TITLE:   GCN CIRCULAR
NUMBER:  33110
SUBJECT: GRB 221226B: VLT/X-shooter redshift and host galaxy
DATE:    22/12/27 09:26:14 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

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 
collaboration:

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 
each.

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 
coordinates

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.