TITLE: GCN CIRCULAR NUMBER: 41289 SUBJECT: GRB 250807B: VLT/X-Shooter spectroscopic redshift z = 1.522 DATE: 25/08/08 14:30:18 GMT FROM: Ines Francesca Giudice ines.giudice@inaf.it
I. F. Giudice (INAF/OACn), M. Garnichey (LUX-Paris Obs.), A. L. Thakur (INAF-IAPS), A. Saccardi (CEA/Irfu), L. Izzo (INAF/OACn & DARK/NBI), A. Martin-Carrillo (UCD), M. De Pasquale (U.Messina), N. R. Tanvir (U. Leicester), S. D. Vergani (LUX-Paris Obs.), D. B. Malesani (DAWN/NBI and Radboud Univ.), report on behalf of the Stargate collaboration:
We observed the optical counterpart (Kingler et al., GCN 41268; van Dalen et al., GCN 41278) of GRB 250807B detected by Swift with ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 A, and consist of 4 exposures of 600 s each. Observation started on 2025-08-08 at 08:42:42 UTC (17 hours after trigger).
In a preliminary reduction of the spectra, we identify several absorption features, which we interpret as Fe II, the Mg II doublet, and Mg I, from which we derive a redshift of z = 1.522. We notice the absence of emission or fine-structure lines. The presence of a continuum in the UVB arm supports the interpretation that this could be the GRB redshift.
The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).
We acknowledge excellent support from the ESO observing staff in Paranal.
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