GCN Circular 5187
Subject
GRB060526: afterglow fitting and probable jet break
Event
Date
2006-05-30T16:43:38Z (20 years ago)
From
Christina Thoene at Niels Bohr Institute,DARK Cosmo Ctr <cthoene@astro.ku.dk>
D. A. Kann (TLS Tautenburg) and C. C. Thoene (DARK Cosmology Center)
report:
Gathering further data from the GCN (Khamitov et al., GCN 5183, 5186,
Baliyan et al., GCN 5185) in addition to those referenced in Kann &
Hoegner (GCN 5182), we perform fits to the R afterglow light curve.
A single power law fit to all data beyond 6000 seconds yields alpha = 1.04
+/- 0.01, but the late-time data beyond two days lie significantly beneath
the fit.
A broken power law gives a better fit, with alpha_1 = 0.98 +/- 0.10,
alpha_2 = 2.06 +/- 0.13, and break time t_b = 1.76 +/- 0.1 days. The break
smoothness parameter n is fixed to 10.
Still, the fit is not acceptable, with strong deviations around 0.5 days.
Fitting separate parts of the light curve, we find a better fit with a
double broken power law. The first part (0.19 to 1.1 days) is described by
alpha_1 = 1.35 +/- 0.07, alpha_2 = 0.85 +/- 0.08 and t_b = 0.44 +/- 0.08
days, n = -10 fixed. The fit is marginally acceptable, with
deviations due to scatter (different zero points and comparison stars).
The second part (0.53 days to the end) is described by alpha_1 = 0.76 +/-
0.04, alpha_2 = 1.87 +/- 0.07 and t_b = 1.42 +/- 0.08 days, n = 10 fixed.
This fit is acceptable. Alpha_2 of the first part and alpha_1 of the
second part are identical within errors, as expected. We note that the
extension of this last fit to early times is consistent with the first
ROTSE data point (Rykoff et al., GCN 5166) and marginally consisten with
the very early Watcher data (French & Jelinek, GCN 5165).
This strengthens the picture that the break first described by Thoene et
al. (GCN 5179) is the jet break. The post-break decay is relatively
smooth.
We caution that these fits do not include the dense data set presented by
Halpern, Armstrong & Mirabal (GCN 5176) and the optical flare they
detected. Similar to early fits of GRB 030329 presented in the GCN, these fits may
be the result of inhomogenous coverage of a highly variable light curve.
Further deep monitoring is encouraged.
The plots for the double broken power law fits can be found at
www.astro.ku.dk/~cthoene