TITLE: GCN CIRCULAR NUMBER: 39650 SUBJECT: GRB 250309B/AT2025dws : COLIBRÍ Detection of the Optical Counterpart DATE: 25/03/10 11:48:47 GMT FROM: F. Fortin at IRAP ffortin.sci.edu@gmail.com
Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Kin Ocelotl López (UNAM), Francesco Magnani (CPPM), Stéphane Basa (UAR Pytheas), Benjamin Schneider (LAM), Alan M. Watson (UNAM), William H. Lee (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Noémie Globus (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Antonio de Ugarte Postigo (LAM)
We observed the field of GRB 250309B (Preis et al., GCN 39629) using the OGSE engineering camera on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.
We observed from 2025-03-10T07:34:55 to 2025-03-10T08:46:13 UTC (23h 56min after the trigger) and obtained 3000 seconds of exposure in the r filter. The data were analyzed in STDWeb/STDPipe (Karpov 2021), 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.
In our stacked image, after the subtraction, we clearly detect a source at the position of the optical candidate AT2025dws (reported by Stein et al., GCN 39639, Pérez-Fournon et al. GCN 39643, Moskvitin et al. GCN 39645, Perley et al. GCN 39646) with magnitude:
r = 20.76 +/- 0.10
Further observations are planned to monitor the evolution of the transient.
We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.
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