GCN Circular 2017

P.A. Price, B.P. Peterson and B.P. Schmidt (RSAA, ANU) report:

Synthesis of observations from Rykoff et al. (GCN #1995), Gal-Yam et
al. (GCN #1999), Martini et al. (GCN #2012) and from the SSO 40-inch
(GCN #1987) yeilds the following decay:

	R/mag ~ 15.8 + 2.4 log (t/days)

with corresponding power-law index alpha = 0.97 +/- 0.03.
Hence, though the afterglow is bright, the decay is not unusually
shallow (eg, 010222 had alpha1 ~ 0.80).

The spectral index, calculated from the B-I colour from Burenin et
al. (GCN #2001) and Rumyantsev et al. (GCN #2005) is beta ~ 1.2.

Using the measured redshift (Martini et al., GCN #2013; Della Ceca et al.,
GCN #2015), optical decay and spectral index, we calculate the R-band
absolute magnitude of the optical afterglow for t=1 day in the source
frame, M_R,1 = -26.7 mag.  This is therefore the most intrinsically bright
optical afterglow observed to date (with 000301C and 000418 tied for
second at M_R,1 = -26.1 mag).  The low redshift and the large intrinsic
brightness combined to produce an optical afterglow with a large apparent
brightness.

The redshift of z=0.57 and measured fluence (Vanderspek et al., GCN #1997)
implies an isotropic-equivalent energy release of 1.1 x 10^53 erg (30-400
keV).  The Frail et al. "standard energy" result implies a jet-break time
of around 4 days.  At this time, the afterglow should be R ~ 17.3 mag.
We encourage polarimetric observations to be made on this timescale to
constrain the jet dynamics.

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