GCN Circular 5018

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

We have analyzed the first orbit data of GRB060428A
(Mangano et al 2006, GCN 5014
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 ).
The XRT data set consists of 39 s exposure in Windowed
Timing (WT) mode followed by 1670 s exposure in Photon
Counting (PC) mode.

The refined position of the source is

    RA  (2000) =  08h 14m 10.98s 
    Dec (2000) = -37d 10' 10.3"

with an uncertainty of 3.6 arcsec (90% confinement).
This position is only 1.5 arcsec from the onboard
detected position (Mangano et al 2006, GCN 5014
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 ).

The 0.2-10 keV X-ray light curve shows an initial very
fast decay with slope -6.3 +/- 0.4, a break at 144 +/- 6 s
from the trigger and a flattening with a decay slope
of -0.11 +/- 0.03. A small flare with a gaussian profile
is present at 676 +/- 56 s after the trigger.

The WT and PC spectra were fitted separately with an
absorbed power law model and showed photon indices of
3.2 +/- 0.4 and 1.9 +/- 0.2, respectively (90% confidence level).
The WT spectrum, covering the time interval 77-116 s, mostly
accounts for the pre-break phase of the light curve.

Both spectra are highly absorbed, but do not show evidence of
an absorption column in excess with respect to the Galactic
value of 8.5 x 10^21 cm^-2.
The 0.2-10 keV unabsorbed fluxes of the two spectra are
2.9 x 10^-9 and 7.6 x 10^-11 erg cm^-2 s^-1, respectively.

If decaying at the present rate the source will be at the
flux level of 2.7 x 10^-11 erg cm^-2 s^-1 (corresponding to
a count rate level of 0.18 counts s^-1) after one day.

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