TITLE:   GCN CIRCULAR
NUMBER:  39754
SUBJECT: GRB 250317B: SVOM/F-GFT (COLIBRÍ) Detection of a Fading Optical Counterpart
DATE:    25/03/17 06:31:31 GMT
FROM:    Sarah Antier at OCA <sarah.antier@oca.eu>

Alan M. Watson (UNAM),  Sarah Antier (OCA/IJCLAB), Jean-Luc Atteia (IRAP), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Antonio de Ugarte Postigo (LAM), Benjamin Schneider (LAM) and C. Wu (NAOC) report:

We imaged the field of the SVOM/ECLAIRs GRB 250317B (Zhao et al. GCN Circ. 39752, Zhao et el. GCN Circ. 39753) with the OGSE test camera imager on the COLIBRÍ (SVOM/F-GFT) telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.

We observed from 2025-03-17 04:21 to 04:56 UTC (2.15 to 2.73 hours after the trigger) and obtained 23 minutes of exposure in the r filter at high airmass. The data were coadded with custom software and analyzed in STDWeb/STDPipe (Karpov 2024), with photometric calibration against Pan-STARRS DR1 and image subtraction against Pan-STARRS DR2. Our photometry is in the AB system and is not corrected for Galactic extinction.

We detect an uncatalogued source at RA = 211.64367 degrees and Dec = 40.07953 degrees (J2000) with a magnitude of

r = 18.18 +/- 0.03

The source is observed to fade in later observations and is not present in Legacy Survey DR10, which strongly suggests it is the afterglow of the GRB.

This source is 81 arcsec from the MXT position reported by Zhao et al. (GCN Circ. 39752) and significantly beyond the reported 25 arcsec 90% confidence radius.

Further observations are planned.

We acknowledge excellent support from the staff of the Observatorio Astronómico Nacional.