TITLE: GCN CIRCULAR NUMBER: 17630 SUBJECT: GRB 150323C: Swift-XRT refined Analysis DATE: 15/03/24 05:15:04 GMT FROM: Phil Evans at U of Leicester V. D'Elia (ASDC), M. de Pasquale (INAF-IASFPA), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), A. Maselli (INAF-IASFPA) and A. Amaral-Rogers report on behalf of the Swift-XRT team: We have analysed 6.1 ks of XRT data for GRB 150323C (Amaral-Rogers et al. GCN Circ. 17621), from 160 s to 12.6 ks after the BAT trigger. The data comprise 312 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 best available XRT position (using the promptly downlinked event data, the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 192.6169, 50.1918 which is equivalent to: RA (J2000): 12 50 28.05 Dec(J2000): +50 11 30.4 with an uncertainty of 2.1 arcsec (radius, 90% confidence). The late-time light curve (from T0+5.1 ks) can be modelled with a power-law decay with a decay index of alpha=1.0 (+/-0.5). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.34 (+/-0.05). The best-fitting absorption column is 1.55 (+0.13, -0.12) x 10^21 cm^-2, in excess of the Galactic value of 1.2 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.52 (+0.25, -0.23) and a best-fitting absorption column of 6.5 (+4.5, -3.9) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.7 x 10^-11 (3.5 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6.5 (+4.5, -3.9) x 10^20 cm^-2 Galactic foreground: 1.2 x 10^20 cm^-2 Excess significance: 2.2 sigma Photon index: 2.52 (+0.25, -0.23) If the light curve continues to decay with a power-law decay index of 1.0, the count rate at T+24 hours will be 3.7 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.8 x 10^-14 (1.3 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00636005. This circular is an official product of the Swift-XRT team.