A. Saccardi (GEPI/Paris Obs.), G. Leloudas (DTU Space), A. J. Levan (Radboud Univ. and Warwick Univ.), L. Izzo (INAF Napoli and DARK/NBI), A. de Ugarte Postigo (OCA), B. P. Gompertz (Birmingham), P. Jonker (Radboud Univ.), J. Palmerio (GEPI/Paris Obs.), E. Pian (INAF/OAS), G. Pugliese (Amsterdam), K. Wiersema (Univ. Herfordshire), R. A. M. J. Wijers (Amsterdam), report on behalf of a larger collaboration:
Following the discovery of GOTO23baj/AT2023vuc (Gompertz et al. 2023, GCN 34878; Iglesias-López et al., GCN 34881) within the error box of GRB 231024A (Fermi GBM Team, GCN 34876), we obtained a spectrum of the optical counterpart candidate with X-shooter. Observations started at 07:58 UT on 2023 Oct 25.
The spectrum is dominated by emission lines, that can be interpreted as strong He II 4686 (with a blue shoulder possibly associated with NIII / CIII) together with H-alpha and H-beta on top of a blue continuum, at a common redshift z = 0.056. The spectrum is consistent with those of very young supernovae that demonstrate flash-ionisation features a few hours to days after shock breakout (e.g. Gal-Yam et al. 2014, Nature, 509, 471). This is consistent with the non detection of the transient by ATLAS just 8 hours prior to the GOTO discovery. A comparison with SNID (Blondin & Tonry 2007, ApJ, 666, 1024) shows that the best matching template is that of SN 1998S at 12 days before maximum. Based on photometry reported by Iglesias-López et al. (GCN 34881), the transient absolute magnitude is around M_r = -18.8 (AB).
Considering the large GRB uncertainty region (12.6 deg radius) and its short duration, GOTO23baj/AT2023vuc is likely unrelated to GRB 231024A. However, this is a very young supernova and further observations are encouraged.
We acknowledge expert support from the ESO observing staff in Paranal, in particular Jonathan Smoker, Matias Jones and Felipe Gaete.