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GCN Circular 6337

Subject
GRB 070420: Swift XRT refined analysis
Date
2007-04-20T18:34:49Z (17 years ago)
From
Giulia Stratta at ASDC <stratta@asdc.asi.it>
G. Stratta (ASDC), M. Perri (ASDC), D.N. Burrows (PSU) and
M. Stamatikos (NASA/ORAU) report on behalf of the Swift XRT
team:

We have analyzed the first four orbits of Swift-XRT data on
GRB 070420 (Stamatikos et al. GCN 6330), with a total exposure
of 573 s in Windowed Timing (WT) mode and of 7.5 ks in Photon
Counting (PC) mode. This provides a refined XRT position at
RA,Dec(deg)=121.230194683,-45.5562815181  which is:

RA(J2000)=08h 04m 55.22s
Dec(J2000)=-45d 33' 22.6"

with an estimated error radius of 3.5 arcseconds (90% confidence).
This position is 62.3 arcseconds from the BAT position
(Stamatikos et al. GCN 6330), 0.8 arcseconds from the initial XRT
position (Stamatikos et al. GCN 6330) and 2.6 arcseconds from the
UVOT refined optical counterpart position
(Holland et al. GCN 6336).

The 0.3-10 keV X-ray light curve presents a steep decay between
T+106 s and T+300 s followed by a plateau phase up to about
T+2ks (end of the first orbit). The last three orbits of data
taken in PC mode show a new steepening of the light curve,
from T+5ks to T+18.5 ks (end of the third orbit).
The light curve from T+106 s to T+18.5 ks is well fit by a
double broken power law model with decay indices:
d1=-4.4+/-0.2, d2=-0.2+/-0.1 and d3=-1.5+/-0.1 and with the
first break at about t1=230s and the second break at about
t2=3200s.

The 0.3-10.0 keV X-ray spectrum from the XRT/WT data during the
decay phase is well fit by an absorbed power law with a photon
index of 2.6 +/- 0.1 and a total column density of
NH=(4.1+/-0.3)e21 cm**-2. The X-ray spectrum from the XRT/PC
data during the flat decay phase (corrected from the pileup
effects) can be fit by an absorbed power law with a photon index
of 2.0+/-0.2 and a total column density of
NH=(5.0+/-1.0)e21 cm**-2. We note that the Galactic column
density in the direction of the source is 3.7e21 cm**-2
(Dickey & Lockman 1990).

Assuming the X-ray emission continues to decline with decay index
d3, we predict a 0.3-10.0 keV XRT count rate of 1.3e-2 count/s at
T+24h and 4.7e-3 count/s at T+48h, which corresponds to an
observed 0.3-10.0 keV flux of 7e-13 erg/cm**2/s and
2.5e-13 erg/cm**2/s, respectively.

This circular is an official product of the Swift Team.
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