Following the INTEGRAL/JEM-X detection of the unidentified source 1RXS J194211.9+255552 (ATel #3816) on December 18, we asked for a Swift/XRT follow-up observation. Swift observed the source field on December 21, 2011 at 06:10:09.7 (UTC), with a net exposure of 1756 s. Within the ROSAT error circle there is only one pointlike source, at the following position (J2000): RA(hh mm ss.s) = 19h42m11.13s, Dec(dd mm ss.s) = +25:56:07.32 (3.6 arcsec error radius). The source light curve is variable on time scale of about 1000 s (similar to the Jem-X light curve). A fit to the XRT spectrum (1-10 keV) with a power law model resulted in a good fit (reduced chi_sq = 1.101 for 36 degrees of freedom) with an absorption of NH = 1.28E22 cm-2 (+0.57, -0.52) (consistent with the total Galactic value), a photon index of 0.64 (+0.30, -0.28) and an unabsorbed flux (1-10 keV) of 7.88E-11 ergs/cm2/s. A black body model resulted in a much worse fit (reduced chi_sq = 1.367 for 36 dof). The source is a factor of 2.5 fainter than during the Jem-X detection (ATel #3816), two days before. This is the first characterization of the source X-ray spectrum. A search at other wavelengths resulted in a possible counterpart in the 2MASS All-Sky Catalog of Point Sources, 19421116+2556056 (J=13.479, H=12.834, K=12.793 mag; although we note that the 2MASS image indicates a blend of two stars, the brighter more consistent with the XRT position), as well as a single star in the USNO-B1.0 catalog, 1159-0412038, at 1.62 arcsec offset from the XRT coordinates, with B1=20.55, R1=17.34, B2=19.85, R2=17.12, I=15.44 mag. Although the hard power law spectrum could indicate a high mass X-ray binary, other observations are needed to fully identify the nature of this X-ray transient. Further multiwavelength observations are encouraged. We would like to thank the Swift Team for making these observations possible, in particular Neil Gehrels, the duty scientists and the science planners.
|Publication date||21 Dec 2011|
|Publication status||Published - 21 Dec 2011|
|Series||The Astronomer's telegram|