TITLE:   GCN CIRCULAR
NUMBER:  40337
SUBJECT: GRB 250502A: Montarrenti Observatory optical detection 
DATE:    25/05/03 21:54:31 GMT
FROM:    Simone Leonini at Montarrenti Observatory (Siena, Italy) <s.leonini@iol.it>

S. Leonini, M. Conti, P. Rosi, L.M. Tinjaca Ramirez (Montarrenti Observatory, Siena, Italy, part of UAI/SSV-GRB section), M.G. Dainotti (National Astronomical Observatory of Japan), Y. Niino (Tokyo University, Institute of Astronomy) and K. Kalinowski (Aarhus University, Department of Physics and Astronomy) report:

we observed the field of  GRB 250502A detected by SVOM/ECLAIRs (Wang et al., GCN 40313) with the automated and remoted 0.53m Ritchey-Chretien telescope at Montarrenti Observatory (Siena, Italy, IAU code C88).
  
The observations were started at 2025-05-02 20:42:17 UT (approximately 12 hours after burst) stacking a set of  Rc-band CCD images. Observations were performed under thin cloud cover in the second part of the night.

The optical afterglow (Rakotondrainibe et al., GCN 40315; An et al., GCN 40319; Li et al., GCN 40320; Gosh et al., GCN 40322; de Ugarte Postigo et al.  with a redshift of  z = 2.163, GCN 40328 ; Perez-Fournon et al., GCN 40330; Odeh & Guessoum, GCN 40331;  Bochenek & Perley, GCN 40333; Corcoran et al., GCN 40334 and Dichiara et al., GCN 40336) was barely detected (S/N =2.1) at the following position:

RA    (J2000.0)  13h 46m 05.60s  +/-0.23 
Decl. (J2000.0) -10° 45' 31.70"  +/-0.16

Preliminary photometry was obtained using nearby PanSTARRS stars as follows: 

Observation  Mid-Time    T-T0 (hr)       Exposure        Filter      Mag.         Err.       
2025-05-02  21:31:19 UT   12.75           140x40s          Rc       20.70       +/- 0.15       

Magnitude was calibrated with the nearby PanSTARRS stars converted using Lupton (2005) equations. No correction for galactic dust extinction was applied.