GCN Circular 32765

L. Izzo (DARK/NBI), A. Saccardi (GEPI, Observatoire de Paris), J. P. U. 
Fynbo (DAWN/NBI), J. Palmerio (GEPI, Paris obs.), D. B. Malesani 
(Radboud Univ. and DAWN/NBI), J. F. Agui Fernandez (IAA/CSIC), D. A. 
Kann (Goethe Univ.), A. Melandri (INAF/OAR), S. D. Vergani (GEPI, Paris 
obs.), K. Wiersema (Lancaster univ.), report on behalf of the Stargate 
consortium:

We observed the optical counterpart of GRB 221009A (Dichiara et al., GCN 
32632
Not found
 
 
; Veres et al., GCN 32636
Not found
 
 
; and very many other GRB satellites) 
using the ESO VLT UT3 (Melipal) equipped with the X-shooter 
spectrograph. A 40-min spectrum was secured covering the wavelength 
range 3000-25000 AA, with a mean time 2022 October 14.02 UT (4.46 days 
after the Fermi/GBM trigger).

 From the acquisition image, we measure for the optical counterpart a 
magnitude i = 19.89 +- 0.05 (AB; calibrated against Pan-STARRS), 4.45 
days after the Fermi/GBM trigger.

The continuum is still dominated by the afterglow. However, narrow 
emission lines can be seen from the host galaxy. We identify H alpha in 
the optical and Pa alpha in the near-infrared, at redshift z = 0.151, 
consistent with the one measured from the afterglow absorption features 
(de Ugarte Postigo et al., GCN 32648
Not found
 
 
).

The H alpha FWHM is about 110 km s^-1. From the H alpha flux, corrected 
for Galactic extinction (A_V = 4.2 mag), we infer a SFR > 0.25 M_Sun 
yr^-1 (this value is a lower limit due to unaccounted host extinction 
and slit losses).

We acknowledge expert support from the ESO observing staff in Paranal, 
in particular Zahed Wahhaj and Matias Jones.