TITLE:   GCN CIRCULAR
NUMBER:  29757
SUBJECT: GRB 210402A: NOT optical afterglow candidate
DATE:    21/04/04 08:23:51 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

D. Xu (NAOC), Z.P. Zhu (NAOC,HUST), D. B. Malesani (DTU Space) report on 
behalf of a larger collaboration:

We reanalysed our previously reported NOT data (Zhu et al., GCN 29745), 
given that the position of the Swift/XRT error circle has significantly 
changed (Page et al., GCN 29743; Evans et al., GCN 29750).

An optical source, S1, is detected at the western edge of the refined 
XRT error circle (Evans et al., GCN 29750) at coordinates: R.A.(J2000) 
=13:13:02.04 and Dec.(J2000)=+44:30:52.69, with r = 24.0 +/- 0.2 and z = 
22.6 +/- 0.3, calibrated with nearby PanSTAR stars. S1 is also detected 
by the LBT observations (Rossi, GCN 29753), and the magnitudes are 
consistent with each other with 1sigma error. In addition, the GRB field 
is covered by the Legacy Survey, and forced aperture photometry of the 
S1 position gives r ~ 24.5 and z ~ 23.3, both fainter than the NOT 
measurements. Thus, S1 is the optical afterglow candidate of the burst.

A second source, S2, is detected at the south-east and outside of the 
refined XRT error circle at coordinates: R.A.(J2000) =13:13:02.45 and 
Dec.(J2000)=+44:30:50.31, as pointed by the LBT observations. It has r 
= 23.5 +/- 0.1 and z = 23.0 +/- 0.2 from the NOT images, and again no 
r-band variability between the NOT and LBT epochs. Moreover, forced 
aperture photometry of the S2 position from the Legacy Survey gives r ~ 
23.6 and z ~ 23.0, being consistent with the NOT measurements. Thus, 
we're inclined to conclude that S2 is unlikely the optical afterglow of 
the burst.

Further observations are planned to asses the brightness variability.