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
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.