TITLE: GCN CIRCULAR NUMBER: 39643 SUBJECT: AT 2025dws, likely optical counterpart of GRB 250309B: LCO confirmation of fast fading DATE: 25/03/10 01:28:25 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias ipf@iac.es
I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, I. Correa-Plasencia, and A.E. Hernández-Díaz (ULL)
We report Las Cumbres Observatory Global Telescope network (LCOGT) observations of ZTF25aaitvjt / AT 2025dws, that has been proposed by Stein et al. (GCN circ. 39639) as the candidate optical counterpart of the Fermi GBM likely long Gamma-Ray Burst GRB 250309B (Preis & Greiner, GCN circ. 39629; Fermi GBM team, GCN circ. 39635; and McDermott et al., GCN circ. 39642), that may be related with the IceCube high-energy neutrino IceCube-250309A (The IceCube Collaboration, GCN circ. 39631).
We observed the field of ZTF25aaitvjt / AT 2025dws with one of the LCOGT 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at Sutherland Observatory (South Africa). We obtained a 180-sec exposure in each of the SDSS g', r', and i' filters starting at about 21.77 hours after the Fermi trigger. We detect ZTF25aaitvjt / AT 2025dws in the three filters.
We measure the following magnitudes, calibrated against Pan-STARRS DR2 stars and not corrected for Galactic extinction:
Date | UT start | t_mid - t0 (hours) | mag | error | filter | ---------------------------------------------------------------------- 2025-03-09 22:28:06 21.79 20.38 0.15 g' 2025-03-09 22:31:46 21.85 20.12 0.14 r' 2025-03-09 22:35:16 21.91 19.87 0.14 i'
The fast fading in the g' and r' bands compared with the Zwicky Transient Facility (ZTF) detections reported by Stein et al. (GCN circ. 39639 and TNS Astronomical Transient Report No. 247224) supports that ZTF25aaitvjt / AT 2025dws is the likely optical afterglow of GRB 250309B.
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCOGT observing programme IAC2025A-009, SGLF).
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