GCN Circular 31107

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
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) 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
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) 
and the Legacy Survey object (Pozanenko et al., GCN 30972
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), we detect 
the GRB host galaxy (Sakamato et al. GCN 31038
Not found
 
 
, Rastinejad et al. GCN 
31002
Not found
 
 
). 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
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, and http://gcn.gsfc.nasa.gov/notices_s/1080859/BA/ 
<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.