TITLE: GCN CIRCULAR NUMBER: 16332 SUBJECT: Swift detection of a bright X-ray source in M31 DATE: 14/05/27 22:15:08 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), S. B. Cenko (GSFC), V. D'Elia (ASDC), D. Malesani (DARK/NBI), V. Mangano (PSU), F. E. Marshall (NASA/GSFC) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team: At 21:24:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located a source in M31 (NGC224) (trigger=600114). Swift slewed immediately to the target. The BAT on-board calculated location is RA, Dec 10.402, +41.556, which is RA(J2000) = 00h 41m 37s Dec(J2000) = +41d 33' 23" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). This is an image trigger of 64 second duration, with significance 6.18 sigma, which is lower than typical blind detection significance. The BAT light curve does not show obvious structure, which is typical for image triggers. The XRT began observing the field at 21:27:05.4 UT, 157.5 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 10.4292, 41.5734 which is equivalent to: RA(J2000) = 00h 41m 43.02s Dec(J2000) = +41d 34' 24.3" with an uncertainty of 2.9 arcseconds (radius, 90% containment). This location is 96 arcseconds from the BAT onboard position, within the BAT error circle. The XRT position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. This position is 4.0 arcseconds from that of a known X-ray source: 3XMM J004143.0+413420 in the XMM-NEWTON XMMSSC catalogue. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.81 x 10^21 cm^-2 (Willingale et al. 2013). The associated source in the 3rd XMM catalog has a spectrum and variability which is consistent with a low-mass X-ray binary (Stiele 2010). However, the target is also spatially consistent with a globular cluster FMZ2005-7 of M31. There may be multiple X-ray binaries within the same globular cluster. The XRT count rate is approximately 30 counts per second. Assuming a typical conversion factor to flux, the 0.3-10 keV flux is approximately 1e-9 erg/cm2/s (uncertain to a factor of 2x). At the distance of Andromeda, this would be a luminosity of 7e40 erg/s (uncertain to the same factor). This luminosity is clearly super-Eddington for a ~solar mass compact object, so either the object is of ULX class, or it is significantly beamed. UVOT took a finding chart exposure of 150 seconds with the White filter starting 162 seconds after the BAT trigger. UVOT detects a 16.9 mag source at the position RA (J2000) = 00:41:43.10 = 10.42957 Dec(J2000) =+41:34:20.4 = 41.57232 with a 90% confidence error radius of 0.61 arc sec. This position is 4 arc seconds from the center of the XRT error circle and consistent with the position of the globular cluster. No correction has been made for the expected extinction corresponding to E(B-V) of 0.06.