TITLE:   GCN CIRCULAR
NUMBER:  33696
SUBJECT: GRB 230427A : OHP/T193 optical upper limit
DATE:    23/04/27 23:52:46 GMT
FROM:    Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>

D. Turpin (CEA Paris-Saclay), C. Adami (LAM), B. Schneider (MIT),
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 230427A (Troja et al., GCN 33687) 
using the T193cm telescope at Observatoire de Haute-Provence (France) 
equipped with the MISTRAL spectro-imager. 18 exposures were obtained 
in the r-band (16x300s + 2x60s) from 2023 27 April 20:36:47 UT to 
2023 27 April 22:59:06 UT (mid time ~11.27h after trigger). In the combined 
frame, we do not detect any source at the UVOT position reported by 
Troja et al., GCN 33687 and the enhanced XRT position (Beardmore et al.,
GCN 33688) down to the following 3-sigma limit:

r > 22.3 mag (AB)

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

We acknowledge the excellent support from Observatoire de Haute-Provence, 
in particular Jean Balcaen.