{
  "bibcode": "2021GCN.29757....1X",
  "body": "D. Xu (NAOC), Z.P. Zhu (NAOC,HUST), D. B. Malesani (DTU Space) report on \nbehalf of a larger collaboration:\n\nWe reanalysed our previously reported NOT data (Zhu et al., GCN 29745), \ngiven that the position of the Swift/XRT error circle has significantly \nchanged (Page et al., GCN 29743; Evans et al., GCN 29750).\n\nAn optical source, S1, is detected at the western edge of the refined \nXRT error circle (Evans et al., GCN 29750) at coordinates: R.A.(J2000) \n=13:13:02.04 and Dec.(J2000)=+44:30:52.69, with r = 24.0 +/- 0.2 and z = \n22.6 +/- 0.3, calibrated with nearby PanSTAR stars. S1 is also detected \nby the LBT observations (Rossi, GCN 29753), and the magnitudes are \nconsistent with each other with 1sigma error. In addition, the GRB field \nis covered by the Legacy Survey, and forced aperture photometry of the \nS1 position gives r ~ 24.5 and z ~ 23.3, both fainter than the NOT \nmeasurements. Thus, S1 is the optical afterglow candidate of the burst.\n\nA second source, S2, is detected at the south-east and outside of the \nrefined XRT error circle at coordinates: R.A.(J2000) =13:13:02.45 and \nDec.(J2000)=+44:30:50.31, as pointed by the LBT observations. It has r \n= 23.5 +/- 0.1 and z = 23.0 +/- 0.2 from the NOT images, and again no \nr-band variability between the NOT and LBT epochs. Moreover, forced \naperture photometry of the S2 position from the Legacy Survey gives r ~ \n23.6 and z ~ 23.0, being consistent with the NOT measurements. Thus, \nwe're inclined to conclude that S2 is unlikely the optical afterglow of \nthe burst.\n\nFurther observations are planned to asses the brightness variability.",
  "circularId": 29757,
  "createdOn": 1617524631000,
  "email": "dxu@nao.cas.cn",
  "subject": "GRB 210402A: NOT optical afterglow candidate",
  "submitter": "Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>",
  "eventId": "GRB 210402A"
}