TITLE:   GCN CIRCULAR
NUMBER:  29638
SUBJECT: GRB 210306A: Early Spectroscopy of the Optical Afterglow
DATE:    21/03/09 18:35:39 GMT
FROM:    Paul S. Smith at Steward Observatory  <psmith@as.arizona.edu>

P.S. Smith, Z. Chen, P. Milne, D. Sand, N. Smith, D. Stark, and G.G.
Williams (Steward Observatory, University of Arizona)

Observations of the optical counterpart of GRB 210306A using the B&C
Spectrograph and the University of Arizona, Steward Observatory 2.3m Bok
Telescope located on Kitt Peak, Arizona began at 07:07:50 on March 6, 2021
UTC and ended at 08:23:13 UT.  Three, 1500 s exposures were obtained
centered at 3.384, 3.859, and 4.279 hours after the Swift/BAT detection of
the burst (D’Elia et al.; GCN Circ. 29597).  Comparison with a
spectrophotometric standard star observed immediately after the optical
afterglow observations suggest that the counterpart was at an approximate
V magnitude of 19.18, 19.33, and 19.46 for the three exposures,
respectively. All spectra show a featureless blue continuum from 350-760nm.
A fit to the least noisy portion (450-750nm) of the scaled median spectrum
from the three observations yields a slope of about -1.25 in log(F_lambda)
vs. log(wavelength). No corrections for Galactic extinction and reddening
have been made.