TITLE:   GCN CIRCULAR
NUMBER:  27827
SUBJECT: GRB 200522A: HST F125W new point source detection
DATE:    20/05/26 16:30:44 GMT
FROM:    Wen-fai Fong at Northwestern U  <wfong@northwestern.edu>

W. Fong (Northwestern), C. D. Kilpatrick (UCSC), J. Rastinejad (Northwestern) and E. Berger (Harvard) report on behalf of a larger collaboration:

"We report on further HST/WFC3 observations of GRB200522A (Evans et al., GCN 27778) taken under Program 15964 in the F125W filter. A total of 5223-sec of exposure time was obtained roughly at the same mid-time of ~3.6 days. We detect sources A and B (Fong et al., GCN 27826), as well as an additional point-like source C at:

RA(J2000) = 00:22:43.62
Dec(J2000) = -00:16:57.56

This position is ~1.2" away from the center of the LCO faint source position (Strausbaugh et al., GCN 27794), ~1.5" offset from Source A, and at the edge of the XRT position (90% confidence). While there is some contamination from the host galaxy that introduces uncertainty, we estimate a magnitude for Source C of m(F125W)~26.8 +/- 0.3 AB mag. Regarding the LCO source, the positional offset, coupled with the fact that Source C is considerably fainter than extrapolations based on afterglow evolution, suggests that the potential LCO source and Source C are not related.

The color of Source C is blue, with F125W-F160W < -0.4 mag. This color is more consistent with those of afterglows (-0.3 mag) than the corresponding redder color of the kilonova of GW170817 (~0.2-0.4 mag; comparing to rest-frame z-J assuming z~0.4). However, further observations are necessary to determine the nature of this source, assess its variability, and its relation to GRB 200522A."