TITLE: GCN CIRCULAR NUMBER: 42054 SUBJECT: GRB 251001B: COLIBRÍ optical observations DATE: 25/10/02 10:03:11 GMT FROM: Benjamin Schneider at MIT bschn@mit.edu
Benjamin Schneider (LAM), Camila Angulo (UNAM), Rosa L. Becerra (U Roma), Antonio de Ugarte Postigo (LAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (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), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Fredd Sánchez Álvarez (UNAM):
We imaged the field of the GRB 251001B (Fermi GBM team, GCN Circ. 42038; Beardmore et al. GCN Circ. 42039) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-02T06:13:31 to 08:49:34 UTC (from 15.94 to 18.54 hours after the trigger) and obtained 64 minutes of simultaneous exposure in the r and z filters.
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 detected the optical counterpart previously reported by Gompertz et al. GCN Circ. 42040; An et al. GCN Circ. 42041; Strausbaugh & Cucchiara GCN Circ. 42043; O'Neill et al. GCN Circ. 42046; A. de Ugarte Postigo et al., GCN Circ. 42050; Ma et al., GCN Circ. 42051 and Sanchez-Ramirez et al., GCN 42053 at preliminary magnitudes of:
r = 21.56 +/- 0.06 z = 21.28 +/- 0.13
Our measured r-band magnitude is consistent with the magnitude reported by the NOT (Ugarte Postigo et al., GCN Circ. 42050), indicating a possible flattening in the optical afterglow.
Further observations and analysis are ongoing in g, r, i, z and y filters.
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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