TITLE: GCN CIRCULAR NUMBER: 42097 SUBJECT: GRB 251001B: COLIBRÍ further observations and evidence of a multi-wavelength shallow decay DATE: 25/10/03 17:59:09 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma rosa.becerra@roma2.infn.it
Rosa L. Becerra (U Roma), Benjamin Schneider (LAM), Camila Angulo (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), 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), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM):
We reobserved 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 2025-10-03 12:00 to 12:19 UTC (from 45.7 to 46.0 hours after the trigger) and obtained 16 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with custom software 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), de Ugarte Postigo et al. (GCN Circ. 42050), Ma et al. (GCN Circ. 42051), Sanchez-Ramirez et al. (GCN Circ. 42053), Schneider et al. (GCN Circ. 42054), and Breeveld et al. (GCN Circ. 42055) at preliminary magnitudes of:
r = 22.28 +/- 0.14.
z = 22.08 +/- 0.41 (with a 3-sigma detection limit of z = 21.85).
By comparing the optical data reported by the aforementioned facilities and COLIBRÍ, we estimate a spectral index of approximately -0.5. This shallow decay is consistent with the temporal evolution observed in the X-ray band after 10ks from the trigger (https://www.swift.ac.uk/xrt_curves/01400467/).
Further observations are planned.
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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