GCN Circular 29433

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
Not found
 
 
; Li et 
al., GCN 29392
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; Hurley et al., GCN 29408
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; Waratkar et al., GCN 29410
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; 
Frederiks et al., GCN 29415
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) 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
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) 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
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 ; Belkin et 
al., GCN29417
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; Teja et al., GCN 29414
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) 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.