GCN Circular 5031

E. Troja (INAF-IASFPA), V. Mangano (INAF-IASFPA),
P. Romano (INAF-OAB), D. N. Burrows (PSU)
report on behalf of the Swift XRT Team:


We have analyzed the XRT data of the first four orbits of
GRB060428B (Campana et al 2006, GCN 5017
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 ).
The data set consists of 205 s exposure in Windowed
Timing (WT) mode followed by  8.4 ks exposure in Photon
Counting (PC) mode.

The refined position of the source is

     RA (2000) = 15h 41m 25.57s
     Dec (2000) = +62d 01' 28.94"

with an uncertainty of 3.8 arcsec (90% confinement).
This position was evaluated from PC data of the orbits 
2-4,
that are not affected by pile-up.
This position is 3.35 arcsec from the onboard
detected position (Campana et al 2006, GCN 5017
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 ) and
1.4 arcsec from the optical afterglow candidate found by
Price et al 2006 (GCN 5019
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 ). To evaluete this offset
we used the KAIT coordinates from Li et al.(GCN 5027
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 ).

The 0.2-10 keV X-ray light curve shows an initial very
fast decay with slope -4.4 +/- 0.07, a break at 689 +/- 28 
s
from the trigger and then a decay slope of -0.89 +/- 0.06.

The WT spectrum from the first orbit was fitted with an
absorbed power law model and showed a photon index of
2.82 +/- 0.08 (90% confidence level).
The WT spectrum covers the 212-418 s time interval.
The spectrum shows evidence of an absorption column at the
level of (6.7 +/- 0.7) x 10^20 cm^-2 in excess with 
respect
to the Galactic value of 1.7 x 10^20 cm^-2.
The 0.2-10 keV unabsorbed flux is 9.8 x 10^-10 ergs cm^-2 
s^-1.

The PC spectrum of the orbits 2-4 (time interval 4.4-18.5 
ks) is well
fit by an absorbed powerlaw model with photon index 2.08 
+/- 0.16.
The 0.2-10 keV unabsorbed flux is 1.6^-12 ergs cm^-2 s^-1.


If decaying at the present rate the source will reach the
flux level of  2.6^-13 erg cm^-2 s^-1 (corresponding to
a count rate level of 5 x 10^-3 counts s^-1)
after one day.

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