GCN Circular 37950

J. F. Agui Fernandez (CAHA), A. de Ugarte Postigo (LAM/OCA, CNRS), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Martin-Carrillo (UCD), G. Lombardi (GTC), C. C. Thoene (ASU-CAS, AbAO), N. R. Tanvir (U. Leicester), L. Izzo (INAF-OACn & DARK/NBI), S. Geier (GTC), D. Perez Valladares (GTC) report:

We observed the afterglow of GRB 241029A (Fermi GBM team, GCN 37932
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; Schanne et al., GCN 37934
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; Chenwei Wang et al., GCN 37935
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; Evans et al., GCN 37936
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; Jiang et al., GCN 37937
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; Weikang Lin et al., GCN 37939
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; Jiang et al., GCN 37940
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; Francile et al., GCN 37941
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; Ackley et al., GCN 37943
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; Odeh et al., GCN 37944
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; Myers et al., GCN 37945
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; Bernardini et al., GCN 37946
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, Adami et al., GCN 37948
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) using OSIRIS+ on the 10.4 m GTC telescope, at the Roque de los Muchachos Observatory (La Palma, Spain). The observation consisted of 4 acquisition images in r-band, followed by a 4x1200 s spectrum. The spectrum was obtained with grism R1000B, covering the spectral range between 3650 and 7800 AA at a resolving power of 600.

The acquisition image, obtained at 2024-10-29T21:32:51 UT, 19.21 hr since the Fermi trigger, shows the afterglow at r(AB) = 21.40 +/- 0.04 mag, as compared to SDSS field stars. The spectrum has a strong continuum throughout the complete range with absorption features due Fe II, FeII*, Mn II, Mg II, Mg I and the [O II] doublet at z = 1.072, which we propose as the redshift of GRB 241029A.