TITLE:   GCN CIRCULAR
NUMBER:  34740
SUBJECT: AT2023sva: redshift from OSIRIS+/GTC of the ZTF afterglow
DATE:    23/09/19 23:02:56 GMT
FROM:    Antonio de Ugarte Postigo at OCA <deugarte@oca.eu>

A. de Ugarte Postigo (CNRS-OCA), D. B. Malesani (Radboud Univ. and DAWN/NBI), J. F. Agui Fernandez (IAA-CSIC), C. C. Thoene (ASU-CAS), S. Geier (GTC) report:

We observed the afterglow candidate AT2023sva (Vail et al., GCN 34730; Malesani et al. GCN 34731), temporally and spatially coincident with the Fermi-GBM trigger bn230916144 / 716527670, using the OSIRIS+ instrument on the 10.4m GTC telescope, in La Palma (Spain). The observation started on the 19th September 2023 at 03:56 UT (3.020 days after the Fermi-GBM trigger) consisted of a 30 s r-band acquisition followed by 3x900 s spectra with grism R1000B, covering the wavelength range between 3700 and 7800 angstroms.

The spectrum displays a clear continuum across the entire spectral range, with two absorption systems. This is consistent with the object being a GRB afterglow. The first system, at a redshift of 1.307, shows clear absorption from Fe II and Mg II. A higher redshift system is revealed by a trough consistent with Lyman-alpha at a redshift of 2.281. At the same redshift, we identify weak tentative absorptions of O I, Si II, C IV and Fe II, although several other typically strong lines are missing. We propose z = 2.281 as the redshift of the GRB afterglow.