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GCN Circular 7499

Subject
GRB080319B: Time-Independent XRT Spectrum
Date
2008-03-21T16:41:56Z (16 years ago)
From
Nat Butler at MIT/CSR <nrbutler@space.mit.edu>
Nat Butler (UC Berkeley) reports:

I have applied our time-dependent pileup correction code (Butler &
Kocevski 2007; ApJ, 663, 407) to the XRT spectrum of GRB080319B (see, Racusin
et al., GCN 7459).  Although I find similar temporal properties in the     
afterglow decay to those quoted by the XRT team, I find significantly
different spectral properties.  Notably, there is no significant evidence
for a time-variation in N_H and no significant need for a thermal component.
The WT model (0.4-10 keV) and PC mode (0.3-10 keV) spectra fitted jointly
and well (chi^2/nu=799.70/788) with an absorbed powerlaw yields:

Gamma = 1.84+/-0.01
N_H = 1.7+/-0.1 x 10^21 cm^(-2) @ z=0.937 (Vreeswijk et al., GCN 7451), in 
  addition to the expected Galaxy contribution along the line of sight.  Here 
  we assume the solar chemical abundances from Anders & Ebihara (1982).
  This value of N_H implies ~2 mag extinction in the observer frame V band
  assuming the mean Galactic dust+gas properties.

Allowing the photon indices to vary separately: Gamma1=1.84^{+0.1}_{-0.2} and 
Gamma2=1.85+/-0.05, closely consistent.  The fractional possible increase in 
rest-frame N_H between the WT mode data (t=660s to 4.95 ksec) and the PC mode 
data (t=4.95 to 174 ksec) is modest and weakly significant: 
0.74^{+0.43}_{-0.37}%.  I suspect the indicated variations are purely due to 
calibration uncertainties in the WT mode data.  The hard WT mode spectral fit 
in GCN 7459 can be reproduced by relaxing our pileup correction.

Additional evidence disfavors an interpretation of an evolving X-ray
spectrum:

Time resolved fits to the WT mode data show no significant evidence for
spectral evolution in the WT mode data considered alone. See,
http://astro.berkeley.edu/~nat/080319B_spec.jpg

The light curve is a simple (broken) powerlaw for the WT mode data, with
no flaring.  Afterglows with such light curves typically exhibit weak
or no spectral evolution (e.g., Butler & Kocevski 2007, ApJ, 668, 400).
There is no significant variations in the X-ray hardness ratio for this event.
See, http://astro.berkeley.edu/~nat/080319B_hardness.jpg

A fit to BAT data (see also, Cummings et al., GCN 7462) in a time     
region (58s to 303s; chi^2/nu=36.46/55) overlapping the XRT observation has
a roughly consistent powerlaw (Gamma=2.1+/-0.1), and also allows for a
smooth extrapolation in flux between BAT and XRT assuming a simple powerlaw
model connecting the flux in both instruments.

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