GCN Circular 27594
P. A. Evans (U Leicester) reports on behalf of the Swift-XRT team: We have analysed 677 s of XRT data for GRB 200416A (Evans et al. GCN Circ. 27591), from 84 s to 773 s after the BAT trigger. The data comprise 43 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 refined XRT position is RA, Dec = 335.6986, -7.5179 which is equivalent to: RA (J2000): 22 22 47.66 Dec(J2000): -07 31 04.3 with an uncertainty of 3.5 arcsec (radius, 90% confidence). The light curve can be modelled with an initial power-law decay with an index of alpha=2.9 (+/-0.5), followed by a break at T+167 s to an alpha of 0.25 (+0.18, -0.25). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.86 (+0.22, -0.20). The best-fitting absorption column is 9.6 (+6.3, -3.1) x 10^20 cm^-2, consistent with the Galactic value of 6.5 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 9.6 (+6.3, -3.1) x 10^20 cm^-2 Galactic foreground: 6.5 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.86 (+0.22, -0.20) The GRB entered Swift's Moon-proximity observing constraint shortly after the trigger, and will not be observable until early on Monday April 20th; observations are planned for that time. The light curve appears to be in the plateau phase common to X-ray afterglows and is expected to break to a steeper decay at some point within the next ~24 hours, so we do not attempt to predict the flux at T0+24 hours. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00966554. This circular is an official product of the Swift-XRT team.