TITLE: GCN CIRCULAR NUMBER: 45004 SUBJECT: GRB 260610B: VLT/FORS2 spectroscopic observation suggests the non-thermal nature of the optical rebrightening DATE: 26/06/19 14:42:59 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie> G. Corcoran (UCD), A. Martin-Carrillo (UCD), L. Izzo (INAF/OAC and DARK/NBI), J. An (NAOC), D. B. Malesani (DAWN/NBI and Radboud), B. P. Gompertz (Birmingham), A. J. Levan (Radboud and Warwick), A. de Ugarte Postigo (LAM), report on behalf of the Stargate collaboration: We performed spectroscopic observations of GRB 260610B discovered by Fermi (Fermi GBM team, GCN 44901; Godwin & Meegan GCN 44931) and SVOM (Yu et al., GCN 44944), whose optical afterglow has recently experienced an optical rebrightening (O’Neill et al., GCNs 44903, 44914; Watson et al., GCN 44905; Zhu et al., GCN 44909; Gillanders et al., GCNs 44910, 44955; Akl et al., GCNs 44911, 44930; Moskvitin et al., GCNs 44918, 44929; Jackson-Horne et al., GCN 44919; Izzo et al., GCN 44920; Li et al., GCN 44921; Bochenek et al., GCNs 44927, 44932; Pankov et al., GCN 44946; Angulo et al., GCN 44958; Moskvitin et al., GCN 44963; Pankov et al., GCN 44977; Gassert et al., GCN 44981; Gupta et al., GCN 44993). We used the ESO Very Large Telescope (VLT) UT1 (Antu) equipped with the FORS2 spectrograph with the 300V grism, covering the wavelength range 4400-8650 AA. The observation consisted of 4 exposures of 600 s each. The observation started at 00:41:19 UT on June 17 2026 (6.04 days after the GRB). In our acquisition image, we measured an AB magnitude r ~ 20 (calibrated against Pan-STARRS, not corrected for Galactic extinction). Based on the reported light curve, this seems to indicate that our observation occurred during the peak of this late flaring episode. The spectrum of the optical counterpart reveals a power-law continuum across the entire covered range suggesting that the observed recent activity is likely caused by late jet activity. Faint undulations are seen overimposed to the continuum, at a flux level consistent with the emergence of an early, still faint SN, but further observations will be required to conclude about its presence. Additionally, through the detection of the Ca II doublet in absorption and [O II] doublet in emission, we confirm the redshift reported by O’Neill et al. (GCN 44914), measuring z = 0.474. Further spectroscopic observations of the source are planned to monitor the possible emergence of the supernova. We acknowledge excellent support from the ESO observing staff in Paranal, in particular Martina Baratella and Leo Rivas, for rapidly executing these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/45004. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...