TITLE:   GCN CIRCULAR
NUMBER:  30337
SUBJECT: GRB 210626A: Ondrejov D50 optical limit
DATE:    21/06/28 09:59:16 GMT
FROM:    Martin Jelinek at Astro.Inst-AVCR,Ondrejov  <martin.jelinek@asu.cas.cz>

Martin Jelinek, Jan Strobl, Rene Hudec and Cyril Polasek (ASU CAS Ondrejov,
CZ), report:

We observed the Swift-improved localization (Tohuvavohu et al. GCN 30325,
Perri et al. GCN 30329) of the GBM-detected GRB 210626A (Fermi GBM Team,
GCN 30323) with the 50 cm robotic telescope (D50) of Ondrejov observatory
in Czech Republic. We obtained a set of 120 s Sloan-i' exposures starting
at 21:10 UT, i.e.  0.5386 d after the initial trigger.

The magnitude limit of our 34 x 120 s image with a mean time T-T0 0.5626 d
post burst is i' > 21.2 (AB). The XRT (Perri et al. GCN 30329) error box
does not appear completely clean in the image, there is a noise fluctuation
or possibly a candidate at the level of i' = 21.3 +/- 0.4. While we mention
this, we also note that this is in a light halo of a nearby bright star, so
artefacts of this kind may not be excluded.