GCN Circular 29502

D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, 
DARK/NBI), C. C. Thoene, M. Blazek, J. F. Agui Fernandez (all 
HETH/IAA-CSIC), P. Minguez, and S. Pedraz (both CAHA) report:

We again observed the afterglow (Laha et al., GCN #29444, Lipunov et 
al., GCN #29447, de Ugarte Postigo et al., GCN #29450, Breeveld & Laha, 
GCN #29457, Strausbaugh & Cucchiara, GCN #29459, Jelinek et al., GCN 
#29469, Kann et al., GCN #29476, Dimple et al., GCN #2948 of GRB 210210A 
(Laha et al., GCN #29444) with CAFOS mounted on the 2.2m Calar Alto 
telescope (Almeria, Spain). We obtained 6 x 600 s image in Rc. We detect 
the optical afterglow faintly in the stacked Rc image.

Using the same comparison star used by de Ugarte Postigo et al., and 
transforming to Rc following the equations of Lupton (2005), then 
transforming back to AB magnitudes, we derive a preliminary magnitude:

Rc = 23.05 +/- 0.14 mag at 3.129261 days after the trigger.

Using the R-band magnitudes from the GCNs listed above, now also 
including Dimple et al., GCN #29488, we confirm the steeper decay found 
by Kann et al., GCN #29476, finding, with a host-galaxy magnitude as a 
free parameter:

alpha_1 = 1.06 +/- 0.18,
alpha_1 = 2.50 +/- 0.90,
t_b = 0.45 +/- 0.23 days,
m_h = 23.19 +/- 0.27 mag (AB).

This fit can be improved with a dedicated host-galaxy observation. We 
note that the X-ray light curve also shows evidence for a possible jet 
break at 0.35 +0.27 -0.13 days, in agreement with our result.