TITLE:   GCN CIRCULAR
NUMBER:  20004
SUBJECT: GRB 160927A: GTC observations
DATE:    16/10/06 15:58:07 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>

A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), L. Izzo (IAA-CSIC), 
D.A. Kann (TLS Tautenburg), C.C. Thoene (IAA-CSIC), Z. Cano 
(IAA-CSIC), D. Reverte-Paya (GRANTECAN, IAC, ULL) report on 
behalf of a larger collaboration:

We observed the afterglow of the short-hard GRB 160927A (Gibson 
et al., GCN 19952) with OSIRIS on the 10.4m GTC telescope. 
Observations consisted in 10x190s exposures in r-band, with a mean 
epoch on 28 September 2016 at 20:26 UT (26.52 hr after the burst). 
The optical counterpart (Tkachenko et al., GCN 19954; D’Avanzo et 
al., GCN 19956; Wiseman et al. GCN 1959; Xu et al., GCN 19960; 
Fong et al., GCN 19964) is detected at a magnitude of r_AB = 
25.3+/-0.2, as compared to several SDSS field stars.

Combining this observation with the rest of the available GCN data, 
the light curve indicates an early evolution with a shallow decay, 
followed by a faster decay (as already mentioned by Wiseman et al. 
GCN 1959 and Xu et al., GCN 19960), with the break at around the 
TNG epoch, after which we estimate a decay rate of with alpha = 
-1.03+/-0.08 (where F~t^alpha).This decay is significantly shallower 
than the one reported by (Xu et al., GCN 19960), indicating that the 
light curve may be flattening at the time of the GTC observation. This 
could be due to the contribution of an additional component (either 
the host galaxy, or more unlikely the contribution of a kilonova) or just 
due to the uncertainties of the different photometric measurements.