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GCN Circular 7511

Subject
GRB 080319B : Apparent spectral evolution in very early Swift/XRT WT mode data: intrinsic or pile-up effect?
Date
2008-03-24T22:35:33Z (16 years ago)
From
Binbin Zhang at UNLV <zbb@physics.unlv.edu>
Bin-Bin Zhang (University of Nevada Las Vegas), Enwei Liang (Guangxi 
University, China) and Bing Zhang (University of Nevada Las Vegas) report:

We have processed the Swift XRT data of GRB 080319B, paying special 
attention to the possible spectral evolution in the WT mode data 
(Racusin et al. GCN 7459; cf. Butler GCN 7499). We perform a 
time-dependent spectral analysis using the method described in (Zhang, 
Liang & Zhang 2007, ApJ, 666, 1002). Since the early data are strongly 
affected by photon pile-up, we use a box annulus region for the WT mode 
data (outer radius 40*20, inner radius 8*20; see also Racusin et al. GCN 
7459) and time-dependent circle annulus regions for the PC mode data to 
extract spectra and lightcurves. We fit the time-dependent spectra using 
a simple power-law model with the absorption from the MilkyWay Galaxy 
(NH_G=1.12e20 cm^{-2} ) and from the host galaxy (NH_host=7.3e20 
cm^{-2}, obtained from fitting to the integrated 1st orbit WT mode 
spectrum). We confirm Butler (GCN 7499) that the apparent spectral 
evolution after 200 seconds is due to instrumental "pile up" effect. 
However, in the very early time t ~ (68-100) seconds, an apparent weak 
but significant hard-to-soft spectral evolution sustains even if we take 
into account the pile-up corrections. The photon index evolves from 1.67 
�� 0.02 to 1.77 �� 0.02 during this period. Our results can be found at 
http://grb.physics.unlv.edu/~xrt/xrtweb/080319B/080319B.html.

To make sure that this early-time spectral evolution is not due to the 
pile-up effect, we extract the time-dependent spectra with box annuli 
having different sizes. By excluding the central regions, we enlarge the 
outer radius up to 80 pixel * 20 pixel to make sure that there are 
enough photons for the spectral analysis. Our tests show that even if 
the inner box size is as large as 30 pixel * 20 pixel (spectra in annuli 
with such a large inner radius is not possible to be affected by the 
pile-up effect), the early time (before 200 seconds) XRT WT data still 
show significant spectral evolution. We therefore cautiously conclude 
that this early spectral evolution is likely intrinsic.

Strong hard-to-soft spectral evolution has been seen in the early steep 
decay phase of many GRB X-ray afterglows (e.g. Zhang et al. 2007, 666, 
1002), which points towards a non-forward-shock origin of the emission. 
We notice that the lightcurve before 200 seconds show several weak 
flaring/flicking features, which is more easily seen in linear scale 
(see http://grb.physics.unlv.edu/~xrt/xrtweb/080319B/earlylc.png). In 
view that some steep decay segments with overlapping flares typically 
show hard-to-soft spectral evolution (Group C in Zhang et al. 2007), we 
suspect that the weak spectral evolution in this burst is also related 
to the weak flaring/flicking features. It is however puzzling why this 
segment naturally transforms to a smooth decay after 200 seconds which 
show no further spectral evolution.

Throughout our fit we have fixed the NH_host values. Another possibitly 
is that the apparent spectral evolution is caused by a varying NH_host 
value (Racusin et al. GCN 7459). We test such a scenario by fixing the 
photon index to \Gamma=1.76 (average value after 200s) and fit the 
time-dependent spectra before 200s using the same model 
(wabs*zbwas*zwabs) but allowing NH_host to be a free parameter.  We 
obtain acceptable fits, and found that the NH_host in the early time 
evolves dramatically to one half of its initiall valve (from ~ 1.1e21 
cm^{-2} to ~ 6.6e20 cm^{-2}). The time evolution of the NH_host value 
can be found at http://grb.physics.unlv.edu/~xrt/xrtweb/080319B/nh.png. 
This is another plausible physical scenario, although a model for the 
rapid depletion of NH_host is called for.

The reduced chi2 in our fitting to the wabs*zwabs*powerlaw model is 
typically ~1. Although a possible thermal component has been suggested 
(cf. Racusin et al. GCN 7459), in our fitting no thermal component is 
required by the data.

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