TITLE: GCN CIRCULAR NUMBER: 31107 SUBJECT: GRB 211023B: LBT likely spectroscopic redshift of the host galaxy DATE: 21/11/23 16:00:07 GMT FROM: Andrea Rossi at INAF A. Rossi, E. Palazzi, L. Amati, E. Pian (INAF-OAS), D. B. Malesani (Univ. Radboud and DAWN/NBI), S. Savaglio (UNICAL), P. D'Avanzo (INAF/OABr), and V. D'Elia (SSDC & INAF-OAR), M. De Pasquale (University of Messina),  report on behalf of the CIBO collaboration: We report the results of the photometric and spectroscopic follow-up of the short GRB 211023B (Dichiara et al., GCN Circ. 30960) obtained with the Multi-Object Double Spectrographs (MODS) instrument mounted on the 2x8.4-m LBT telescope (Mt Graham, AZ, USA). Data were obtained at the mid time of 12:40 UT on 2021-11-06, ~13.6 days after the burst trigger. At the position of the UVOT localization (Kuin & Dichiara, GCN 30975) and the Legacy Survey object (Pozanenko et al., GCN 30972), we detect the GRB host galaxy (Sakamato et al. GCN 31038, Rastinejad et al. GCN 31002). We measure the following AB magnitudes: g = 24.02+-0.17 r = 23.99+-0.15 calibrated against Pan-STARRS field stars and not corrected for Galactic extinction. Spectroscopy of the source was obtained for a total of 4550s, covering the wavelength range 3200-10000 AA.Continuum is faintly detected over the wavelength range 3200-10000 AA, and a single emission line is visible at ~6940 AA. The most likely interpretation is [O II] at z = 0.862. At this redshift, [O III] would unfortunately fall on top of a region affected by sky lines, so its non-detection is not very constraining. Other interpretations are less likely. We can disregard Lyalpha, since there is no drop in the continuum blueward of the line. We deem Halpha unlikely, as the corresponding redshift z = 0.057 would imply an extremely faint host galaxy (absolute magnitude ~ -13), and we don't see the expected [O III] and Hbeta lines. The interpretation of the observed feature as [O III] 5008 seems to be also unlikely as we would expect to detect hints of [O III] 4959 as well as Halpha which would fall in a wavelength region clean of skylines. Assuming the redshift z=0.862, standard cosmology (Planck 2016) and Swift/BAT refined and on-line automated spectral analysis (Laha et al., GCN 30979, and http://gcn.gsfc.nasa.gov/notices_s/1080859/BA/ ) we estimate an isotropic-equivalent radiated energy Eiso (1-10000 keV) of about (0.9-1.2)x E^51 erg, which is typical of short GRBs. However, we note that: a) despite its short duration, the burst has a rather soft spectrum (simple power with index ~2) which is consistent with a spectral peak energy Ep close to (or even below) the BAT low-energy threshold and would imply a rest-frame peak energy Ep,i < ~30-40 keV, thus locating this GRB in the region of the Ep,i - Eiso plane populated by long GRBs. The preliminary automatic modelling with a cut-off power-law or a Band function is consistent with a larger Ep,i, which would make this burst more similar to other short GRBs, but with a very large uncertainty. b) the soft spectra and short duration put GRB 211023B closer to the region populated by long GRBs than to to the one populated by short events also in the HR-T90 diagram, even though it is still consistent with belonging to the tail of the distribution of short GRBs in this plane. We acknowledge the excellent support from the LBTO and LBT-INAF staff, particularly O. Kuhn, S. Allanson, F. Cusano, S. Bisogni, and D. Paris, in obtaining these observations.