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; Yassine et al., GCN 21380; Stanbro & Meegan, GCN 21383) 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, GCN 21373; Heintz et al., GCN 21374; Guidorzi et al., GCN 21375; Watson et al., GCN 21389). Given the rapid variability and positional coincidence with the X-ray (Osborne et al., GCN 21377) 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."