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
Not found
 
 
).
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
Not found
 
 
) and
1.4 arcsec from the optical afterglow candidate found by
Price et al 2006 (GCN 5019
Not found
 
 
). To evaluete this offset
we used the KAIT coordinates from Li et al.(GCN 5027
Not found
 
 
).

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.