TITLE:   GCN CIRCULAR
NUMBER:  29852
SUBJECT: GRB 210420B: Multiband photometry from NOT
DATE:    21/04/21 01:42:40 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>

A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. B. Malesani (DTU
Space), E. Knudstrup (NOT, Aarhus Univ.) report on behalf of a larger
collaboration:

We observed the afterglow of GRB 210420B (Moss et al. GCN 29844;
Lipunov et al. GCN 29845; Oksanen GCN 29846; Hu et al. GCN 29847; 
Sun et al. GCN 29848; Leonini et al. GCN 29851) with ALFOSC at the 
2.5 m Nordic Optical Telescope at Roque de los Muchachos Observatory 
(La Palma, Spain). The observation consisted of 3x100 s imaging in g, r 
and i bands, and 5x100 s in z-band and started at 00:13 UT.

The afterglow is well detected in individual images in all bands at
the following coordinates (J2000) calibrated with Gaia, and with an
uncertainty estimated at 0.3":

R.A. = 16:57:18.00
Dec. = +42:34:12.1

In a preliminary analysis we measure the following AB magnitudes, as
compared with 3 field stars from the SDSS catalogue:

g = 19.45 +/- 0.04
r =  19.32 +/- 0.04
i = 19.18 +/- 0.04
z = 19.03 +/- 0.07