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GCN Circular 6675

Subject
GRB 070724B: Swift-XRT afterglow likely candidate
Date
2007-07-27T17:52:01Z (17 years ago)
From
Pat Romano at OAB-Swift <patrizia.romano@brera.inaf.it>
P. Romano, C. Guidorzi(Uni-Bicocca and OAB), A. Moretti(OAB),
M. Chester(PSU) report on behalf of the Swift-XRT team:

We have analysed all downlinked Swift-XRT data obtained for
GRB 070724B detected by SuperAGILE (Feroci et al., GCN Circ.
6668), for a total of 7973s of Photon Counting (PC) mode data,
beginning on 2007-07-25 18:32:39 UT.
Of the two sources proposed in Romano et al. (GCN Circ. 6669),
we can exclude number 2 (RA, Dec= 17.51024, 57.81898) as an
afterglow candidate since no fading is found in X-ray data
and there is a catalogued V=11.6 star within the XRT error
circle (2.9 arcsec away).

We propose source number 1 as the most likely afterglow candidate.
The Photon Counting mode image provides a refined XRT
position at RA,DEC(J2000)=17.48497, 57.67616, which is

RA(J2000)  =  01 09 56.39
Dec(J2000) = +57 40 34.2

with an uncertainty of 4.0 arcsec (radius, 90% containment).
This is 1.5 arcsec from the initial X-ray position (Romano et
al., GNC Circ. 6669) and 4.6  arcmin from the initial SuperAGILE
position (Feroci et al., GCN Circ. 6668).
The light curve exhibits a fading behaviour, with a power-law
slope of 1.1+/-0.3.

The spectrum (150 photons) can be fit using Cash statistics
with an absorbed power law with a photon index of 2.6+/-0.5
and a column density of (9+/-3)E21 cm^-2 in excess
of the Galactic value (3.1E21 cm^-2; Kalberla et al. 2005).
The absorbed (unabsorbed) 0.3-10.0keV flux for the spectrum is
1.2E-12 (4.4E-12) ergs cm^-2 s^-1, at mean time of
T+1.54d.

Since the second set of XRT data were taken with an offset with
respect to the first set (GCN Circ. 6669), more of the
SuperAGILE error circle was covered. A new, weak source was
found (Source number 3) at RA,DEC(J2000)=17.24912, 57.79786, or
RA(J2000)  =  01 08 59.79,
Dec(J2000) = +57 47 52.3
with an uncertainty of 6.0 arcsec (radius, 90% containment).
This position lies 14.2 arcmin from the SuperAGILE position
(GCN Circ. 6668). The current observed 0.3-10 keV count rate is
(5.3+/-1.6)E-03 counts/s, which translates into a 0.3-10 keV flux
of ~2.6E-13 erg/cm2/s (assuming a power-law with photon index=2
absorbed by a column of NH=E21 cm-2).

Further observations with Swift are planned.

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