Z.P. Zhu (NAOC, HUST), L. Izzo (DARK/NBI), S.Y.Fu, D. Xu (NAOC), J.P.U.
Fynbo (DAWN/NBI), E. Paraskeva, S. Vitali (NOT), report on behalf of a
We observed the bright optical afterglow of GRB 210822A (Page et al.,
GCN 30677; Wang et al., GCN 30678; Ohno et al., GCN 30687) using the
Nordic Optical Telescope (NOT) equipped with the ALFOSC camera.
We obtained 3x120s frames in the Sloan r-filter, starting at 23:11:50 UT
on 2021-08-22 (i.e., ~ 13.89 hr after the BAT trigger). The previously
reported optical afterglow (e.g., Page et al., GCN 30677; Zheng &
Filippenko, GCN 30679; Butler et al., GCN 30680; Lipunov et al., GCN
30681; Fu et al., GCN 30684) has decayed to m(r) = 19.71 +/- 0.02,
calibrated with the PS1 field.
Subsequently 4x1200 s spectroscopy was carried out. A continuum is
detected over the whole wavelength range of 3700-9500 AA, superimposed
with some absorption features, among which prominent Fe II, Mg II, Mg I,
and Al III lines were identified at a common redshift of z = 1.736. We
conclude that this is the redshift of the burst.