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GCN Circular 5168

GRB 060526: Swift/XRT Team refined analysis
2006-05-26T22:53:23Z (18 years ago)
Sergio Campana at INAF-OAB <>
S. Campana (INAF-OAB), A. Moretti (INAF-OAB), C. Guidorzi
(Bicocca Univ. & INAF-OAB), G. Chincarini (Bicocca Univ. &
INAF-OAB), D.N. Burrows (PSU) on behalf of the Swift-XRT team

We have analysed the first orbit of Swift/XRT data of GRB 060526
(Campana et al., GCN 5162 and GCN 5163) summing up 1773 s of data
in Photon Counting (PC) and 334 s in Window Timing (WT) mode,
respectively. We find a refined XRT position for this burst of:

RA(J2000)  = 15 31 18.41
Dec(J2000) = +00 17 05.7

with an error of 3.6 arcseconds radius (90% containment).
This position is 2.8 arcseconds from the XRT position quoted by
Campana et al. (GCN 5163) and 1.1 arcseconds from the UVOT position.

The afterglow shows several states: the XRT started observing the
GRB at T+73 s. For the first 53 s (up to T+126 s) the XRT remained
in WT mode with a slowly decaying count rate (power law decay of
-1.4+/-0.9, 90% confidence). Then it switched to PC mode continuing
its slow decay up to T+206 s when a bright flare carry back the
XRT in WT mode. The flare is made of two flares with FRED-like
shape. The first peak reached >335 c/s (being slightly piled-up at
level count rate level) around T+240 s and the second about 200 c/s
around T+295 s. The decay from the first peak is very steep (as
measured from T), with an power law index of -7.7+/-0.7 and also
from the second with -8.6+/-0.2. The second flare shows some
structures close to the peak.
The PC data show the latest stages of the flare and then a break
to a much flatter decay (-0.7+/-0.5) around 600 s (further data
will improve the determination of the time of this break).
This does not allow a first prediction of the flux at one day.

 From the spectral point of view, PC data before and after the flare
are consistent with a simple power law (photon index 1.8+/-0.2) at
a column density consistent with the Galactic value NH<1x10^21 cm^-2
(NH_g=6x10^20 cm^-2).
Flux in the first WT data is 1.7x10^-10 erg s^-1 cm^-2 (unabsorbed
0.3-10 keV) and in the last PC data 1.6x10^-10 erg s^-1 cm^-2.
Considering pre- and post-flare WT and PC data some spectral evolution
is observed, this can be explained with a spectral softening or a
column density decrease.
The spectrum of the first flare (excluded the brightest piled-up
portion) is consistent with a simple power law with photon index
around 1.8 and a column density larger than the values reported above,
likely indicating a fast evolving cut-off power law.
The second flare is much softer with with a power law photon index
around 2.6. Also in this case the column density is slightly larger
than the previous values. Being not piled-up we can estimate a mean
0.3-10 keV unabsorbed flux of 8x10^-9 erg s^-1 cm^-2 (from T+215 to
T+304 s).

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