The brightest GRB ever detected: GRB 221009A as a highly luminous event at z=0.151

<p dir="ltr"><i>Context.</i> The extreme luminosity of gamma-ray bursts (GRBs) makes them powerful beacons, thus effective probes of the distant Universe. The most luminous bursts are typically detected at moderate and high redshift, where the volume for seeing such rare events is maximized and the star-formation activity is greater than at <i>z</i> = 0. For distant events, not all observations are feasible, such as those at TeV energies.</p><p dir="ltr"><i>Aims.</i> 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.</p><p dir="ltr"><i>Methods.</i> We used the X-shooter spectrograph at the ESO Very Large Telescope (VLT) to obtain simultaneous optical to near-infrared (NIR) spectroscopy of the burst afterglow 0.5 days after the explosion.</p><p dir="ltr"><i>Results.</i> The spectra exhibit both absorption and emission lines from material in a host galaxy at <i>z</i>_GRB = 0.15095 ± 0.00005. Thus, GRB 221009A was a relatively nearby burst with a luminosity distance of <i>d</i>_L = 745 Mpc. Its host galaxy properties (star-formation rate and metallicity) are consistent with those of long GRB hosts at low redshift. This redshift measurement yields information on the energy of the burst. The inferred isotropic energy release, <i>E</i>_iso > 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 and once per millennium.</p>