TITLE: GCN CIRCULAR NUMBER: 40577 SUBJECT: GRB 250530A: SVOM/COLIBRÍ (FM-GFT) optical counterpart candidate DATE: 25/05/30 08:26:50 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS adeugartepostigo@gmail.com
Antonio de Ugarte Postigo (LAM), Enrique Moreno Méndez (UNAM), Stéphane Basa (UAR Pytheas), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), and Y. Wang (PMO, CAS):
We imaged the field of the SVOM GRB 250530A (Wang et al., GCN Circ. 40576) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-05-30 06:40:04 to 06:41:59 UTC (from 489 s after the burst onset) and obtained 3x30 s of exposure in the i-band in our first stack of images.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
We detect an uncatalogued source (after the image subtraction using Legacy Survey/PanSTARRS as template and) consistent with the ECLAIRs error circle (Wang et al., GCN Circ. 40576) at:
RA(J2000) = 12:26:56.38 = 186.7349 degrees Dec(J2000) = -2:41:56.900 = -2.6991 degrees with an uncertainty of 0.5 arcsec.
The preliminary magnitude derived for that source is:
i = 19.49 +/- 0.07 mag
Compared with subsequents stacks, we observe the fading of the brightness in this candidate, suggesting that this is indeed the afterglow of the GRB.
Further analysis and observations are ongoing.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
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