TITLE:   GCN CIRCULAR
NUMBER:  29863
SUBJECT: GRB 210420B: Tautenburg observations 
DATE:    21/04/21 15:01:05 GMT
FROM:    Sylvio Klose at TLS Tautenburg  <klose@tls-tautenburg.de>

B. Stecklum, A. Nicuesa Guelbenzu, S. Klose, S. Melnikov, and U. Laux
(all Tautenburg) report:

We observed the field of GRB 210420B (Moss et al., GCN 29844) with the 
Tautenburg 1.34m Schmidt telescope equipped with the TAUKAM 6k x 6k CCD 
camera. Observations started on April 20 at 20:13:21 UT, 99 min after the 
BAT trigger (Lien et al., GCN 29854). Altogether four observing runs (5x5 
min each) were performed (Sloan r'). The afterglow (Lipunov et al., GCN 
29845; Oksanen, GCN 29846; Hu et al., GCN 29847; Sun et al., GCN 29848; 
Leonini et al., GCN 29851; de Ugarte Postigo et al., GCNs 29852, 29853; 
Watson et al., GCN 29858) was clearly detected in all frames. Magnitudes 
were calibrated against SDSS.

The data show that between the first (mean time: 20:26:10 UT; r' = 18.30 
+/- 0.03) and the second observing run (mean time: 21:47:07 UT; r' = 18.75 
+/- 0.03) ) the light curve was steepening to a decay with alpha2 = 0.99 
+/- 0.04 (break time tb = 0.091 +/- 0.017 days). Before the break the 
decay index is not well constrained (alpha1 = 0.35 +/- 0.33).