TITLE: GCN CIRCULAR NUMBER: 40175 SUBJECT: GRB 250407A: GTC/OSIRIS+ spectroscopic redshift z = 1.36 DATE: 25/04/19 13:09:50 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University d.malesani@astro.ru.nl
B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. de Ugarte Postigo (LAM), A. Martin-Carrillo (UCD), J. F. Agui Fernandez (CAHA), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), N. R. Tanvir (Univ. Leicester), C. C. Thoene (AbAO), A. Perez (GTC) and R. Scarpa (GTC), report:
We observed the optical afterglow of the GRB 250407A (Fermi GBM team, GCN 40104; Preis & Greiner, GCN 40105; Mukherjee & Meegan, GCN 40114; Molkov et al., GCN 40110; Dichiara et al., GCN 40113; Ducoin et al., GCN 40117; Zheng et al., GCN 40119; Wang et al., GCN 40120; Frederiks et al., GCN 40121; Williams et al., GCN 40123; Pérez-Fournon et al., GCN 40125; Kozyrev et al., GCN 40127; Pankov et al., GCN 40133; deGraw et al., GCN 40135) using the 10.4 m GTC located at the Roque de los Muchachos Observatory in the island of La Palma (Spain) equipped with the OSIRIS+ instrument.
In a 60 s acquisition image in the r-band, starting on 2025-04-17 at 21:27:11 UT (10.24 after the Fermi trigger), an object is detected at the position of the optical afterglow (Zheng & Filippenko, GCN 40116), with a magnitude r = 22.48 +/- 0.06 (AB, calibrated against nearby Pan-STARRS objects). At a consistent location, the Legacy Survey also reveals a source, the likely GRB host galaxy, with a somehow fainter magnitude r = 22.65. In our observation the target flux is thus likely dominated by the host, possibly with some transient contribution.
Our spectroscopic observations started on 2025-04-17 at 21:32:03 UT (10.24 days after the Fermi trigger) and consisted of 3x900 s exposures with grism R1000R, covering the range between 5100 and 10,000 AA.
In a preliminary reduction of the spectra, we clearly detect a continuum over the entire covered range. From the detection of multiple absorption features, including Fe II, Mg II, and Mg I, we infer a redshift of z = 1.36. At a consistent redshift, an emission line is also detected, which we interpret as the [O II] doublet from the host galaxy.
We thus conclude that GRB 250407A is at redshift z = 1.36.
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