GCN Circular 44124
Subject
GRB 260310A: OSIRIS+/GTC spectroscopic detection of the associated BL-Ic, SN 2026fgk
Event
Date
2026-03-27T07:39:02Z (2 days ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
A. de Ugarte Postigo (LAM), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), D. González González (GTC) report:
We observed the optical counterpart (AT2026fgk, a.k.a. GOTO26buh, O’Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument.
A total of 3 spectra of 900 s were secured, starting on 2026-03-27 at 04:25:40 UT (16.98 days after trigger), using grism R1000B. Continuum is visible over the full range 3610-7880 AA. The spectrum has evolved from the power-law continuum reported in de Ugarte Postigo et al. (GCN 43984) and now shows undulations expected in a SN spectrum. Indeed, using SNID to classify the SN we get a best match to a type Ic-BL SN close to maximum light.
Our results allow us to conclusively identify the presence of a SN, linking the event to a massive stellar collapse.