Z.P. Zhu (NAOC,HUST), D. Xu (NAOC), D. B. Malesani (DTU Space) report on
behalf of a larger collaboration:
We continued to observe the field of GRB 210402A (Page et al., GCN
29743) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC
camera. We obtained 9x360 s frames in the Sloan r-filter, starting at
23:04:56 UT on 2021-04-04, i.e., 2.04 days after the burst.
The optical afterglow candidate, S1, (Xu et al. GCN 29757, see also
Rossi et al., GCN 29753) is still detected in the stacked image. S1 now
has r = 24.3 +/- 0.1, calibrated with nearby Pan-STARRS stars, while it
had r = 24.0 +/- 0.2 in the last NOT epoch. The magnitudes are basically
consistent with each other within 1\sigma error, although there might be
slight decay, which indicates that S1 has been dominated by the
potential host galaxy since the first NOT epoch.
Meanwhile, the unlikely optical afterglow source, S2, is well detected
in the stacked image with r= 23.5 +/- 0.1, the same as the last NOT
epoch. We thus conclude that S2 is not associated to the GRB.