Swift reports the detection of the transient source XMMSL1 J182155.0-134719

ATel #3933; 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 16 Feb 2012; 22:22 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 J182155.0-134719 and also the unidentified INTEGRAL source IGR J18219-1347 (Krivonos et al. 2010, A&A, 523, 61). 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 under an alternate name, Swift J1821.8-1348.

In the current epoch, XMMSL1 J182155.0-134719 was first detected by the BAT in a 4-day integration covering 2012 Jan. 31 through Feb. 3 (MJD 55957 - 55960) at an average rate of 0.00060 +/- 0.00009 counts/sec/cm2 (3 mCrab) in the 15-50 keV band. It rose in brightness to an average rate of 0.0035 +/- 0.0015 counts/sec/cm2, where it remained for ~15 days, before dropping to ~0.001 counts/sec/cm2 in recent days. The peak rate was 0.0067 +/- 0.0020 on 2012 Feb. 6. If one assumes a photon index of 2.5, the average BAT count rate transforms to roughly 2 X 10^-10 erg/s/cm2 in the 17-60 keV band. This is a factor of more than 25 times the flux reported for IGR J18219-1347 in Krivonos et al. 2010 (7.1 +/- 1.3 X 10^-12 erg/s/cm2).

A 1400-second Swift target of opportunity observation was performed starting at 06:52:12.52 on 2012 Feb. 15. The XRT observed in Photon Counting mode and a UVOT-enhanced position was determined at:

RA (J2000): 18 21 54.86 (275.47856 deg)
Dec (J2000): -13 47 26.5 (-13.79070 deg)
90% Error radius: 1.7"

We note that this is on the galactic plane at L=17.32466, B=+0.13414.

The XRT data are low statistics and completely dominated by very high absorption, which means that the photon index cannot be reliably fitted. The fit parameters were (Cstat = 167 for 192 dof):

N_H = 4.4 (+2.6, -1.9) x 10^22 cm^-2
Gamma = 0.18 +/- 0.56
Flux (0.3-10 keV) = 4.1 +/- 0.72 x 10^-11 erg/s/cm2

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 0.074 to 0.93 ct/s, with an average rate of 0.28 +/- 0.09 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). There was no detection in the UVOT B filter, which is not surprising given the large extinction (Galactic column density 1.35 x 10^22 cm-2; Kalberla et al. 2005).

The location of the XRT source is consistent with XMMSL1 J182155.0-134719, which was detected on 2006 Sep. 26 at 3.28703 +/- 1.20088 X 10^-11 erg/s/cm2, consistent with the currently measured XRT flux. However, in Swift/XRT observations in March 2010, Landi et al, (ATel #3272) report an upper limit of 8.0 x 10^-13 erg/s/cm2 (0.2-12 keV). There is a source in the Spitzer/GLIMPSE catalog within the XRT error circle, G017.3244+00.1344 at RA=275.478202, dec=-13.790732, and quoted magnitudes in 3.6 um: 12.93, 4.5 um: 12.34, 5.8 um: 11.89. 2MASS 18215463-1347232 lies within the BAT and XMM error circles). However, it is 4.6 arcsec from the XRT position and hence inconsistent.

The source intensity varies by at least a factor of 25 in the hard X-ray band and at least 50 in the soft X-ray band. However, the detection of this source in the 7-year INTEGRAL survey (Dec. 2002 - July 2009) near threshold suggests that either its quiescent flux is around 7 X 10^-12 erg/s/cm2, or that it had other outbursts during the survey period, which were not detected, but which, when integrated together led to a hard X-ray detection.

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 approved.

Swift/BAT Hard X-ray Transient light curve for XMMSL1 J182155.0-134719