D. Xu (NAOC), Y. Qin (Geneva Observatory), Y.D. Hu (IAA-CSIC), H.J.
Wang, H. Wu, M. He, H.X. Feng, Z.P. Zhu (NAOC) report:
We observed the optical afterglow of GRB 170202A (Racusin et al., GCN
20575) using the 2.16-m telescope at Xinglong, Hebei, China, equipped
with the BFOSC camera. The observation started at 18:59:32 UT on
2017-02-02 (i.e., 31.5 mins after the BAT trigger), 4x120s R-band
photometry was obtained, followed by 1x3600s spectroscopy using the
G4+385LP grism setting, covering the wavelength of ~3600 - 8500 AA.
The optical afterglow has m(R)~18.0 mag from the first R-band exposure,
calibrated with the nearby SDSS stars.
Preliminary analysis shows that the spectrum is featured by a prominent
Lyman_alpha trough, a Lyman forest blue to the trough, together with Si
IV, C IV and other potential absorption lines red to the trough, all at
a common redshift of z = 3.65. This redshift value is consistent with
measurement in de Ugarte Postigo et al. (GCN 20584).