TITLE: GCN CIRCULAR NUMBER: 45283 SUBJECT: GRB 260708A / SN 2026stf: Spectroscopic host redshift z = 0.055 and confirmation of an associated type Ic-BL supernova DATE: 26/08/04 08:19:29 GMT FROM: Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie> G. Corcoran (UCD), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), L. P. Xin (NAOC) report on behalf of the Stargate collaboration: We observed the counterpart of GRB 260708A detected by Fermi-GBM, Fermi-LAT, Glowbug and GECAM (Fermi-GBM Team, GCN 45115; Holzmann Airasca et al., GCN 45117; Malacaria, GCN 45124; Ren et al., GCN 45126; Cheung et al., GCN 45130), AT 2026stf (Shilova et al., GCN 45123; Freeberg et al., GCN 45127; Shi et al., GCN 45140, Sheng et al., GCN 45147; Li et al., GCN 45150) with a possible emerging supernova (Shi et al., GCN 45280) using the FORS2 spectrograph with the 300V grism and the GG435 order blocker on the ESO VLT UT1 (Antu). Observations started at 23:13:36 UT on 2026-08-02 (~25.5 days after the GRB trigger). Our spectra cover the wavelength range ~4560-8690 AA and consist of 4 exposures of 600 s each. In an R-band acquisition image we measure a magnitude of R=19.12+/-0.09 (AB) calibrated using nearby stars in the SkyMapper catalog and not corrected for Galactic extinction. We note that this is likely contaminated with host light as the candidate host has an r-band magnitude of r=20.06+/-0.02 from the DECam Local Volume Exploration Survey (DELVE, Drlica-Wagner et al. 2021, doi:10.3847/1538-4365/ac079d). In a stack of the quicklook spectra provided on the ESO archive, we identify clear narrow emission features from the candidate host galaxy consistent with a redshift of z = 0.055. In addition, using SNID-SAGE (Stoppa et al. 2026, doi:10.1093/mnras/stag1066) we identify clear supernova features consistent with a broad-lined type Ic (Ic-BL) supernova post peak at a consistent redshift. A comparison figure of our spectrum (blue line) to a template Ic-BL SN is available at the following link: https://shorturl.at/rzxgB We acknowledge the excellent support from the observing staff in Paranal, in particular Elisa Rita Garro and Edouard Gonzalez. View this GCN Circular online at https://gcn.nasa.gov/circulars/45283. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...