TITLE: GCN CIRCULAR NUMBER: 9163 SUBJECT: GRB 090418: Swift XRT refined analysis DATE: 09/04/19 16:55:19 GMT FROM: Vanessa Mangano at INAF-IASFPA Mangano V., Sbarufatti B. (INAF-IASFPA) report on behalf of the Swift XRT Team: We have analysed the first four orbits of Swift-XRT data obtained from GRB 090418 (trigger 349510; Mangano, et al., GCN Circ. 9149), comprising 116 s taken in Windowed Timing (WT) mode, from T+102 s to T+218 s, and a total exposure of 8.5 ks in Photon Counting (PC) mode from T+219 s to the end of the observation at T+18 ks. The best position of the X-ray afterglow is the UVOT enhanced XRT position given in Goad, et al., GCN Circ. 9154, that is RA, Dec = 269.31329, +33.40607 which is equivalent to: RA (J2000): 17h 57m 15.19s Dec (J2000): +33d 24' 21.8" with an uncertainty of 1.7 arcsec (radius, 90% confidence). The 0.3-10 keV X-ray light curve is well fitted by a doubly broken pawer law, with a initial decay slope alpha1=1.7+/-0.1, a first breal at about T+245 s, an intermediate decay slope alpha2=0.35 -0.1+0.06, a second break at about T+2 ks and a final decay slope alpha3=1.39+/-0.06 If decaying at this rate, the afterglow will reach a count-rate of 6.1E-3 counts/s at T+48h. The average WT spectrum (corresponding to the initial steep decay phase) is best fitted by an absorbed power-law model, with photon index 2.0+/-0.2, and intrinsic NH=(6+/-3)E21 cm-2 (at the redshift z=1.608, Chornock, et al., GCN circ. 9151) in excess with respect to the galactic absorption value of 3.6E20 cm-2 (Kalberla et al. 2005). The average observed(unabsorbed) flux in the 0.3-10 keV band is 3.5(4.6)E-10 ergs cm-2 s-1. The average PC spectrum of the first orbit (covering the intermediate slowly decaying phase) is well fitted by an absorbed power-law model, with photon index 2.1+/-0.1, intrinsic NH=(1.2+/0.3)E22 cm-2 and average observed(unabsorbed) flux in the 0.3-10 keV band of 1.5(2.2)E-10 ergs cm-2 s-1. Finally, the average PC spectrum from T+4.2 ks to T+18 ks (orbits 2-4) is well fitted by an absorbed power-law model, with photon index 2.0+/-0.1, intrinsic NH=(1.1+/0.2)E22 cm-2 and average observed(unabsorbed) flux in the 0.3-10 keV band of 1.3(1.8)E-11 ergs cm-2 s-1. The average count-rate to observed(unabsorbed) flux conversion factor is 4.5(6.5)E-11. All quoted errors are at 90% confidence level. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00349510. This circular is an official product of the Swift-XRT team.