Context: The extreme luminosity of gamma-ray bursts (GRBs) makes them
powerful beacons for studies of the distant Universe. The most luminous bursts
are typically detected at moderate/high redshift, where the volume for seeing
such rare events is maximized and the star-formation activity is greater than
at z = 0. For distant events, not all observations are feasible, such as at TeV
energies.
Aims: Here we present a spectroscopic redshift measurement for the
exceptional GRB 221009A, the brightest GRB observed to date with emission
extending well into the TeV regime.
Methods: We used the X-shooter spectrograph at the ESO Very Large Telescope
(VLT) to obtain simultaneous optical to near-IR spectroscopy of the burst
afterglow 0.5 days after the explosion.
Results: The spectra exhibit both absorption and emission lines from material
in a host galaxy at z = 0.151. Thus GRB 221009A was a relatively nearby burst
with a luminosity distance of 745 Mpc. Its host galaxy properties
(star-formation rate and metallicity) are consistent with those of LGRB hosts
at low redshift. This redshift measurement yields information on the energy of
the burst. The inferred isotropic energy release, Eiso>5×1054 erg, lies at the high end of the distribution, making GRB 221009A one
of the nearest and also most energetic GRBs observed to date. We estimate that
such a combination (nearby as well as intrinsically bright) occurs between once
every few decades to once per millennium.Comment: 9 pages, 4 figures, submitted to Astronomy & Astrophysic