GCN Circular 39723

M. A. Williams (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAR), T.
Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), S. Dichiara (PSU), J.A.
Kennea (PSU) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 250313A, collecting  6.3 ks of Photon
Counting (PC) mode data between T0+30.7 ks and T0+82.7 ks. 

Seven uncatalogued X-ray sources have been detected, however none of
them is above the RASS limit or shows definitive signs of fading.
Therefore, at the present time we cannot identify which, if any, is the
afterglow. Details of these sources are given below:

Source 1:
  RA (J2000.0):  278.5986  =  18:34:23.66
  Dec (J2000.0): +53.8117  =  +53:48:42.1
  Error: 5.7 arcsec (radius, 90% conf.)
  Count-rate: (3.20 [+1.07, -0.89])e-3 ct s^-1	 
  Distance: 360 arcsec from Fermi/LAT position.
  Flux: (1.13 [+0.38, -0.32])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 2:
  RA (J2000.0):  278.4773  =  18:33:54.54
  Dec (J2000.0): +53.6792  =  +53:40:45.1
  Error: 5.9 arcsec (radius, 90% conf.)
  Count-rate: (1.23 [+0.71, -0.54])e-3 ct s^-1	 
  Distance: 184 arcsec from Fermi/LAT position.
  Flux: (5.1 [+2.9, -2.2])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 3:
  RA (J2000.0):  278.7435  =  18:34:58.45
  Dec (J2000.0): +53.8112  =  +53:48:40.2
  Error: 6.1 arcsec (radius, 90% conf.)
  Count-rate: (1.19 [+0.90, -0.67])e-3 ct s^-1	 
  Distance: 560 arcsec from Fermi/LAT position.
  Flux: (2.6 [+2.0, -1.5])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 4:
  RA (J2000.0):  278.7131  =  18:34:51.13
  Dec (J2000.0): +53.8208  =  +53:49:14.7
  Error: 6.9 arcsec (radius, 90% conf.)
  Count-rate: (2.16 [+0.95, -0.76])e-3 ct s^-1	 
  Distance: 532 arcsec from Fermi/LAT position.
  Flux: (2.38 [+1.04, -0.83])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 5:
  RA (J2000.0):  278.5290  =  18:34:6.96
  Dec (J2000.0): +53.8678  =  +53:52:04.2
  Error: 8.1 arcsec (radius, 90% conf.)
  Count-rate: (1.57 [+0.84, -0.65])e-3 ct s^-1	 
  Distance: 532 arcsec from Fermi/LAT position.
  Flux: (6.4 [+3.4, -2.6])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 6:
  RA (J2000.0):  278.5496  =  18:34:11.90
  Dec (J2000.0): +53.7202  =  +53:43:12.8
  Error: 5.7 arcsec (radius, 90% conf.)
  Count-rate: (1.35 [+0.73, -0.55])e-3 ct s^-1	 
  Distance: 41 arcsec from Fermi/LAT position.
  Flux: (8.6 [+4.7, -3.5])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 7:
  RA (J2000.0):  278.6648  =  18:34:39.55
  Dec (J2000.0): +53.6612  =  +53:39:40.2
  Error: 7.4 arcsec (radius, 90% conf.)
  Count-rate: (7.5 [+6.0, -4.6])e-4 ct s^-1   
  Distance: 356 arcsec from Fermi/LAT position.

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021822.

This circular is an official product of the Swift-XRT team.