TITLE: GCN CIRCULAR NUMBER: 40328 SUBJECT: GRB 250502A: Redshift from OSIRIS+/GTC z = 2.163 DATE: 25/05/03 02:52:49 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS adeugartepostigo@gmail.com

A. de Ugarte Postigo (LAM), J. F. Agui Fernandez (CAHA), C. C. Thoene (AbAO), B. Schneider (LAM), N. A. Rakotondrainibe (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), 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. R. Tanvir (Univ. Leicester), A. Tejero (GTC) and D. Garcia-Alvarez (GTC) report

We observed the afterglow of the SVOM GRB 250502A (Wang et al. GCN 40313; Rakotondrainibe et al. GCN 40315; An et al. GCN 40319; Li et al. GCN 40320; Ghosh et al. GCN 40322) using OSIRIS+ on the 10.4m GTC telescope, at Roque de los Muchachos Observatory (La Palma, Spain). The observation started at 2025-05-03T00:34:49 UT (15.806 hrs after the burst) and consisted of acquisition imaging in r-band followed by 3x1200s exposures of spectroscopy using grism R1000B, covering the range between 3600 and 7800 AA at a resolving power of ~ 600.

In the acquisition image the afterglow is detected at an AB magnitude of r = 21.00 +/- 0.03, as compared to PanSTARRS field stars.

The spectrum shows a clear continuum over the complete spectral range with multiple absorption features that we identify as Ly-alpha, SII, OI, SiII, SiII*, CII, SiIV, CIV, FeII, FeII*, AlII, AlIII at a common redshift of z = 2.163, which we identify as the redshift of the GRB. We also note the presence of a prominent emission of Lyman alpha.

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