TITLE: GCN CIRCULAR NUMBER: 32899 SUBJECT: GRB 221028A: Swift-XRT refined Analysis DATE: 22/10/31 07:29:01 GMT FROM: Phil Evans at U of Leicester pae9@leicester.ac.uk
V. D'Elia (SSDC & INAF-OAR), D.N. Burrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), B. Sbarufatti (INAF-OAB), A. D'Ai (INAF-IASFPA) and A.Y. Lien report on behalf of the Swift-XRT team:
We have analysed 5.6 ks of XRT data for GRB 221028A (Lien et al. GCN Circ. 32874), from 92 s to 28.1 ks after the BAT trigger. The data comprise 23 s in Windowed Timing (WT) mode (the first 2 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 314.2027, +41.0898 which is equivalent to:
RA (J2000): 20 56 48.64 Dec(J2000): +41 05 23.3
with an uncertainty of 3.5 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay index of alpha=2.04 (+0.13, -0.12).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.5 (+0.5, -0.4). The best-fitting absorption column is 2.4 (+1.0, -0.8) x 10^22 cm^-2, in excess of the Galactic value of 7.8 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 8.2 x 10^-11 (1.3 x 10^-10) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 2.4 (+1.0, -0.8) x 10^22 cm^-2 Galactic foreground: 7.8 x 10^21 cm^-2 Excess significance: 3.3 sigma Photon index: 1.5 (+0.5, -0.4)
If the light curve continues to decay with a power-law decay index of 2.04, the count rate at T+24 hours will be 8.3 x 10^-6 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.8 x 10^-16 (1.1 x 10^-15) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01131910.
This circular is an official product of the Swift-XRT team.