GRB 211211A: Nanshan/NEXT optical observations

We observed the field of GRB 211211A detected by Fermi (Fermi GBM team, GCN 31201) and Swift (D'Ai et al., GCN 31202) using the NEXT-0.6m optical telescope located at Nanshan, Xinjiang, China. Observations started at 23:01:09 UT on 2021-12-11, i.e., 9.85 hr after the BAT trigger, and 10x200 s frames in the Sloan r-filter as well as 12x200 s frames in the Sloan z-filter were obtained. The optical afterglow candidate of the GRB reported by KAIT (Zheng & Filippenko, GCN 31203), consistent with the enhanced Swift-XRT position (Beardmore et al., GCN 31205), is detected in our stacked r- and z-band images. Preliminary photometry is as follows: T_mid-T0 (hr) Mag MagErr Filter 10.34 20.29 0.07 r 10.68 19.9 0.3 z both calibrated with nearby PS1 stars. We note that the NEXT's r-band magnitude is comparable to that of KAIT in the clear band, although the time interval of the two observations is ~10 hr.