GCN Circular 30731
Subject
GRB 210820A: CAHA 2.2m Optical Detection
Event
Date
2021-08-27T18:03:36Z (4 years ago)
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.