TITLE: GCN CIRCULAR NUMBER: 38809 SUBJECT: GRB 250103A: Redshift 4.01 from OSIRIS+/GTC DATE: 25/01/03 23:35:51 GMT FROM: Antonio de Ugarte Postigo at LAM/OCA, CNRS deugarte@oca.eu
A. de Ugarte Postigo (CNRS, OCA and LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), C. C. Thoene (ASU-CAS, 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), A. Martin-Carrillo (UCD), N. A. Rakotondrainibe (LAM), Riccardo Scarpa (GTC) and Alvaro Tejero (GTC), report:
We observed the optical counterpart of GRB 250103A (Wang et al. GCN 38786, Li et al. GCN 38795, Qiu et al. GCN 38802, Trigg et al. GCN 38806, Xu et al. GCN 38808) with OSIRIS+, mounted on the 10.4 m GTC, at Roque de los Muchachos Observatory, in the island of La Palma (Spain). The observation started at 2025-01-03T22:03:11.011 UT (12.11 hr after the burst) consisted of an r-band acquisition followed by a single 1200 s spectrum with grism R1000B, covering the range between 3600 and 7800 AA. Although the planned observation was longer, weather and technical issues prevented us from completing the planned observation.
In a preliminary reduction, the spectrum shows a red trace with strong features that we identify as being due to the Lyman-alpha forest, a broad damped Lyman-alpha feature, and strong lines of SiII, SiII*, SiIV, OI, CII, CIV, amongst others, at a common redshift of 4.01. Further analysis is ongoing.
The underlying galaxy mentioned by Xu et al. (GCN 38808) has AB magnitudes in the Legacy survey of g=24.55, r=23.16, i=22.77, z=22.52. We note that this photometry does show a break in the g-band, which would be consistent with the Lyman break at z = 4. However, if this object is indeed the host galaxy, it would have an extraordinary luminosity at z=4.01, with an absolute magnitude M(UV) ~ -23 mag.
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