BAT hard X-ray transient monitor light curve for Swift J1836.6+0341