TITLE: GCN CIRCULAR NUMBER: 42349 SUBJECT: GRB 251016A: VLT/FORS2 Optical Observation DATE: 25/10/18 09:53:25 GMT FROM: Roberto Ricci at INAF-IRA ricci@ira.inaf.it

Roberto Ricci (U Rome), Niccolo Passaleva ( U Rome) and Eleonora Troja (U Rome) report:

We observed the field of the short duration GRB 251016A detected by SVOM/ECLAIRs (Basa et al., GCN 42308), FERMI-GBM (Fermi GBM team, GCN 42309), CALET-GBM (Kawakubo et al., GCN 42321), Glowbug (Woolf et al., GCN 42313) and Astro Sat CZTI (Arya et al., GCN 42327) with the FORS2 imager on the ESO VLT UT1 (Antu). Observations started on Oct 16th, 2025 at 23:45 UT (~9 hours after the SVOM trigger) and were carried out in the I_BESS filter with an average airmass of ~1.3.

The GRB lies within a very crowded region at low Galactic latitude. Several optical sources are detected within the localization of Swift/XRT Source #1 (Evans et al., GCN 42311). Objects brighter than 24 AB mag have counterparts in the GAIA DR3 catalogue (Babusiaux et al., 2023), however several fainter (>24 AB mag) sources remain unmatched.

Further observations are planned to determine variability.

We thank the staff at the VLT for the rapid execution of these observations.

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