TITLE: GCN CIRCULAR NUMBER: 6675 SUBJECT: GRB 070724B: Swift-XRT afterglow likely candidate DATE: 07/07/27 17:52:01 GMT FROM: Pat Romano at OAB-Swift 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.