Swift reports the detection of a new transient source Swift J1741.5-6548

ATel #4902; H. A. Krimm (CRESST/GSFC/USRA), S. T. Holland (STScI), J. A. Kennea (PSU), S. B. Cenko (UC Berkeley) S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), N. Gehrels (GSFC), A. Y. Lien (NASA/GSFC/ORAU), C. B. Markwardt (GSFC), D. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU/GSFC), T. Ukwatta (MSU)
on 20 Mar 2013; 21:22 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 previously unknown transient source. Swift X-Ray Telescope (XRT) observations have confirmed the detection and produced a refined position.

The new source, Swift J1741.5-6548, was first detected by the BAT in a 16-day integration covering the days 2013 February 26 - March 13 (MJD 56349 - 56364) at an average rate of 0.0025 +/- 0.0003 counts/sec/cm2 (11 mCrab) in the 15-50 keV band. It has remained at roughly the same brightness since that time. Examination of archival data shows that the source peaked on 2013 March 1 at a rate of 0.005 +/- 0.001 counts/sec/cm2, or about 25 mCrab. The significance of this detection (4.0-sigma) was below the 5-sigma threshold for an alert from a previously unknown source.

A 3200-second Swift target of opportunity observation was performed starting at 2013 Mar 19 at 19:42:45 UT. The XRT observed in Photon Counting mode and a position (not UVOT-enhanced) was determined at:

RA (J2000): 17 41 23.47 (265.3478 deg)
Dec (J2000): -65 47 27.6 (-65.7910 deg)
90% Error radius: 3.5" (statistical + systematic)

This corresponds to galactic coordinates of glon = 327.1855, glat = -17.8224.

There are no sources in Simbad, Vizier or the 2MASS or USNO catalogs within the XRT error circle. There is however, a possible detection of a UVOT counterpart in the B band at the edge of the XRT error circle at a position of:

RA (J2000) 17 41 24.11 = (265.35046 deg)
Dec (J2000) -65 47 27.4 = (-65.79094 deg)
90% Error radius: 0.43" (statistical + systematic)

There is also a possible uncataloged quiescent source at this position in the DSS image. Confirmation of whether the UVOT source is indeed the counterpart will await further observations to determine long-term variation and fading. The UVOT source does show significant variation over five exposures:

The magnitudes are:

Date Time Exposure Mag Err
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Mar 19 19:42:50 125 19.20 0.16
Mar 19 19:45:00 1255 19.02 0.07
Mar 19 20:06:20 156 18.64 0.15
Mar 19 21:20:13 129 18.56 0.11
Mar 19 21:22:27 1290 18.65 0.06
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The XRT data are well fitted by an absorbed power-law model (Cstat = 483.2 for 552 dof) with the following parameters:

N_H = 1.51 +/- 0.31 x 10^21 cm-2
Gamma = 1.54 +/- 0.093
Flux (0.3-10 keV; unabsorbed) = 1.78 +/- 0.079 x 10^-10 erg/s/cm-2
There is no evidence of any lines or other deviations from a smooth spectrum. The XRT flux in the 0.3-10 keV band is slightly higher (4.23 +/- 0.72 ct/s) in the first pointing compared to the second (3.63 +/- 0.41 ct/s), but the overall variation is less than for the UVOT source. 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 much less likely given the faintness of the optical counterpart. Another speculation is that Swift J1741.5-6548 could be a similar tidal disruption event to Swift J164449.3+573451 (Burrows et al, 2011, Nature 476, 421) or Swift J2058+0516 (Cenko et al, 2012, ApJ 753, 77), given its high galactic latitude (-17.8 degrees), a low-level BAT light curve and the faintness of the possible quiescent counterpart, even when there is relatively low absorption in the direction toward the source. However, the lack of variability in the XRT is an argument against this interpretation.

Further observations with Swift have been approved starting on 2013 March 24.

Swift/BAT Hard X-ray Transient light curve for Swift J1741.5-6548