TITLE:   GCN CIRCULAR
NUMBER:  18158
SUBJECT: GRB 150817A: LCOGT observations
DATE:    15/08/18 18:03:36 GMT
FROM:    Simone Dichiara at Ferrara U/Italy  <dichiara@fe.infn.it>

S. Dichiara, C. Guidorzi (U. Ferrara), C.G. Mundell (U. Bath),
S. Kobayashi (LJMU), A. Gomboc (U. Ljubljana) on behalf of
a large collaboration report:

The 1-m telescopes in Cerro Tololo (Chile) and McDonald (Texas)
 began observing Swift GRB 150817A (V. D'Elia, et al., GCN 18143).
on August 17 at 03:16:31 UT, i.e. ~70 minutes since the BAT trigger,
with the r' and i' filters. Further observations with the i' filter
were performed using the 2-m Faulkes Telescope North ~400 min after
the trigger.

Within the enhanced XRT error circle (Osborne et al. GCN Circ. 18146)
we find no optical counterpart to the following limiting magnitude:

Mid time from  Total Exp   Filter    Magnitude
trigger (min)   (s)
-------------------------------------------------
75.9         120x9       r'        > 19.6
86.3         120x8       i'        > 18.5
107.6        120x5       r'        > 19.7
119.2        120x5       i'        > 19.1
408.1        120x5       i'        > 20.7
-------------------------------------------------

Our results are consistent with the previous observations reported by
Butler et al. (GCN Circ. 18144) and Cenko et al. (GCN Circ. 18153).

Magnitudes are calibrated against nearby USNO-B1 stars (R2 and I).