GCN Circular 9107
Subject
GRB 090407: Swift-XRT refined analysis
Date
2009-04-08T00:10:35Z (15 years ago)
From
Andy Beardmore at U Leicester <apb@star.le.ac.uk>
A.P. Beardmore (U. Leicester) and H. Ziaeepour (UCL-MSSL) report on
behalf of the Swift-XRT team:
We have analysed the first five orbits of Swift-XRT data obtained from
GRB 090407 (trigger #348650; Ziaeepour, et al., GCN Circ. 9101),
comprising 273 s taken in Windowed Timing (WT) mode, starting at
T+96.7 s, followed by 10.2 ks in Photon Counting (PC) mode from
T+398 s to T+24.2 ks. The UVOT-enhanced position was given by Evans
et al. (GCN Circ. 9105).
The 0.3-10 keV X-ray light curve shows a number of flares in the first
400 s, the largest occurring from T+120 s to T+180 s, reaching a peak
rate of 90 count s^-1 at T+140 s. This flare is also seen by the
Swift-BAT (Ukwatta et al., GCN Circ. 9104). Further flaring is seen
from ~T+200s to ~T+350 s, ending with a steep decay of slope
~4.0. After T+950 s the X-ray light curve breaks to a much shallower
decay slope of 0.28 +/- 0.06.
The data show spectral evolution during the flaring intervals, after
which the PC mode spectrum can be modelled by an absorbed powerlaw
with a photon index of 2.43 +/- 0.13 and a total absorbing column
density of (2.51 +/- 0.35) x 10^21 cm^-2, which is in excess of the
Galactic value of 3.7 x 10^20 cm^-2 (Kalberla et al. 2005) in the
direction of the burst. The observed 0.3-10 keV flux at this time
(~T+6.2 ks) is (6.29 +0.41 -0.56) x 10^-12 erg cm^-2 s^-1, which
corresponds to an unabsorbed flux of 1.3 x 10^-11 erg cm^-2 s^-1. The
count to observed flux conversion factor for this spectrum is
3.5 x 10^-11 erg cm-^2 count^-1.
If the light curve continues to decay with the same slope of 0.28 we
predict an X-ray count rate of 0.047 count s^-1 at T+24 hours after
the trigger, which corresponds to an observed 0.3-10 keV flux of
1.6 x 10^-12 erg cm^-2 s^-1. However, we note that it is unlikely
the light curve decay will remain this shallow at T+24 hours.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_curves/00348650.
This circular is an official product of the Swift-XRT team.