TITLE: GCN CIRCULAR NUMBER: 41363 SUBJECT: GRB 250813B: VLT X-shooter spectroscopic redshift z = 1.752 DATE: 25/08/14 10:29:14 GMT FROM: Gregory Corcoran at University College Dublin gregory.corcoran@ucdconnect.ie
B. Schneider (LAM), G. Corcoran (UCD), E. Le Floc’h (CEA), A. de Ugarte Postigo (LAM), J. An (NAOC), A. L. Thakur (INAF-IAPS), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration:
We observed the optical afterglow (Jelinek et al., GCN 41355; de Ugarte Postigo et al., GCN 41356) of GRB 250813B detected by SVOM/ECLAIRs (Xie et al., GCN 41352; Xie et al., GCN 42354) with ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
In images taken with the acquisition camera on Aug 14 05:40 UT (6.82 hr after the ECLAIRs trigger), the optical afterglow is clearly detected in g, r, and z bands. We measured a preliminary magnitude r = 22.24 +/- 0.06 AB (calibrated against the Pan-STARRS PS1 catalog).
Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600 s each. Observations started on 2025-08-14 at approximately 05:50 UT (6.98 hr after the SVOM trigger). In a preliminary reduction of the spectra, we detect a faint continuum down to ~3590 AA at the blue end. From the detection of a multitude of absorption lines, which we interpret as due to C II, Si IV, Si II, C IV, C I, Al II, Al III, Mn I, Fe II, Fe II*, Mn II, Mg II, Mg I, Ca II, we infer a redshift of z = 1.752.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Celia Desgrange, Rodrigo Romero, and Akke Corporaal. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).
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