GCN Circular 30252

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA),
J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU) and
M. G. Bernardini (INAF-OAB) report on behalf of the Swift-XRT team:

We have analysed 5.2 ks of Photon Counting (PC) mode XRT data for the
Swift-BAT-detected burst GRB 210618A (Bernardini et al. GCN Circ.
30250
Not found
 
 
), collected between T0+65 s and T0+22.6 ks. 

Three uncatalogued X-ray sources have been detected consistent with
being within 296 arcsec of the Swift-BAT position, however none of them
is above the RASS limit or shows definitive signs of fading. Two
sources (Source 1 and Source 4) are consistent within the XRT error
circle with two spectroscopically identified quasars, and the third one
(Source 2) is consistent with a SDSS galaxy with photo_z=0.59 and
marginally consistent with a candidate quasar. Therefore, for the
moment the most plausible afterglow candidate is Source 2. Swift will
perform follow-up observations to confirm this candidate.

Details of these sources are given below:

Source 1:
  RA (J2000.0):  235.8870  =  15:43:32.87
  Dec (J2000.0): +46.0253  =  +46:01:31.0
  Error: 8.9 arcsec (radius, 90% conf.)
  Count-rate: (3.82 [+1.19, -1.00])e-3 ct s^-1	 
  Distance: 179 arcsec from Swift-BAT position.
  Flux: (1.17 [+0.36, -0.30])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)
This source is consistent with SDSS J154332.15+460133.2,
spectroscopically classified as a quasar at z = 0.657. 

Source 2:
  RA (J2000.0):  235.7698  =  15:43:4.75
  Dec (J2000.0): +46.0691  =  +46:04:08.6
  Error: 9.3 arcsec (radius, 90% conf.)
  Count-rate: (4.0 +/- 1.0)e-3 ct s^-1	 
  Distance: 156 arcsec from Swift-BAT position.
  Flux: (1.35 +/- 0.35)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV) 
This source is positionally consistent with SDSS J154305.56+460405.0, a
galaxy with photo_z = 0.590 +/- 0.153 and 12.2 arcsec away from SDSS
J154305.40+460418.7, a candidate quasar (Richards et al., 2015, ApJS,
219, 39).

Source 4:
  RA (J2000.0):  235.7183  =  15:42:52.39
  Dec (J2000.0): +46.0336  =  +46:02:01.0
  Error: 8.7 arcsec (radius, 90% conf.)
  Count-rate: (1.48 [+0.78, -0.60])e-3 ct s^-1	 
  Distance: 256 arcsec from Swift-BAT position.
  Flux: (7.2 [+3.8, -2.9])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)
This source is positionally consistent with SDSS J154251.96+460206.1,
spectroscopically classified as a quasar at z = 2.130.

Six uncatalogued sources were also detected too far from the GRB
position to be likely afterglow candidates.

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/01056426.

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