[vsnet-grb-info 18512] GRB 160927A: NOT optical observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19960
SUBJECT: GRB 160927A: NOT optical observations
DATE:    16/09/28 05:44:46 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu at nao.cas.cn>

D. Xu (NAOC/CAS), D. Malesani (DARK/NBI and DTU Space), H. Dahle, B. 
Racine (Oslo U.), T. Pursimo (NOT), J. P. U. Fynbo (DARK/NBI), report on 
behalf of a larger collaboration:

We observed the field of GRB 160927A (Gibson et al., GCN 19952) using 
the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC 
camera. Observations started at 21:24:39 UT on 2016-09-27 (i.e., 3.33 hr 
after the trigger) and 5x300s SDSS r-band frames were obtained.

The previously reported optical afterglow of the burst (Tkachenko et 
al., GCN 19954; D'Avanzo et al., GCN 19956; Wiseman et al., GCN 19959) 
is clearly detected in our stacked r-band image. It has m(r)=23.07 +/- 
0.10 at a median time of 3.56 hr post-burst, calibrated with nearby SDSS 
stars.

Combined with the RTT150, TNG, and GROND data points reported in the 
above GCN Circulars, we confirm the steepening of the afterglow 
light-curve (Wiseman et al., GCN 19959). A rough estimate gives rise to 
alpha_1~0.6, a break time of ~3.1 hr post-burst, and alpha_2~1.7 (but 
poorly constrained).