GCN Circular 39160
Subject
GRB 250205A: Redshift from OSIRIS+/GTC z = 3.55
Event
Date
2025-02-06T00:28:38Z (a year ago)
From
Antonio de Ugarte Postigo at LAM/OCA, CNRS <deugarte@oca.eu>
Via
email
A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), J. F. Agui Fernandez (CAHA), N. R. Tanvir (Univ. Leicester), L. Izzo (INAF/OACn and DARK/NBI), J. P. U. Fynbo (DAWN/NBI), S. Geier (GTC), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), A. M. Garcia Rodriguez (GTC), D. González González (GTC) report,
We have observed the counterpart of GRB 250205A (Saccardi et al. GCN 39154, also detected by EP as trigger id 01709131283), also discovered in parallel by Gompertz et al. (GCN 39156) and Schneider et al. (GCN 39157) and detected by SVOM/VT (Palmerio et al. GCN 39159), with OSIRIS+ on the 10.4 m GTC, located at the Roque de los Muchachos Observatory in the island of La Palma (Spain). Our observation consisted of a 30 s acquisition in r-band followed by 4x900s of spectroscopy using grism R1000B, which covers the spectral range between 3600 and 7800 AA.
The optical counterpart is well detected in the acquisition obtained at 23:10:36 UT (mean epoch 1.7661 hrs after the burst) with an r-band AB magnitude of 21.78 +/- 0.06 mag, as compared with 4 field stars from the Sloan catalogue.
The spectra show a faint trace with clear absorption features. In a preliminary reduction, we identify features of Ly-alpha, SII, SiII, SiII*, OI, CII, SiIV, CIV, FeII, FeII* at a common redshift of 3.55, which we identify as the redshift of the GRB. Further analysis is ongoing.