TITLE:   GCN CIRCULAR
NUMBER:  40068
SUBJECT: GRB 250403A: Montarrenti Observatory optical detection
DATE:    25/04/04 21:35:28 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 250403A detected by Fermi/GBM (Fermi team, GCN 40025),  SVOM/ECLAIRs (Julakanti et al., GCN 40026) and SWIFT (Dichiara et al., GCN 40037; Shilling et al., GCN 40048) with the automated and remoted 0.53m Ritchey-Chretien telescope at Montarrenti Observatory (Siena, Italy, IAU code C88).
  
The observations were started at 2025-04-03 21:56:50 UT (approximately 7 hours after burst) stacking a set of  Rc-band CCD images. The elevation of the field decreased from 19 degrees above the horizon.

The optical afterglow (Starling et al., GCN 40028; Wu et al., GCN 40029; Du et al., GCN 40031; Li et al., GCN 40032; Julakanti et al., GCN 40034; Ghosh et al, GCN 40039;  Jiang et al., GCN 40041; Perez-Fournon et al., GCN 40043; Brivio et al., GCN 40044; Xin et al, GCN 40045 and Zheng et al., GCN 40065) was barely detected (S/N=2.4) at the following position:

RA    (J2000.0)  12h 58m 27.72s  +/-0.15 
Decl. (J2000.0) -24° 22' 46.5"   +/-0.19

Preliminary photometry was obtained using nearby PanSTARRS stars as follows: 

Observation  Mid-Time     T-T0 (hr)       Exposure        Filter      Mag.         Err.        UL (3-sigma)
2025-04-03  21:57:10 UT     6.68           94x40s           Rc       20.10       S/N =2.4        >19.90

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