{
  "bibcode": "2021GCN.31107....1R",
  "body": "A. Rossi, E. Palazzi, L. Amati, E. Pian (INAF-OAS), D. B. Malesani \n(Univ. Radboud and DAWN/NBI), S. Savaglio (UNICAL), P. D'Avanzo \n(INAF/OABr), and V. D'Elia (SSDC & INAF-OAR), M. De Pasquale (University \nof Messina),�� report on behalf of the CIBO collaboration:\n\nWe report the results of the photometric and spectroscopic follow-up of \nthe short GRB 211023B (Dichiara et al., GCN Circ. 30960) obtained with \nthe Multi-Object Double Spectrographs (MODS) instrument mounted on the \n2x8.4-m LBT telescope (Mt Graham, AZ, USA). Data were obtained at the \nmid time of 12:40 UT on 2021-11-06, ~13.6 days after the burst trigger.\n\nAt the position of the UVOT localization (Kuin & Dichiara, GCN 30975) \nand the Legacy Survey object (Pozanenko et al., GCN 30972), we detect \nthe GRB host galaxy (Sakamato et al. GCN 31038, Rastinejad et al. GCN \n31002). We measure the following AB magnitudes:\ng = 24.02+-0.17\nr = 23.99+-0.15\ncalibrated against Pan-STARRS field stars and not corrected for Galactic \nextinction.\n\nSpectroscopy of the source was obtained for a total of 4550s, covering \nthe wavelength range 3200-10000 AA.Continuum is faintly detected over \nthe wavelength range 3200-10000 AA, and a single emission line is \nvisible at ~6940 AA. The most likely interpretation is [O II] at z = \n0.862. At this redshift, [O III] would unfortunately fall on top of a \nregion affected by sky lines, so its non-detection is not very \nconstraining. Other interpretations are less likely. We can disregard \nLyalpha, since there is no drop in the continuum blueward of the line. \nWe deem Halpha unlikely, as the corresponding redshift z = 0.057 would \nimply an extremely faint host galaxy (absolute magnitude ~ -13), and we \ndon't see the expected [O III] and Hbeta lines. The interpretation of \nthe observed feature as [O III] 5008 seems to be also unlikely as we \nwould expect to detect hints of [O III] 4959 as well as Halpha which \nwould fall in a wavelength region clean of skylines.\n\nAssuming the redshift z=0.862, standard cosmology (Planck 2016) and \nSwift/BAT refined and on-line automated spectral analysis (Laha et al., \nGCN 30979, and http://gcn.gsfc.nasa.gov/notices_s/1080859/BA/ \n<http://gcn.gsfc.nasa.gov/notices_s/1080859/BA/>) we estimate an \nisotropic-equivalent radiated energy Eiso (1-10000 keV) of about \n(0.9-1.2)x E^51 erg, which is typical of short GRBs. However, we note that:\na) despite its short duration, the burst has a rather soft spectrum \n(simple power with index ~2) which is consistent with a spectral peak \nenergy Ep close to (or even below) the BAT low-energy threshold and \nwould imply a rest-frame peak energy Ep,i < ~30-40 keV, thus locating \nthis GRB in the region of the Ep,i - Eiso plane populated by long GRBs. \nThe preliminary automatic modelling with a cut-off power-law or a Band \nfunction is consistent with a larger Ep,i, which would make this burst \nmore similar to other short GRBs, but with a very large uncertainty.\nb) the soft spectra and short duration put GRB 211023B closer to the \nregion populated by long GRBs than to to the one populated by short \nevents also in the HR-T90 diagram, even though it is still consistent \nwith belonging to the tail of the distribution of short GRBs in this plane.\n\nWe acknowledge the excellent support from the LBTO and LBT-INAF staff, \nparticularly O. Kuhn, S. Allanson, F. Cusano, S. Bisogni, and D. Paris, \nin obtaining these observations.",
  "circularId": 31107,
  "createdOn": 1637683207000,
  "email": "andrea.rossi@inaf.it",
  "subject": "GRB 211023B: LBT likely spectroscopic redshift of the host galaxy",
  "submitter": "Andrea Rossi at INAF  <andrea.rossi@inaf.it>",
  "eventId": "GRB 211023B"
}