GCN Circular 41348
Subject
GRB 250812A: VLT X-shooter spectroscopic redshift confirmation z = 2.571
Event
Date
2025-08-13T18:02:09Z (a year ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
A. de Ugarte Postigo (LAM), B. Schneider (LAM), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), J. An (NAOC), E. Le Floc’h (CEA), A. Martin-Carrillo (UCD), M. De Pasquale (Univ. of Messina) report on behalf of the Stargate collaboration:
We observed the optical afterglow (He et al., GCN 41324; Xin et al., GCN 41326; Rakotondrainibe et al., GCN 41328; Freeberg et al., GCN 41331; Siegel et al., GCN 41344; Santos et al., GCN 41346; van Dalen et al., GCN 41347) of GRB 250812A detected by SVOM/ECLAIRs (Xin et al., GCN 41322) with ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
In a 30 s image taken with the acquisition camera on Aug 13 at 09:25 UT (30.65 hr after the ECLAIRs trigger), the optical afterglow is clearly detected for which we measure a preliminary magnitude r = 19.5 +/- 0.2 AB (calibrated against 1 star from SkyMapper catalog).
Our spectra cover the wavelength range 3000-21000 AA, and consist of 2 exposures of 600 s each. Observations started on 2025-08-13 at approximately 09:26 UT (30.7 hr after the SVOM trigger). In a preliminary reduction of the spectra, we detect a continuum over the entire covered wavelength range. From the detection of a broad trough as well as numerous narrow absorption lines, which we interpret as due to Lya, NV, Si II, Si II*, O I, C II, C II*, Si IV, Si II, C IV, Al II, Al III, Fe II, and Mg II, we infer a redshift of z = 2.571, that we securely identify as the redshift of GRB 250812A confirming, and refining, the result from van Dalen et al. (GCN 41347).
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Akke Corporaal, Rob van Holstein and Rodrigo Romero. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).