TITLE:   GCN CIRCULAR
NUMBER:  22000
SUBJECT: GRB 171010A: NOT optical observations
DATE:    17/10/12 01:44:15 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani@dark-cosmology.dk>

D. Malesani (DARK/NBI), K. E. Heintz (Univ. Iceland and DARK/NBI), P. 
Calissendorff (Stockholm Univ), and I. R. Losada (NOT) report on behalf 
of a larger collaboration:

We observed the optical counterpart (Thorstensen & Halpern, GCN 21987; 
Izzo & Malesani, GCN 21988; Guidorzi et al., GCN 21991) of the Fermi GRB 
171010A (Poolakkil & Meegan, GCN 21992; Omodei et al., GCN 21985) with 
the Nordic Optical Telescope equipped with the StanCam imager. 
Observations were carried out in the Johnson-Cousins BVRI filters.

The optical counterpart is well detected in our images. Using the 
Pan-STARRS catalog as reference, we measure a magnitude r = 19.18 +- 
0.06 AB at an epoch Oct 12.03 UT (29.74 hr after the trigger).

Comparing our measurement with the report by Thorstensen & Halpern (GCN 
21987) we infer a decay index alpha = 1.4 assuming an unbroken power-law 
decay.