TITLE: GCN CIRCULAR NUMBER: 42472 SUBJECT: GRB 251025B: VLT/MUSE spectroscopic redshift z = 2.003 DATE: 25/10/27 11:30:46 GMT FROM: Andrea Saccardi at CEA/Irfu andrea.saccardi@cea.fr
A. Saccardi (CEA/Irfu), L. Izzo (INAF/OACn & DARK/NBI), P. Schady (University of Bath), D. B. Malesani (DAWN/NBI and Radboud), J. An (NAOC), A. A. Chrimes (ESA/ESTEC & Radboud Univ.), G. Corcoran (UCD), A. Martin-Carrillo (UCD), B. Schneider (LAM), N. R. Tanvir (U. Leicester), S. D. Vergani (LUX-Paris Obs.), report on behalf of the Stargate collaboration:
We observed the field of the SVOM/ECLAIRs GRB 251025B (Hussein et al., GCN 42437), using the ESO VLT UT4 (Yepun) equipped with the MUSE spectrograph. Our observation started at 03:27:23 UT on 2025 October 27 (1.54 days after the GRB trigger), and consisted of 4 exposures of 700 s each.
The optical counterpart (Wu et al., GCN 42438; Gress et al., GCN 42439; Wu et al., GCN 42440; Mohan et al., GCN 42447; Hernández Fung et al., GCN 42450; Pereyra et al., GCN 42452; Li et al., GCN 42453; Mo et al., GCN 42459; Rakotondrainibe et al., GCN 42460; Rajabov et al., GCN 42466) is well detected in the wavelength-stacked “white light” image. The afterglow has a synthetic magnitude Rc = 21.22 +/- 0.04 (AB).
Our spectra cover the wavelength range 4750 - 9330 AA. In a preliminary reduction, we detect a continuum over the entire covered wavelength range. From the detection of several absorption features that we identify as due to Al II, Al III, Fe II, Mg II, and Mg I, we infer a common redshift of z = 2.003 for the GRB. Additionally, we note the presence of an intervening absorber at z = 0.73 with Mg II (and likely Mg I) absorption lines.
As first noted by Pereyra et al. (GCN 42452), an archival object is visible in the Legacy Survey underlying the optical afterglow (g = 24.02 +/- 0.15; r = 23.32 +/- 0.21; z = 22.18 +/- 0.16). If this is the GRB host, at z = 2.0 it would have an absolute magnitude of M(2000 Å) ~ -21.6 AB.
We acknowledge expert support from the observing staff in Paranal. 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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