TITLE: GCN CIRCULAR NUMBER: 12028 SUBJECT: GRB 110520A: Swift-XRT refined Analysis DATE: 11/05/21 09:08:51 GMT FROM: Andy Beardmore at U Leicester A.P. Beardmore (U. Leicester) and M. De Pasquale (UCL-MSSL) report on behalf of the Swift-XRT team: We have analysed 11 ks of XRT data for GRB 110520A (De Pasquale et al. GCN Circ. 12020), from 84 s to 25.2 ks after the BAT trigger. The data comprise 74 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN. Circ 12027). The light curve initially follows a power-law decline with decay index alpha=2.0, then shows a small flare from T+137 s to T+516 s, reaching a peak count rate of 12 c/s at T+254 s, after which it decays with an index of 1.42 (+0.12, -0.11). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.77 (+/-0.17). The best-fitting absorption column is 2.9 (+/-0.6) x 10^21 cm^-2, in excess of the Galactic value of 2.7 x 10^20 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.0 x 10^-11 (7.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.9 (+/-0.6) x 10^21 cm^-2 Galactic foreground: 2.7 x 10^20 cm^-2 Excess significance: 7.4 sigma Photon index: 1.77 (+/-0.17) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00453747. This circular is an official product of the Swift-XRT team.