TITLE:   GCN CIRCULAR
NUMBER:  18510
SUBJECT: GRB 151027A: LCOGT-McDonald afterglow observations
DATE:    15/10/28 12:16:09 GMT
FROM:    Cristiano Guidorzi at Ferrara U,Italy  <guidorzi@fe.infn.it>

S. Dichiara (U. Ferrara, ICRANet), D. Kopac (U. Ljubljana), C. Guidorzi 
(U. Ferrara), S. Kobayashi (LJMU), A. Gomboc (U. Ljubljana) on behalf of 
a larger collaboration report:

The 1-m LCOGT McDonald observatory began observing Swift GRB 151027A 
(Maselli et al. GCN 18478) on October 28, 01:29:27 UT (0.90 days after 
the burst trigger) with SDSS R and I filters. We clearly detect the 
optical afterglow (Maselli et al.; Zheng & Filippenko GCN 18479; Elenin 
et al. GCN 18480; Perley & Cenko GCN 18481; Xu et al. GCN 18485) with 
the following magnitudes:

Mid Time      Exposure       Filter       Magnitude
(days)           (s)
-------------------------------------------------------
0.90            5x120         R           16.87 +- 0.04
0.91            6x120         I           17.26 +- 0.05
-------------------------------------------------------

Calibration is done against nearby USNOB-1 star RA(J2000)=18:09:53.742, 
DEC(J2000)=+61:20:57.27, assuming R2=17.21 mag and I=17.17 mag. We note 
the presence of another USNOB-1 object at RA(J2000)=18:09:56.489, 
DEC(J2000)=+61:21:10.64, lying at 3.1 arcsec from the GRB afterglow.