D. Xu (NAOC), A. Chrimes (Radboud Univ.), B. Schneider (MIT), L. Izzo
(DARK/NBI), A. Saccardi (GEPI, Observatoire de Paris), Z.P. Zhu (NAOC,
HUST), N.R. Tanvir (Univ. Leicester) report on behalf of the Stargate
We observed the field of GRB 221226B detected by Swift (Cenko et al.,
GCN 33108) and Fermi (Fermi GBM team, GCN 33107) using the ESO VLT UT3
(Melipal) equipped with the X-shooter spectrograph. Our spectra cover
the wavelength range 3000-21000 AA, and consist of 4 exposures by 1200 s
The observations started at 01:10:06 UT on 2022-12-27, i.e., ~2.48 hr
after BAT trigger. The optical afterglow of the burst is detected at
R.A.(J2000) = 01:31:38.036
Dec.(J2000) = - 41:31:37.25
with an uncertainty of ~0.3 arcsec and r = 22.0 +/- 0.1 mag, calibrated
with Legacy Survey nearby stars.
A broad absorption feature is visible around 4490 AA, which we identify
as being due to H I. From detection of multiple absorption features,
which we interpret as being due to S II, Si II, O I, C II, Si IV, Al
III, NV, and emission features due to [O II] and Hbeta, we infer a
common redshift of z = 2.694. We conclude this is the redshift of the burst.
Note that at the optical afterglow position, the Legacy Survey presents
an extended source with r = 23.82 mag, which is likely the host galaxy
of the burst, being consistent with the the detection of emission
features in the X-shooter spectra.
We acknowledge excellent support from the ESO observing staff in
Paranal, in particular Pascale Hibon and Ditte Slumstrup.