TITLE: GCN CIRCULAR NUMBER: 36574 SUBJECT: GRB 250429A: Redshift from GTC/OSIRIS+ DATE: 24/05/30 11:30:32 GMT FROM: Antonio de Ugarte Postigo at OCA deugarte@oca.eu
A. de Ugarte Postigo (CNRS, OCA/LAM), C. C. Thoene (ASU-CAS), J. F. Agui Fernandez (CAHA), D. B. Malesani (DAWN/NBI and Radboud U.), Rakotondrainibe (LAM), N. R. Tanvir (U. Leicester), J. P. U. Fynbo (DAWN/NBI), L. Izzo (INAF-OACn & DARK/NBI), G. Lombardi (GTC), S. Geier (GTC), A. Cabrera Lavers (GTC), F. Perez-Toledo (GTC), A. Tejero (GTC) report:
We obtained spectroscopy of the afterglow of GRB 240529A (Eyles-Ferris et al. GCN 36556; Kumar et al. GCN 36559; Joshi et al. GCN 36560; Fu et al. GCN 36561; Shilling et al. GCN 36562; Perez-Garcia et al. GCN 36563; Dutton et al. GCN 36568; Mo et al. GCN 36569; Odeh et al. GCN 36573) using OSIRIS+ at the 10.4 m GTC telescope in the Roque de los Muchachos Observatory (La Palma, Spain). The observation consisted of 3x900s exposures using grism R1000B, covering the range between 3600 and 7800 AA at a resolving power of around 600. The observation had a mean epoch 2024-05-30T04:05:55 UT (1.0468 days after the burst onset). At the time of the afterglow acquisition we measure a magnitude of r(AB) = 19.59 +/- 0.05 mag, as compared to PanSTARRS field stars.
In spite of the late observation, the spectrum still shows a bright continuum with many strong spectral features, which we identify as due to at least three absorption systems. The highest redshift one, which we identify as the one corresponding to the redshift of the GRB is at z=2.695. For this system we identify features of HI, SII, SiII SiII*,SiIV, OI, CI, CII, CIV, FeII, AlII and AlIII.
Additionally we detect two strong intervening systems. The highest redshift one has the most prominent features, corresponding to CIV, AlII, AlIII, and FeII at a redshift of z=2.035. The second system, at z=1.695 has features of CIV, AlIII, FeII, MgI, MgII.
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