TITLE: GCN CIRCULAR NUMBER: 42004 SUBJECT: GRB 250925A: COLIBRÍ photometric redshift DATE: 25/09/26 15:11:23 GMT FROM: Ny Avo Rakotondrainibe at LAM nyavo.rakotobe@gmail.com
N. A. Rakotondrainibe (LAM), Enrique Moreno Méndez (UNAM), Alan M. Watson (UNAM), Benjamin Schneider (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), 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), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM):
COLIBRÍ performed follow-up observations of the Swift GRB 250925A (Dichiara et al., GCN Circ. 41985, Moreno Méndez et al., GCN Circ. 41995) from 2025-09-26 04:47 to 06:46 UTC (from 11.7 to 13.7 hours after the trigger) and obtained 16/16 minutes of exposure in the g/y filters. We report a detection in the y-filter, with a magnitude of y = 20.64 +/- 0.13 mag and a non-detection of the burst in the g-filter with a 3-sigma upper limit of g > 23.75 mag. The photometry was performed using STDWeb/STDPipe (Karpov 2025), was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
After correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.212 mag (Schlafly & Finkbeiner 2011) and fitting a power-law model to the grizy-bands using the LMC extinction curve, we derive a photometric redshift of z = 4.20+0.34-0.52 (1-sigma c.l.), according to the g- and r-band dropouts with g-r > 0.68 mag and r-i ~ 1.45 mag. Our result is consistent with the GTC/OSIRIS+ redshift of z=3.899 (Castro-Tirado et al., GCN Circular 41997). 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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