B. Schneider (MIT), A. Saccardi (GEPI, Observatoire de Paris), L. Izzo (INAF-Naples & DARK/NBI), A. J. Levan (Radboud), D. B. Malesani (DAWN/NBI), N. R. Tanvir (Univ. Leicester), G. Pugliese (Amsterdam), report on behalf of the Stargate collaboration:
We observed the field of the Swift/Fermi GRB 231118A (Fermi GBM Team, GCN 35100; Laha et al., GCN 35101) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and consist of
4 exposures of 600s each. The observation mid-time is 01:23:787 UT on Nov 19 2023 (8.1 hr after the Fermi trigger).
In a 30s image taken with the acquisition camera on Nov 19 00:51:23 UT, we clearly detect the optical afterglow (Dutton et al., GCN 35103, GCN 35108; Lipunov et al., GCN 35109; Strausbaugh et al., GCN 35110; Shrestha et al, GCN 35113), for which we measure an AB magnitude r = 19.88 +/- 0.04 mag (calibrated against one nearby star from Legacy Survey).
In a preliminary reduction, we clearly detect a continuum over the entire wavelength range. From detection of multiple absorption features, which we interpret as being due to FeII, MnII, MgII, MgI and FeII*, ZnII, CrII, and CaII, we infer a common redshift of z = 0.8304. We conclude this is the redshift of the burst. We also detect multiple emission lines (Halpha, Hbeta, OIII doublet) at a consistent redshift, which we interpret as being due to the GRB host galaxy. We also note the presence of additional absorption features likely due to multiple intervening systems.
We acknowledge expert support from the ESO staff in Paranal, in particular Thomas Rivinius and Matias Jones.