TITLE: GCN CIRCULAR NUMBER: 9451 SUBJECT: GRB090530: Swift XRT refined analysis DATE: 09/05/30 21:09:24 GMT FROM: Vanessa Mangano at INAF-IASFPA Mangano V., Sbarufatti B. (INAF-IASFPA), J. K. Cannizzo (NASA/UMBC) report on behalf of the Swift XRT Team: We have analyzed the first five orbits of Swift-XRT data of GRB 090530 (trigger 353567; Cannizzo, et al., GCN Circ. 9438), consisting of 22 s in Windowed Timing (WT) mode from T+81 to T+103 s and 9 ks in Photon Counting (PC) mode from T+105 s to T+24.4 ks. The best position for the X-ray afterglow is the XRT UVOT-enhanced position: RA, Dec = 179.41846, +26.59341 which is equivalent to: RA (J2000): 11h 57m 40.43s Dec (J2000): +26d 35' 36.3" with an uncertainty of 1.5 arcsec (radius, 90% confidence), as given by Beardmore, et al., GCN Circ 9445. The 0.3-10 keV X-ray light curve is best fitted by a broken powerlaw with early decay index of about -5.7, late decay index -0.60+/-0.04 and a break at about 140 s after the trigger. In the first orbit a small amplitude flare is detected at T+300 s. Flaring activity is also visible along the later decay. If decaying at the present rate, the predicted rate after 24h from the trigger is 2.0e-2 counts/s. However, we remark that a second break followed by a steepening of the lightcurve is possibly expected. The issue will be clarified with following Swift observations that will resume on Sunday May 31st. The average initial WT spectrum (covering the initial steep decay) can be modeled as an absorbed power-law with index 2.3 (+0.5-0.4), absorbing column NH < 11E20 cm-2 (three sigma upper limit) and observed(unabsorbed) average flux in the 0.3-10 keV energy range of 2.9(3.6)E-10 ergs cm-2 s-1. The average PC spectrum (roughly covering the flatter part of the afterglow) is best fitted by an absorbed power-law with index 2.00+/-0.15 The absorbing column is NH = (8.2 -2.9+3.6)E20 cm-2 in excess with respect to the Galactic value of 1.78E+20 cm-2 (Kalberla et al. 2005). The average observed(unabsorbed) flux is 4.0(4.9)E-12 ergs cm-2 s-1. The count-rate to flux conversion factor is 5.4E-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/00353567. This circular is an official product of the Swift-XRT team.