TITLE:   GCN CIRCULAR
NUMBER:  41997
SUBJECT: GRB 250925A: 1.5m OSN optical detection and redshift with the 10.4m GTC/OSIRIS+
DATE:    25/09/26 09:41:57 GMT
FROM:    Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>

A. J. Castro-Tirado, A. Sota, R. Sanchez-Ramirez, S. Guziy, M.D. Caballero-Garcia, S.-Y. Wu and I. Perez-Garcia (IAA-CSIC), D. Garcia-Alvarez (GTC, IAC), A. Perez-Romero (GTC), A. Cabrera-Lavers (IAC, GTC), S. B. Pandey (ARIES), M. Gritsevich (Univ. of Helsinki), Y.-D. Hu (GXI), A. Nicuesa Guelbenzu (Tautenburg Obs.), L. Piro (INAF/IAPS) and B.-B. Zhang (NJU), on behalf of a larger collaboration, report:

Following the detection of GRB 250925A by Swift (Dichiara et al. GCNC 41985) we conducted optical follow-up observations with the 1.5m OSN at Observatorio de Sierra Nevada (Granada), starting on Sep 25 at 21:47 UT. At the position of the X-ray afterglow (Page et al. GCNC 41988), we detect the optical afterglow (with I = 19.7 +/- 0.1) previously discovered at the NOT (Corcoran et al. (GCNC 41990)  and also reported by means of other ground-based telescopes (Bochenek et al. GCNC 41991, Masi et al. GCNC 41992, Moreno Mendez et al. GCNC 41995).

Additional observations were conducted with the 10.4m GTC telescope, at the Spanish Roque de los Muchachos Observatory, on the island of La Palma, equipped with the OSIRIS+ instrument. The spectroscopic observations consisted of 900s exposures using grisms R1000B and R2500I, with a joint spectral coverage between 3,600 and 10,000 A. The observations started on Sep 25, 21:57 UT (i.e. 4.9 h after the burst trigger).

We detect a strong DLA at ~6000 A, among a plethora of metal absorption lines, that we interpret as coming from NV, SiII, SiII*, OI, CII, SiIV, CIV, FeII, FeII*, AlII, AlII, NiII, MgII, MgI at z=3.899, consistent with the COLIBRI photometry as suggested (Moreno Mendez et al. GCNC 41995). The detection of fine structure features confirms the association of this absorption system to GRB 250925A.