GCN Circular 5009

A. D. Falcone, D. N. Burrows, D. Morris, J. Racusin (PSU), P. T.
O'Brien, J. P. Osborne (U Leicester), N. Gehrels (GSFC) report on behalf
of the Swift XRT team:

We have performed more analysis on recent Swift XRT data from GRB 060418
(Falcone et al., GCN4966
Not found
 
 
).  As reported by Falcone et al. (GCN4973
Not found
 
 
), the
early afterglow decay has bright flaring.  This flaring made it
difficult to estimate the afterglow decay index at the time of the
previous report, which made use of only the initial flare-dominated
data.  Here we give a better estimate of this decay, along with some
preliminary spectral information.

 From 78 s until about 115 s after the burst trigger time, the light
curve had a steep power law decay index of alpha = 4.6 +/- 0.2.  A large
flare begins at T+115 s, peaking at T+135 s (where T is the BAT trigger
time) with a maximum count rate of ~560 c/s.  Flaring continues to at
least T+6000 s.  No flares are evident after T+40,000 s, at which point
the count rate has dropped to less than 0.01 c/s.  The temporal decay of
the underlying afterglow was fit using two time regions that showed no
evidence of flaring (T+350 to T+530 s, and T+40,000 to T+190,000 s).
The underlying afterglow has a power law decay index of alpha = 1.4 �
0.1 from T+350 s until the last detection at T+700,000 s.  There is no
evidence for a break in the underlying decay curve up to 9 days after
the trigger.  In particular, we do not detect the break predicted by
Ghisellini et al. (GCN 4991
Not found
 
 
), although we have only a single 2 sigma
data point after the predicted break time.  Due to a normal orbital gap,
we have no data during the time of the potential second burst reported
by Konus-Wind (Golenetskii et al. GCN4989
Not found
 
 
).

A preliminary light curve can be viewed at
http://www.astro.psu.edu/users/afalcone/grb060418/falcone_grb060418.gif

The observed spectra of the underlying afterglow can be described by a
simple absorbed power law, with photon index 2.04 +/- 0.13 and total N_H
= (19 +/- 4)e20 cm^-2, with a chi2/dof of 1.22 (75 dof).  This fit
assumes all absorption is local to the observer.  The Galactic N_H at
this position is ~9e20 cm^-2 (Dickey and Lockman 1990).  We fit the time
period during the large flare with a spectral model that was the sum of
two power laws; one was the underlying afterglow frozen to the values
shown above (with the normalization changed based on the measured
temporal decay) and the other was a power law that was free to vary.
This fit resulted in a chi2/dof of 1.07 (321 dof), with the following
flare power law component parameters: photon index 2.04 +/- 0.05 and N_H
= (29 +/-2)e20 cm^-2.  For the flare component, we also tried a cutoff
power law, a Band function, and a blackbody model.  In all cases, the
fits were equivalently acceptable, with chi2/dof always falling in the
range from 1.05 to 1.07.  The cutoff power law was equivalent to the
simple power law since the cutoff reached a maximum value in excess of
500 keV.  The Band function and the blackbody models both resulted in
harder energy spectral indices, with an N_H that was consistent (within
1 sigma error bars) with that of the underlying afterglow.  This is in
contrast to the simple power law that implies an N_H increase during the
flare (relative to the underlying afterglow value).

This Circular is an official product of the Swift XRT Team.