TITLE: GCN CIRCULAR NUMBER: 39758 SUBJECT: GRB 250316A: Swift-XRT afterglow detection DATE: 25/03/17 09:48:06 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
A.P. Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (PSU), J.P. Osborne (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has conducted further observations of the field of the Swift/BAT-detected burst GRB 250316A. The observations now extend from T0+21.7 ks to T0+51.0 ks and have a total exposure time of 4.6 ks. . The source previously reported, "Source 1", is believed to be the afterglow. Using 3006 s of PC mode data and 2 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 219.59644, +14.89108 which is equivalent to:
RA (J2000): 14h 38m 23.15s Dec(J2000): +14d 53' 27.9"
with an uncertainty of 3.0 arcsec (radius, 90% confidence). This position is 118 arcsec from the Swift/BAT position. The source is fading with alpha >0.9.
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.7 (+0.9, -0.7). The best-fitting absorption column is 5.1 (+4.0, -2.8) x 10^21 cm^-2, in excess of the Galactic value of 1.5 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.0 x 10^-11 (8.4 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 5.1 (+4.0, -2.8) x 10^21 cm^-2 Galactic foreground: 1.5 x 10^20 cm^-2 Excess significance: 2.9 sigma Photon index: 2.7 (+0.9, -0.7)
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021823. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021823.
This circular is an official product of the Swift-XRT team.
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