TITLE:   GCN CIRCULAR
NUMBER:  33537
SUBJECT: GRB 230328B: OHP/T120 optical observations
DATE:    23/03/29 14:52:49 GMT
FROM:    Benjamin Schneider at MIT  <bschn@mit.edu>

C. Adami (LAM), B. Schneider (MIT), M. Birlan, T. Hromakina 
(Obs. de Paris), S. Basa (LAM), E. Le Floc'h, D. Turpin, D. Götz 
(CEA Paris-Saclay), S. D. Vergani (GEPI, Obs. de Paris), report 
on behalf of a larger collaboration:

We observed the field of GRB 230328B (Gropp et al., GCN 33527; 
Osborne et al., GCN 33529; Dalessi et al., GCN 33531; Waratkar et al.
GCN 33532) using the T120cm telescope at Observatoire de Haute-Provence 
(France). Six exposures were obtained in the Rc band (2x300s + 4x600s) 
from 28 March 2023 18:51:01 UT to 28 March 2023 20:13:46 UT (mid time 
~4.5h after trigger). In the combined frame, we clearly detect the source 
and obtain the following magnitude:

Rc = 20.03 +/- 0.10 mag (AB)

The photometric calibration was performed using nearby stars from 
the PanSTARRS catalog converted in Rc and the magnitude is not 
corrected for Galactic extinction.

The value is consistent with early observations obtained by Pankov et al. 
(GCN 33528), Belkin et al. (GCN 33530) and Lu et al. (GCN 33534), and the 
upper limit reported by Catapano & Izzo (GCN 33535) at a similar epoch.

We acknowledge the excellent support from Observatoire de Haute Provence.