GCN Circular 31107
Subject
GRB 211023B: LBT likely spectroscopic redshift of the host galaxy
Event
Date
2021-11-23T16:00:07Z (4 years ago)
From
Andrea Rossi at INAF <andrea.rossi@inaf.it>
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/
<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.