{
  "bibcode": "2015GCN.17536....1E",
  "body": "Phil Evans (U. Leicester) , Alessandro Maselli (INAF-IASFPA), David Burrows (PSU), \nSergio Campana (INAF-OAB), Neil Gehrels (GSFC), Jamie Kennea (PSU), \nAmy Lien (GSFC/UMBC), Lea Hagen (PSU), Daniele Malesani (DARK/NBI), \nFrank Marshall (GSFC), Julian Osborne (U. Leicester), Kim L. Page (U Leicester), \nMassimiliano De Pasquale (INAF-IASFPA) report on behalf of the Swift team:\n\nWe report further XRT observations of the highly unusual transient or GRB detected \nby BAT on March 1 ( trigger 633105 or GRB150301C; Lien et al., GCN Circ 17512; \nStamatikos et al., GCN Circ 17516).  It lasted about 20 seconds in BAT and has a \nlocation in M31, but had very weak X-ray emission. The XRT found a single source \nwithin the BAT error circle, which is coincident with the known X-ray source [PFH2005] 622. \nThis source was previously reported in an XMM-Newton survey of M31 (Pietsch et al., \nA&A, 434, 483, 2005), and was classified as a \"hard\" source based on the hardness ratio (see \ndefinition in Pietsch et al. 2005). The paper suggests that these \"hard\" sources  may be\nX-ray binaries, Crab-like SNRs, or AGNs.\n\nUsing the faint-source-optimised detection system from the 1SXPS catalogue (Evans et al., \nApJS 210, 8, 2014), the XRT source is detected with a likelihood of 18.3 in the first observation \nof the BAT error region. This corresponds to a \"Good\"-flagged object, i.e. a >3-sigma detection.\n\nThe refined XRT position is RA,Dec = 11.3107, +41.8431 degrees which corresponds to\n\nRA (J2000):   00h 45m 14.57s\nDec (J2000): +41d 50' 35.1\"\n\nwith an uncertainty of 5.7 arcsec (radius, 90% confidence). This is 4 arcsec from the catalogued \nposition of [PFH2005] 622, which is at RA=00h 45m 14.87,Dec=+41d 50' 33'' with an uncertainty \nof 1.5 arcsec. Hence, the XRT position is consistent with the position of [PFH2005] 622.\n\nDuring the first XRT observation, starting at T0+105s and lasting 1.9 ks, the source is detected with \na count-rate of 6.0 (+2.3, -1.8) x10^-3 ct/sec. In the subsequent observations, from T0+44.9 ks and \ntotalling 6ks exposure, the source is undetected, with an upper limit of 1.2 x10^-3 ct/sec, consistent \nwith fading.\n\nSwift-XRT has previously observed this location for a total of ~4ks, corresponding to \"Stacked image \n7598\" in the 1SXPS catalogue. The source was not detected in this dataset, with an upper limit of \n2.2x10^-3 ct/sec, significantly below the level at which we have detected it in this observation. \nAdditionally, XMM-Newton observations (Pietsch et al., 2005) detected this X-ray source in 2005 \nwith a flux of 6.9x10^-15 erg/cm^2/s. The peak rate in our observations corresponds to \n~2x10^-13 erg/cm^2/s, i.e. ~35 times brighter than the XMM detection. At the distance of M31, \nthese fluxes correspond to a luminosity of ~5x10^35 erg/s (0.2-4.5 keV) in the XMM-Newton observations, \nand ~1.4x10^37 erg/s (0.3-10 keV) in the XRT observations. For comparison, the average luminosity \nin BAT at this distance was 1.25x10^42 erg/s in 15-150 keV (Stamatikos et al., GCN Circ 17516).\n\nWe therefore conclude that XRT has detected a bursting source that is likely the counterpart to \nBAT trigger 633105.",
  "circularId": 17536,
  "createdOn": 1425338376000,
  "email": "amy.y.lien@nasa.gov",
  "subject": "GRB 150301C: XRT detection of a potential counterpart of BAT trigger 633105",
  "submitter": "Amy Lien at GSFC  <amy.y.lien@nasa.gov>",
  "eventId": "GRB 150301C"
}