TITLE:   GCN CIRCULAR
NUMBER:  29762
SUBJECT: GRB 210402A: Further NOT optical observations
DATE:    21/04/05 15:18:50 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

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.