Kasper E. Heintz (Univ. of Iceland, DAWN/NBI), Johan P. U. Fynbo (DAWN/NBI), and Tapio Pursimo (NOT), report on behalf of a larger collaboration:
We observed the field of the Swift/BAT-detected GRB 210321A (Dichiara et al., GCN #29677), using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Starting at 20:26:25.6 UTC on March 21, we obtained 3x300 sec exposures in the Sloan r-band. The observations were carried out under good conditions, with seeing around 0.8". We confirm the optical afterglow reported by Siegel et al. (GCN #29679), and measure r = 21.6 +/- 0.15 mag at 13.5 hr post-burst, calibrated against nearby field stars in the Pan-STARRS catalog.
Following the imaging sequence, we obtained spectroscopy of the afterglow with ALFOSC using grism 4 (320 to 960 nm) and for 4x1200 sec total integration. A faint trace is detected but we do not identify any particular absorption or emission features to infer the redshift from. However, since the continuum is detected down to 380 nm, we put a limit on the redshift of z < 2.