GCN Circular 6659
Subject
GRB 070724: Swift-XRT Team refined analysis
Event
Date
2007-07-24T17:29:58Z (18 years ago)
From
Kim Page at U.of Leicester <kpa@star.le.ac.uk>
K.L Page (U. Leicester) and H. Ziaeepour (UCL-MSSL) report on behalf of
the Swift-XRT team:
We have analysed the first three orbits of Swift-XRT data obtained for GRB
070724 (Ziaeepour et al., GCN Circ. 6654), which includes ~60 s of
Windowed Timing (WT) mode data and ~5.2 ks of Photon Counting (PC) data.
Using 399 s of overlapping XRT PC mode and UVOT V-band data, we find an
astrometrically corrected X-ray position (using the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue) of RA, Dec (J2000) =
27.80815, -18.59448 which is equivalent to:
RA (J2000): 01 51 13.96
Dec (J2000): -18 35 40.1
with an uncertainty of 2.2 arcsec (radius, 90% confidence). This is 2.7
arcsec from the initial XRT position (GCN Circ. 6654) and 77.8 arcsec from
the ground-calculated BAT position (Parsons et al., GCN Circ. 6656).
The first orbit of XRT data shows flaring activity, with the underlying
power-law decay asserting itself from about 500 seconds after the trigger.
This decay slope is poorly constrained with the data currently available:
alpha ~ 0.4 +0.3/-0.2
Fitting the PC data from the second and third orbits (~4-12 ks after the
trigger) in order to avoid any spectral evolution during the flares, the
spectrum can be modelled by a power-law of Gamma = 1.3 +0.8/-0.6, absorbed
by the Galactic column in this direction of 1.43e20 cm^-2. The 0.3-10 keV
observed (unabsorbed) flux during this time is 5.18e-13 (5.27e-13) erg
cm^-2 s^-1.
If the decay continues with a slope of alpha ~ 0.4, the count rate at 24
hours is predicted to be 4e-3 count s^-1; this ranges between 2e-3 and
8e-3 count s^-1, taking into account the uncertainties on the slope. Using
the spectral fit detailed above, this corresponds to an observed flux of
(2 +7/-1)e-13 erg cm^-2 s^-1 (over 0.3-10 keV).
This circular is an official product of the XRT team.