D. Xu, S.Q. Jiang, S.Y. Fu, X. Liu (NAOC), Z.P. Zhu (NAOC, HUST), T.H.
Lu (NAOC), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:
We observed the field of the optical counterpart of Swift J1913.1+1946
or GRB 221009A detected by Swift (Dichiara, GCN 32632), using the
NEXT-0.6m telescope located at Nanshan, Xinjiang, China. Observations
started at 14:25:55 UT on 2022-10-09, i.e., 762 s after the Swift/BAT
trigger. A series of frames in the Sloan r- and z- filters were obtained.
The optical counterpart is clearly detected in each of our images, which
decays from r ~ 14.93 to r ~ 16.50 within ~ four hours, with a decay
index of \alpha ~ 0.5 (F_t ~ t^-\alpha), calibrated the nearby PS1
stars. The decaying behavior is consistent with that for conventional
Spectroscopy of the optical counterpart was performed at the 2.16m
telescope equipped with the BFOSC camera at Xinglong, Hebei, China. The
spectrum covers the wavelength range 3800-9000 AA. The observation mid
time is 2022 Oct 10.63 UT, i.e., 0.95 hr after the BAT trigger.
The spectrum is dominated by a red continuum due to very high Galactic
extinction, and no prominent absorption feature can be identified. Given
the detections by Swift/UVOT as well as spectral continuum, the event
could be real GRB 221009A at a rather low redshift.
We thank the great support of the Xinglong-2.16m staff, in particular
Junjun Jia, Min He, Aiying Zhou, and Jie Zheng.