TITLE:   GCN CIRCULAR
NUMBER:  33712
SUBJECT: GRB 230427A: NOT optical upper limit
DATE:    23/04/29 15:45:11 GMT
FROM:    Dong Xu at NAOC/CAS <dxu@nao.cas.cn>

Z.P. Zhu (NAOC, HUST), D. B. Malesani (Radboud Univ. and DAWN/NBI), X. Liu, D. Xu (NAOC), report on behalf of a larger collaboration:

We observed the field of GRB 230427A detected by Swift (Troja et al., GCN 33687) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 23:06:58 UT on 2023-04-27, i.e., ~12.6 hr after the Swift/BAT trigger, and 3x300 s Sloan r-filter images and 5x200 s Sloan z-filter images were obtained, respectively.

No new source is detected in our stacked images at the Swift/UVOT position (Troja et al., GCN 33687), and preliminary photometric results are as follows:

T_mid-T0 (hr)    Filter   Upper_Limit (3-sigma)
    12.7                r          >23.5
    13.0                z          >22.5

calibrated with the nearby Pan-STARRS field and not corrected for Galactic extinction.