TITLE:   GCN CIRCULAR
NUMBER:  30723
SUBJECT: GRB 210822A: CAHA 2.2m observations
DATE:    21/08/27 00:27:10 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, I. Hermelo, and P. Martin (all CAHA) 
report:

We observed the bright GRB 210822A (Swift detection: Page et al., GCN 
#30677; GECAM detection: Wang et al., GCN #30678; Fermi-LAT detection: 
Ohno et al., GCN #30687;  AstroSat CZTI detection: Prasad et al., GCN 
#30693; Konus-Wind detection: Frederiks et al., GCN #30694; GRBAlpha 
detection: Ohno et al., GCN #30697) at a redshift of z = 1.736 (Zhu et 
al., GCN #30692) with CAFOS mounted on the 2.2m CAHA telescope at Calar 
Alto, Alemria, Spain. The first epoch was obtained under very inclement 
conditions (almost overcast), and one 60 s as well as four 90 s images 
were useful. The second epoch, two nights later, was obtained under 
significantly improved (but still not really good) conditions, and 31 x 
30 s images were obtained.

The optical afterglow (Page et al., GCN #30677; Zheng et al., GCN 
#30679; Butler et al., GCN #30680; Fu et al., GCN #30684; Zhu  et al., 
GCN #30692; Romanov & Lane, GCN #30701; Nicuesa Guelbenzu et al., GCN 
#30703; Siegel et al., GCN #30710; Belkin  et al., GCN #30712; Belkin  
et al., GCN #30714) is faintly detected in the first epoch, but not 
detected in the second epoch.

Against three nearby Pan-STARRS comparison stars, we measure (AB 
magnitudes):

i' = 18.56 +/- 0.14 mag at 0.43283 days after the GRB, and
i' > 21.5 mag at 2.5206 mag after the GRB.