TITLE:   GCN CIRCULAR
NUMBER:  29433
SUBJECT: GRB 210204A: LBT observations and jet break
DATE:    21/02/08 19:23:12 GMT
FROM:    Andrea Rossi at INAF  <andrea.rossi@inaf.it>

A. Rossi (INAF-OAS Bologna), A. Melandri (INAF-OAB) report on behalf of 
the CIBO collaboration:

We observed the field of GRB210204A (Fermi GBM team, GCN 29390; Li et 
al., GCN 29392; Hurley et al., GCN 29408; Waratkar et al., GCN 29410; 
Frederiks et al., GCN 29415) simultaneously in the g'r'i' and z' bands 
with the LBC imager mounted on LBT  (Mt Graham, AZ, USA). We obtained 
20min of imaging for each band at the midtime 08:20 UT on 2021-02-07, 
3.08 days after the burst trigger.

We clearly detect the afterglow (Kool et al., GCN 29405) and we 
preliminary measure the following AB magnitudes:

g = 21.1 +- 0.1
r = 20.7 +- 0.1
i = 20.4 +- 0.1
z = 20.2 +- 0.1

calibrated against SDSS field stars.

Modelling the observations available (Kool et al., GCN29405 ; Belkin et 
al., GCN29417; Teja et al., GCN 29414) with a broken power-law, we find 
that the light curve first decays with index alpha_1=0.60+-0.05, then 
breaks at t_b=2.2+-0.2 days, and afterwards it is described by a decay
index alpha_2~1.7+-0.5. The last one is constrained only by the LBT 
detection, but is ~0.4 magnitude fainter than the brightness predicted 
extrapolating the pre-break decay.

We acknowledge the excellent support from the LBTO and LBT-INAF staff, 
particularly B. Rothberg, F. Cusano and D. Paris, in obtaining these 
observations.