TITLE: GCN CIRCULAR NUMBER: 27331 SUBJECT: GRB 200306A: Swift-XRT refined Analysis DATE: 20/03/07 00:38:06 GMT FROM: Phil Evans at U of Leicester J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (ASDC), B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), A.P. Beardmore (U. Leicester) and K.K. Simpson report on behalf of the Swift-XRT team: We have analysed 2.1 ks of XRT data for GRB 200306A (Simpson et al. GCN Circ. 27320), from 66 s to 6.2 ks after the BAT trigger. The data comprise 186 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. Using 2051 s of PC mode data and 6 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 101.10233, -62.00654 which is equivalent to: RA (J2000): 06h 44m 24.56s Dec(J2000): -62d 00' 23.5" with an uncertainty of 1.5 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=4.05 (+/-0.21). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.93 (+/-0.14). The best-fitting absorption column is 5.2 (+/-0.5) x 10^21 cm^-2, in excess of the Galactic value of 8.6 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.89 (+0.19, -0.18) and a best-fitting absorption column of 4.3 (+1.1, -0.9) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.2 x 10^-11 (6.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.3 (+1.1, -0.9) x 10^21 cm^-2 Galactic foreground: 8.6 x 10^20 cm^-2 Excess significance: 6.0 sigma Photon index: 1.89 (+0.19, -0.18) If the light curve continues to decay with a power-law decay index of 4.05, the count rate at T+24 hours will be 3.6 x 10^-11 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x 10^-21 (2.3 x 10^-21) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00959917. This circular is an official product of the Swift-XRT team.