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

Context.The  extreme  luminosity  of  gamma-ray  bursts  (GRBs)  makes  them  powerful  beacons  for  studies  of  the  distant  Universe.  The  mostluminous bursts are typically detected at moderate/high redshift, where the volume for seeing such rare events is maximized and the star-formationactivity is greater than atz=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 emissionextending 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 ofthe burst afterglow 0.5 days after the explosion.

Results.The spectra exhibit both absorption and emission lines from material in a host galaxy atzGRB=0.15095±0.00005. Thus GRB 221009Awas a relatively nearby burst with a luminosity distance ofdL=745 Mpc. Its host galaxy properties (star-formation rate and metallicity) areconsistent  with  those  of  LGRB  hosts  at  low  redshift.  This  redshift  measurement  yields  information  on  the  energy  of  the  burst.  The  inferredisotropic energy release,Eiso>5×1054erg, lies at the high end of the distribution, making GRB 221009A one of the nearest and also mostenergetic GRBs observed to date. We estimate that such a combination (nearby as well as intrinsically bright) occurs between once every fewdecades to once per millennium.