GCN Circular 42036
Subject
GRB 250910A / EP250910a: VLT limits on the possible associated supernova
Event
Date
2025-10-01T12:16:53Z (a day ago)
From
Ben Rayson at University of Leicester <br155@leicester.ac.uk>
Via
Web form
N. Habeeb (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), B. Rayson (U. Leicester), N. R. Tanvir (U. Leicester), report on behalf of the Stargate collaboration:
We obtained late-time imaging of the GRB 250910A / EP250910a field (Schneider et al., GCN 41794; Li et al., GCN 41798; see also Ma et al., GCN 41769; Liu et al., GCN 41776; Hubert et al., GCN 41842) at z = 0.592 (Saccardi et al., GCN 42031) with the ESO VLT equipped with FORS2. Observations were carried out on 2025 Oct 1.16 UT, i.e. ~21 days after the trigger, in the z and R bands.
At the position of the reported afterglow/NIR counterpart we detect a source in both filters. From photometry we measure z = 23.19 ± 0.11 (AB) and r = 23.97 ± 0.10 (AB) calibrated against nearby SkyMapper stars and not corrected for Galactic extinction (which is small, A_V = 0.05 mag). We note that a source is clearly visible at this location in the Legacy Survey images with magnitudes of z = 23.10 ± 0.20 and r = 23.73 ± 0.10 (both in AB).
Within the present uncertainties and allowing for filter differences, the fluxes are consistent with the pre-existing host-galaxy brightness. Using SN1998bw as the canonical GRB-SN template, the expected associated SN would have a magnitude of z ~ 22.8 at this epoch, implying a total host+SN flux of z ~ 22.2, which is not observed in this case. We therefore find no evidence for an optically bright SN associated with GRB 250910A / EP250910a at ~21 days post-burst (~13 days in the rest frame). The lack of an additional component on top of the host galaxy suggests that the expected SN emission would need to have approximately z < 24.1, corresponding to a SN ~3x fainter than SN1998bw. Without additional epochs for accurate template subtraction, at this moment, we cannot confirm if GRB 250910A / EP250910a truly lacks an associated SN component or if it simply suffers from some extinction.
We acknowledge excellent support from the ESO staff in Paranal, in particular Abel de Burgo Sierra and Cecilia Bustos.