GCN 44105: EP260321a: VLT/FORS2 spectroscopy confirmation of an associated type Ic-BL supernova SN 2026gzf

G. Corcoran (UCD), A. Martin-Carrillo (UCD), L. Izzo (INAF/OACN), A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), M. De Pasquale (U. of Messina), N. R. Tanvir (U. Leicester), R. A. J. Eyles-Ferris (U. Leicester), A.J. Levan (Radboud) report on behalf of the Stargate collaboration:

We observed the optical counterpart (Tanvir et al., GCN 44082; see also Lee et al., GCN 44070; Aryan et al., GCN 44081; Ahumada et al., GCN 44084; Liu et al., GCN 44087, Sankar et al., GCN 44089, de Ugarte Postigo et al., GCN 44091, Xu et al., GCN 44092, Pankov et al., GCN 44103), of EP260321a (Huang et al., GCNs 44068, 44075) with the FORS2 spectrograph on the ESO VLT UT1 (Antu) beginning on 2026-03-24 at 00:52 UT (2.515 days post-trigger). Our observations consisted of 3x300 s exposures in both the 300V and 300I grisms, spanning overall the wavelength range 3300-11000 AA.

From the 30 s R-band acquisition image we measure a magnitude of r = 18.31 +/- 0.09 mag (AB) calibrated using nearby stars from the Pan-STARRS catalogue, not corrected for Galactic extinction and not corrected by the host galaxy contribution at that position.

The acquired spectrum shows a good match to the spectrum of GRB-SN 2006aj (at 5 days pre-peak) using SNID SAGE (Stoppa in prep., https://github.com/FiorenSt/SNID-SAGE). Based on this similarity, we identify the supernova associated with EP260321a as a broad-lined type-Ic SN similar to other SNe accompanying GRBs and FXTs. This object is now named SN 2026gzf.

A figure comparing our spectrum (blue line) to SN 2006aj (red line) is available at the following link: https://shorturl.at/DpROp.

Further analyses and observations are ongoing.

We acknowledge the excellent expert support from the observing staff in Paranal, in particular Linda Schmidtobreick, Alonso Luna Ruiz Fernandez, Lorena Faundez, and Marcela Espinoza.