GCN Circular 9163

Mangano V., Sbarufatti B. (INAF-IASFPA) report on behalf of the
Swift XRT Team:

We have analysed the first four orbits of Swift-XRT data obtained from
GRB 090418 (trigger 349510; Mangano, et al., GCN Circ. 9149
Loading...
 
 ),
comprising 116 s taken in Windowed Timing (WT) mode, from T+102 s to
T+218 s, and a total exposure of 8.5 ks in Photon Counting (PC)
mode from T+219 s to the end of the observation at T+18 ks.

The best position of the X-ray afterglow is the UVOT enhanced XRT
position given in Goad, et al., GCN Circ. 9154
Loading...
 
 , that is
RA, Dec = 269.31329, +33.40607 which is equivalent to:

RA (J2000): 17h 57m 15.19s
Dec (J2000): +33d 24' 21.8"

with an uncertainty of 1.7 arcsec (radius, 90% confidence).

The 0.3-10 keV X-ray light curve is well fitted by a doubly broken
pawer law, with a initial decay slope alpha1=1.7+/-0.1,
a first breal at about T+245 s, an intermediate decay slope
alpha2=0.35 -0.1+0.06, a second break at about T+2 ks and
a final decay slope alpha3=1.39+/-0.06
If decaying at this rate, the afterglow will reach a count-rate of
6.1E-3 counts/s at T+48h.

The average WT spectrum (corresponding to the initial steep decay phase)
is best fitted by an absorbed power-law model, with photon index 2.0+/-0.2,
and intrinsic NH=(6+/-3)E21 cm-2 (at the redshift z=1.608,
Chornock, et al., GCN circ. 9151
Loading...
 
 ) in excess with respect to the galactic
absorption value of 3.6E20 cm-2 (Kalberla et al. 2005).
The average observed(unabsorbed) flux in the 0.3-10 keV band is
3.5(4.6)E-10 ergs cm-2 s-1.
The average PC spectrum of the first orbit (covering the intermediate
slowly decaying phase) is well fitted by an absorbed power-law model,
with photon index 2.1+/-0.1, intrinsic NH=(1.2+/0.3)E22 cm-2
and average observed(unabsorbed) flux in the 0.3-10 keV band of
1.5(2.2)E-10 ergs cm-2 s-1.
Finally, the average PC spectrum from T+4.2 ks to T+18 ks (orbits 2-4)
is well fitted by an absorbed power-law model,
with photon index 2.0+/-0.1, intrinsic NH=(1.1+/0.2)E22 cm-2
and average observed(unabsorbed) flux in the 0.3-10 keV band of
1.3(1.8)E-11 ergs cm-2 s-1.
The average count-rate to observed(unabsorbed) flux conversion factor
is 4.5(6.5)E-11.
All quoted errors are at 90% confidence level.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00349510.

This circular is an official product of the Swift-XRT team.