GCN Circular 39647
Subject
GRB 250309B / AT2025dws: OSIRIS+/GTC spectroscopy z=1.898
Date
2025-03-10T05:47:31Z (5 days ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
D. B. Malesani (DAWN/NBI and Radboud Univ.), A. de Ugarte Postigo (LAM), 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), F. Perez Toledo (GTC), and A. Perez (GTC) report:
We observed the afterglow of GRB 250309B (Preis et al. GCN 39629; Fermi GBM team GCN 39635; Stein et al. GCN 39639; McDermott
et al. GCN 39642; Perez-Fournon et al. GCN 39643; Stain et al. GCN 39644; Moskvitin et al. GCN 39645; Perley et al. GCN 39646) with OSIRIS+, mounted on te 10.4.m GTC telescope at the Roque de los Muchachos Observatory in the island of La Palma (Spain). The observation started at 2025-03-10T03:28:33.011 UT (19.83 h after the burst) and consisted of 2x900 s spectroscopy with grism R1000B, covering the range between 3600 and 7880 AA.
A preliminary reduction shows a continuum across the complete spectral range with multiple spectral features which we interpret as due to SiII, SiIV, OI, CII, CIV, FeII, AlII, AlIII, ZnII, and MnII at a common redshift of 1.898, which we identify as the redshift of the GRB. Additionally, we detect a strong intervening system with features of CIV, AlII, FeII, MgII and MgI at a a redshift of 1.634.