Detection of a new Galactic Center X-ray source: Swift J174444.9-295042

ATel #4111; R. M. Bandyopadhyay (UF), T. J. Maccarone (Southampton), J. Kennea (Penn State)
on 14 May 2012; 16:41 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Reba Bandyopadhyay (reba@alum.mit.edu)

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In conjunction with the Vista Variables in the Via Lactea's (VVV) intensive Bulge monitoring program, we have arranged a Swift monitoring program of a large region of the inner Galactic Bulge; see ATel #4109 for a description of this program, which began on 1 May 2012. In the data for 10 May 2012, Swift detected an X-ray source at 17:44:44.91, -29:50:42.3 (4.1" error radius, 90% confidence), at a count rate of 0.1 cts/sec, and it is detected by Swift down to ~3 keV. The best fit using the pipeline of Evans et al. 2009 (MNRAS, 397, 1177) is a N_H=10^23 cm^-2, Gamma=3.5 power law spectrum with a 0.3-10 keV unabsorbed flux of 5*10^-10 ergs/sec/cm^2, corresponding to a luminosity of 3.8*10^36 erg/sec for 8 kpc, but the fit is poorly constrained due to the relatively low count rate. The absorbed flux is a factor of 70 lower, so a considerable decrease in the flux from the source could result from a different choice of spectral model.

In the catalog of Muno et al. 2009 (ApJS, 181, 110), there is a nearby hard, faint X-ray source, CXOGC 174445.4-295042, located 8.3" away from this Swift source. This Chandra source has a positional uncertainty of 2.8" radius. As the Swift and Chandra positions of the two sources are not consistent within their errors, we believe the two sources are unrelated. If so, then Swift J174444.9-295042 is a new X-ray source that was not detected in the previous deep Chandra surveys of this region of the Galactic Center, clearly suggesting a transient nature. However, we note that both error radii are reported at a 90% confidence limit, so there is a slim chance that these could be two detections of a single source. In that case, Swift would be detecting an outburst of the Chandra source, as the Swift flux is considerably brighter than the ~2 counts/ksec flux reported for CXOGC 174445.4-295042 in the Muno 2009 catalog. In either case, the Swift observation suggests that this source is undergoing a transient outburst.

We have searched for candidate near-infrared counterparts to this potential new Galactic Center X-ray transient using the UKIDSS Galactic Plane Survey (GPS) Catalog (Lucas et al., 2008, MNRAS, 391, 136) Data Release 8. Within the Swift error circle (4.1" radius), there are five infrared stellar sources in the UKIDSS GPS DR8 catalog, with K magnitudes ranging from 12.89 to 16.68 (typical errors ~ +/-0.05). None of the sources within the error circle have JHK magnitudes consistent with that expected from a supergiant at distances of 8 kpc or less, most likely ruling out a Supergiant Fast X-ray Transient (SFXT) nature for this source. All five IR sources are affected by substantial extinction, consistent with their location at or near the distance of the Galactic Center.

Follow-up IR imaging to search for evidence of photometric variability in the five IR candidate counterparts of Swift J174444.9-295042 while it is in an X-ray active phase is ongoing with the VVV Bulge monitoring program; additional IR and X-ray observations are strongly encouraged.