GCN Circular 3669
Subject
GRB 050724: early Swift XRT analysis results
Date
2005-07-24T18:30:47Z (19 years ago)
From
Pat Romano at OAB-Swift <romano@merate.mi.astro.it>
P. Romano, A. Moretti, S. Covino (INAF-OAB), L.A. Antonelli(IANF-OAR),
D.N. Burrows, D. Grupe (PSU), M. Chester (PSU), G. Chincarini, G.
Tagliaferri (INAF-OAB), P. Boyd (GSFC-UMBC) on behalf of the Swift-XRT
Team report.
We have analyzed the Swift XRT data from the first orbit observation of
GRB 050724 (Covino et al., GCN 3665). The new refined coordinates are:
RA(J2000) = 16h 24m 45s Dec(J2000) = -27 32 25.2. This position is 65
arcseconds from the refined BAT position given in GCN 3667 (H. Krimm et
al), and 9 arcseconds from the XRT position given in GCN 3665. We
estimate an uncertainty of 6.3 arcseconds radius (90% containment).
However, we note that there is accumulating evidence of a time-dependent
systematic shift in XRT positions derived from ground-processed data
towards lower declinations than the optical counterparts. This effect is
being investigated but is not yet understood. Extrapolation of earlier
positional errors suggests that the correct position could be
approximately 7 arcseconds north of the position given above, which
would be close to the on-board position given in GCN 3665.
A preliminary spectral fit (simple absorbed power-law) to the WT data
yields a photon index of 1.93-/+0.05 in the [0.5-10] keV band. The
derived NH is (5.7)E21 cm^-2, which is higher than the Galactic value
(1.46E+21 cm-2; Dickey & Lockman 1990). The average (79-342 seconds from
trigger) estimated unabsorbed 0.5-10 keV flux is 5.3E-9 ergs/s/cm2.
The light curves in Windowed Timing (WT) and Photon Counting (PC) mode
start 79 and 343 seconds from the BAT trigger (T0), and they show a
fading afterglow which can be fitted with a broken power law of slopes
-3.2+/-0.1 and -0.8+/-0.2. The unabsorbed 0.5-10.0 keV flux at 24 hours
after the burst is then estimated to 9E-14 ergs/s/cm2.