[vsnet-grb-info 8144] GRB 090715B: break in the optical afterglow decay

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9677
SUBJECT: GRB 090715B: break in the optical afterglow decay
DATE:    09/07/16 11:49:28 GMT
FROM:    Cristiano Guidorzi at Ferrara U,Italy  <guidorzi at fe.infn.it>

C. Guidorzi (U. Ferrara), Z. Cano, A. Melandri, C.G. Mundell,
R.J. Smith, I.A. Steele,  D. Bersier, S. Kobayashi (Liverpool JMU),
A. Gomboc (U. Ljubljana) on behalf of a large collaboration report:

We began observing the Swift GRB 090715B (Vetere et al. GCN Circ. 9668)
with the Liverpool Telescope (LT) on July 15, 21:14:19 UT
(11.0 min post BAT trigger) with the SDSS riz filters,
confirming the detection of the optical afterglow seen by
Swift-UVOT (Smith et al. GCN Circ. 9670; Malesani et al GCN
Circ. 9671; Antonelli et al. GCN Circ. 9675).
Observations with the LT carried on until 5.6 hours post burst.
Our measurements are in agreement with the value reported by
Malesani et al. (GCN Circ. 9671) after correcting for the
different calibration.

We also observed the OT with the Faulkes Telescope North (FTN)
from July 16, 09:45:46 UT (i.e., at 12.71 hours) with the R filter.

The light curve in the r filter from 0.18 to 12.7 hours can be
fitted with a broken power-law with alpha1=0.25 +/- 0.02,
t_break =18 (-3, +12) ks, alpha2~0.7.


Telescope   Mid Time   Exposure  Filter   Mag
            (hours)     (s)
------------------------------------------------------
FTN         12.90      1200        R     20.98 +- 0.05
------------------------------------------------------

Calibration was performed against the SDSS star mentioned
by Malesani et al. (GCN 9671), assuming r'=17.74 and R=17.54.