GCN Circular 21395

W. Fong (University of Arizona), T. Laskar (NRAO/UC Berkeley), K. D.
Alexander (Harvard) and E. Berger (Harvard) report:

"We observed the position of GRB 170728B (Cenko et al., GCN 21371
Not found
 
 
; Yassine
et al., GCN 21380
Not found
 
 
; Stanbro & Meegan, GCN 21383
Not found
 
 
) with the Karl G. Jansky
Very Large Array (VLA) beginning on 2017 Jul 29.075 UT (2.75 hr post-burst)
at a mean frequency of 6 GHz. We obtained a second set of observations
beginning on 2017 Jul 29.811 UT (20.4 hr post-burst) at mean frequencies of
6 and 10 GHz.

We do not detect any radio counterpart in our first set of observations at
2.75 hr, while we clearly detect a radio source in our second set of
observations at both frequencies at 20.4 hr at the position:

RA(J2000) = 15:51:55.45
Dec(J2000) = +70:07:21.2

with an uncertainty of 0.2" in each coordinate. This position is coincident
with the reported optical source (D'Avanzo et al., GCN 21372
Not found
 
 
, GCN 21373
Not found
 
 
;
Heintz et al., GCN 21374
Not found
 
 
; Guidorzi et al., GCN 21375
Not found
 
 
; Watson et al., GCN
21389
Not found
 
 
). Given the rapid variability and positional coincidence with the
X-ray (Osborne et al., GCN 21377
Not found
 
 
) and optical afterglows, we consider this
to be the radio afterglow of GRB 170728B.

Further observations are planned. We thank the VLA staff for quickly
executing these observations."