TITLE:   GCN CIRCULAR
NUMBER:  35009
SUBJECT: GRB 231111A: Afterglow redshift from OSIRIS+/GTC
DATE:    23/11/13 11:53:53 GMT
FROM:    C. C. Thoene at ASU-CAS <christina.thoene@gmail.com>

C. C. Thoene (ASU-CAS), A. de Ugarte Postigo (CNRS/OCA), L. Izzo (INAF/Capodimonte), F. Agui Fernandez (CAHA), F. Perez Toledo (GTC), M. Blazek (CAHA), J. Fynbo (DAWN/NBI), S. Geier (GTC) and N.R. Tanvir (Univ. Leicester) report:

We observed the afterglow of GRB 231111A (Melandri et al. GCN 34981, Jiang et al. GCN 34982) with OSIRIS+ at the 10.4m GTC on Nov. 11, 2023, starting at 20:19:13 UT, 6h after the GRB. We obtained 3x900s exposures under bad seeing conditions (1.9 arcsec) using the R1000B grism, which gives a wavelength coverage between 3700 and 7800 A. At the time of observations, the afterglow had a magnitude of r~20.2 as determined from the acquisition image.

In the combined spectrum we detect the MgII 2796,2803 Å doublet and faint lines of FeII 2585 and 2600 Å, which results in a redshift of z=1.179. TNG (D’Avanzo et al. GCN 35007) also claims the detection of a MgII doublet, however, at their reported redshift of z=1.39, no lines are present in our spectrum. No fine-structure lines are detected, making the redshift of 1.179 strictly seen as a lower limit. However, given the lack of further emission or absorption features in the spectral range and the upper limit of z~2 due to the detection of continuum down to 3650 Å (similar to the claim of D'Avanzo et al. GCN 35007) we expect this to be the actual redshift of the GRB.

We acknowledge the excellent support by the GTC staff.