TITLE:   GCN CIRCULAR
NUMBER:  22029
SUBJECT: GRB 171020A: NOT optical afterglow detection
DATE:    17/10/21 00:19:15 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani@dark-cosmology.dk>

D. Malesani (DARK/NBI), M. Stone (Turku Univ.), K. Karhunen (Turku 
Univ.), A. de Ugarte Postigo (IAA/CSIC and DARK/NBI), and D. Xu 
(NAOC/CAS), report on behalf of a larger collaboration:

We observed the field of GRB 171020A (Page et al., GCN 22028) with the 
Nordic Optical Telescope equipped with the AlFOSC imager and 
spectrograph. Observations were carried out in the r and z filters, 
starting on 2017 Oct 20.979 UT (22.9 min after the trigger).

Within the XRT error circle, we detect an uncatalogued source at 
coordinates (J2000):

RA = 02:36:59.62
Dec = +15:12:16.0

By comparison with nearby stars from the Pan-STARRS catalog, we measure 
r = 20.98 +- 0.03 AB. As this object is not visible in the deeper, 
archival Pan-STARRS images, and given the coincidence with the X-ray 
counterpart, we suggest this object to be the optical afterglow of GRB 
171020A.