GCN Circular 32765
Subject
GRB 221009A: VLT spectroscopic detection of the host galaxy
Date
2022-10-15T16:56:06Z (2 years ago)
From
Daniele B Malesani at Radboud U <d.malesani@astro.ru.nl>
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; Veres et al., GCN 32636; 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).
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.