TITLE: GCN CIRCULAR NUMBER: 40024 SUBJECT: GRB 250225B: VLT host galaxy redshift z = 0.950 DATE: 25/04/03 14:50:19 GMT FROM: Andrea Saccardi at CEA/Irfu andrea.saccardi@cea.fr
A. Saccardi (CEA/Irfu), G. Corcoran (UCD), B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), A. J. Levan (Radboud and Warwick) report on behalf of the Stargate collaboration:
We observed with the ESO VLT UT3 (Melipal) the object visible in the Legacy Survey underlying the position of the optical/NIR afterglow (Schneider et al., GCN 39494; Yang et al., GCN 39495, 39521; Rayson et al., GCN 39535; Yang et al., GCN 39544) of GRB 250225B (Williams et al., GCN 39473; Zhang et al., GCN 39493; Ridnaia et al., GCN 39498; Godwin et al., GCN 39502; Thakur et al., GCN 39517; Finneran et al., GCN 39541; Liu et al., GCN 39830). Due to its spatial coincidence, this is the likely GRB host galaxy.
We obtained two spectra of 1000 s each with the X-shooter instrument, starting at 07:08:05 UT on 2025 April 3, i.e. about 36.5 days after the Swift trigger. The observations were obtained under good conditions, with a measured seeing of 0.5".
Emission lines are detected in the visible and near-infrared arms, which we identify as H-alpha, [O III] 5007 and the [O II] 3726,3729 doublet at a common redshift of z = 0.950. This value is consistent with the photometric solution z_phot = 1.12 +/- 0.18 from the Legacy Survey (Zhou et al. 2021, doi:10.1093/mnras/staa3764). The absolute magnitude of the galaxy is M_B ~ -21, which is at the bright end of the distribution of long-duration (collapsar) GRB hosts. Our redshift measurement rules out an association between the object in the Legacy Survey and ESO 340-26 at z = 0.018. Considering the low chance association probability (Schneider et al., GCN 39494; Yang et al., GCN 39495), we consider this to be the likely redshift of GRB 250225B.
We acknowledge the excellent support from the ESO staff in Paranal, in particular Matias Jones, Rob van Holstein, and Rodrigo Palominos.
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