TITLE: GCN CIRCULAR NUMBER: 32784 SUBJECT: GRB 221016A: Swift-XRT refined Analysis DATE: 22/10/17 15:15:27 GMT FROM: Phil Evans at U of Leicester pae9@leicester.ac.uk
A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , D.N. Burrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU) and K.L. Page report on behalf of the Swift-XRT team:
We have analysed 8.2 ks of XRT data for GRB 221016A (Page et al. GCN Circ. 32774), from 76 s to 44.8 ks after the BAT trigger. The data comprise 83 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 Goad et al. (GCN Circ. 32776).
The light curve can be modelled with an initial power-law decay with an index of alpha=2.20 (+0.22, -0.18), followed by a break at T+254 s to an alpha of 0.78 (+0.04, -0.05).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.49 (+0.23, -0.21). The best-fitting absorption column is 7.9 (+4.2, -3.7) x 10^20 cm^-2, in excess of the Galactic value of 3.5 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.04 (+0.20, -0.19) and a best-fitting absorption column of 1.5 (+0.6, -0.5) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 1.5 (+0.6, -0.5) x 10^21 cm^-2 Galactic foreground: 3.5 x 10^20 cm^-2 Excess significance: 3.5 sigma Photon index: 2.04 (+0.20, -0.19)
If the light curve continues to decay with a power-law decay index of 0.78, the count rate at T+24 hours will be 0.019 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.1 x 10^-13 (8.1 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/01129775.
This circular is an official product of the Swift-XRT team.