TITLE: GCN CIRCULAR NUMBER: 30731 SUBJECT: GRB 210820A: CAHA 2.2m Optical Detection DATE: 21/08/27 18:03:36 GMT FROM: Alexander Kann at IAA-CSIC kann@iaa.es

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

We observed GRB 210820A (Swift detection: Beardmore et al., GCN #30664) with CAFOS mounted on the 2.2m CAHA telescope at Calar Alto, Alemria, Spain, under inclement conditions (bright moonlight, passing clouds, low transparency). We obtained 10 x 90 s each in filters B, V, Rc. Stacks reveal no detection of the afterglow in B (very shallow because of moonlight) and Rc (shallow because of incoming clouds which also prevented further observations). However, the afterglow (Beardmore et al., GCN #30664; Butler et al., GCN #30672; LaPorte & Beardmore, GCN #30686; Nicuesa Guelbenzu et al., GCN #30695) is clearly detected in V, somewhat north of a source also seen in Pan-STARRS and in SDSS.

We find the afterglow to be V = 22.43 +/- 0.15 mag (AB mag, derived from two SDSS field stars transformed to V via the equations of Lupton 2005) at 0.43868 days after the GRB.