Swift reports the detection of the transient source XMMSL1 J184555.4-003941 = Swift J1845.7-0037

ATel #4130; 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), 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 27 May 2012; 00:39 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 galactic transient source with a position consistent with the XMM slew survey source XMMSL1 J184555.4-003941. Swift X-Ray Telescope (XRT) observations have confirmed the detection and produced a refined position. The Swift observations of the source reported here are listed in the Swift data archive under an alternate name, Swift J1845.7-0037.

In the current epoch, XMMSL1 J184555.4-003941 was first detected by the BAT in a 4-day integration covering 2012 May 16 through May 19 (MJD 56063 - 56066) at an average rate of 0.0031 +/- 0.00005 counts/sec/cm2 (15 mCrab) in the 15-50 keV band. It rose in brightness to an average rate of 0.0067 +/- 0.0018 counts/sec/cm2 (~30 mCrab) on May 22 and has declined somewhat since then.

A 2600-second Swift target of opportunity observation was performed starting at 09:27:38.346 UT on 2012 May 24. The XRT observed in Photon Counting mode and a UVOT-enhanced position was determined at:

RA (J2000): 18 45 54.72 (281.47800 deg)
Dec (J2000): -00 39 34.0 (-0.65944 deg)
90% Error radius: 1.5"

We note that this is on the galactic plane at L=31.714956, B=+0.928654.

The XRT data are fitted by an absorbed powerlaw. The fit parameters were (Cstat = 447 for 455 dof):

N_H = 3.57 (+0.92, -0.81) x 10^22 cm^-2
Photon power law index = 0.52 +/- 0.29
Flux (0.3-10 keV) = 8.0 +/- 0.74 x 10^-11 erg/s/cm2 (observed)
Flux (0.3-10 keV) = 1.0 +/- 0.0079 x 10^-10 erg/s/cm2 (unabsorbed)

There is no evidence of any lines or other deviations from a smooth spectrum. The XRT flux showed some variability over four pointings, with a count rate (0.3-10 keV) ranging from 0.30 to 1.1 ct/s, with an average rate of 0.74 +/- 0.03 ct/s. The baseline is not long enough to reliably determine whether there is periodicity. 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). There was no detection in the UVOT B filter, which is not surprising given the large extinction (Galactic column density 1.66 x 10^22 cm^-2; Kalberla et al. 2005).

The location of the XRT source is consistent with XMMSL1 J184555.4-003941, which was detected on 2005 Sep. 20 at 1.96445 +/- 0.666555 X 10^-12 erg/s/cm2 in the 0.2-12 keV band, a factor of 40 weaker than the currently measured XRT flux. The source position was also observed serendipitously in a Swift galaxy survey pointing on 2011 June 27. It was barely detected at 0.008 +/- 0.005 ct/s (0.3-10 keV). Assuming the current spectral index, this would convert to a flux of 1.904 X 10^-12 ergs/s/cm2, consistent with the XMM value. The source intensity thus varies by at least a factor of 40 in the soft X-ray band.

The XRT error circle contains an object in the 2MASS catalog: 2MASS 18455462-0039341, at RA=281.477598, dec=-0.659499 and magnitudes J=15.801, H=14.589 +/- 0.067, K=13.591 +/- 0.059. We therefore suggest that the 2MASS star is the counterpart to XMMSL1 J184555.4-003941/Swift J1845.7-0037.

The current results do not allow us to determine the nature of the source, but given its location and variability it is likely a galactic X-ray binary. Further Swift observations have been requested.

BAT Hard X-ray Transient light curve for XMMSL1 J184555.4-003941