GCN Circular 41160
Subject
GRB 250725A: VLT/FORS2 spectroscopic redshift z = 5.26
Event
Date
2025-07-25T10:24:26Z (5 days ago)
From
Nusrin Habeeb at University of Leicester <nh312@leicester.ac.uk>
Via
Web form
A. L. Thakur (INAF-IAPS), N. R. Tanvir (Univ. of Leicester), A. de Ugarte Postigo (LAM), A. Saccardi (CEA/Irfu), L. Izzo (INAF/OACn and DARK/NBI), A. Kumar (RHUL), S. D. Vergani (LUX-Paris Obs.) report on behalf of the Stargate collaboration:
We observed the optical counterpart (Buckley et al. GCN 41151; Li et al. GCN 41158; Perez-Garcia et al. GCN 41159) of GRB 250725A (Fermi GBM Team, GCN 41150; Evans et al. GCN 41152) using the ESO/VLT UT1 (Antu) equipped with the FORS2 spectrograph.
In a 60 s acquisition image obtained in I-band starting on 2025 July 25 at 07:44:31 UT (5.88 hrs after the burst), the afterglow is well detected with I(AB) = 20.5+/-0.1 mag, calibrated against nearby stars from the SkyMapper catalog.
We obtained a single exposure of 1800 s using grism 300I+11 starting on 2025 July 25 at 08:04:47 UT (6.22 hrs after the burst). In a preliminary reduction of the spectrum, a trough due to Lyman-alpha is visible at 7610AA. From the detection of metals absorption features, which we interpret as due to O I and Si II, plus a tentative Si II* fine structure line, we infer a redshift of z = 5.26. At that same redshift we also detect an absorption feature consistent with Lyman-beta from the host galaxy, within the Lyman forest.
We acknowledge the excellent support from the ESO observing staff in Paranal, in particular Carlos La Fuente and Lorena Faundez.