S.Y. Fu (NAOC), K.E. Heintz (DAWN/NBI), D.B. Malesani (Radboud, DAWN/NBI), D. Xu (NAOC), M. Linares (NTNU & UPC), M. Turchetta (NTNU) report on behalf of a larger collaboration:
We observed the field of GRB 230420A detected by Swift (Klingler et al., GCN 33658) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 00:31:32 UT on 2023-04-21, i.e., ~14.2 hr after the Swift/BAT trigger, and 3 x 300 s images in the Sloan r-filter and 5 x 200 s images in the Sloan z-filter were obtained, respectively.
We detect an uncatalogued source in the stacked images within the enhanced XRT error circle (Osborne et al., GCN 33661) at coordinates:
R.A. (J2000) = 10:44:11.55
Dec. (J2000) = +32:06:28.34
with an uncertainty of ~0.3 arcsec. The source has m(r) = 23.2 +/- 0.2 (AB) at 14.34 hr post-burst and m(z) = 22.3 +/- 0.2 (AB) at 14.64 hr post-burst, calibrated with nearby field stars from the Pan-STARRS catalog.
This source is likely the same as the one observed by BOOTES-5/JGT (Hu et al., GCN 33663), as both sources lie within the very small XRT error circle. The fading confirms that this source is the optical afterglow of GRB 230420A.