TITLE:   GCN CIRCULAR
NUMBER:  28501
SUBJECT: GRB 200925B: OSN Afterglow Candidate Discovery
DATE:    20/09/25 23:14:27 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. A. Kann, C. C. Thoene,
M. Blazek, J. F. Agui Fernandez (all HETH/IAA-CSIC), and F. Aceituno 
(IAA-CSIC) report:

We observed the location of the Swift GRB 200925B (Cenko et al., GCN 
#28499) with the 1.5m telescope of the Observatorio Sierra Nevada, in 
Granada, Spain. We obtained a 300 s image each in the BVRc bands and 
multiple 100 s images in Ic, beginning 2020-09-25 22:27:07 UT.

We detect a source in all filters at

RA (J2000) = 16:27:08.50
Dec. (J2000) = +78:23:24.9

with an error of 0".5, within the UVOT-enhanced XRT position (Evans, GCN 
#28500). We note this source is also clearly detected in the initial 
Swift UVOT white finding chart, at a significantly brighter magnitude 
relative to the nearby field star.

This strongly suggests the source is fading and is therefore the 
afterglow of GRB 200925B.

At 0.034294 days after the GRB, we measure r' = 19.46 +/- 0.1 (AB 
magnitude) vs. nearby SDSS field stars.

[GCN OPS NOTE(26Sep20): Per the author's request: in the Title the "25A" was changed to "25B",
the affiliation of A. de Ugarte Postigo is "*-CSIC" not "*-CSIS",
"Thoene" not "Thone", and in the first sentence, the "200906A" should be "200925B".]