GCN Circular 23285
Subject
GRB 180930A: Swift-XRT refined Analysis
Event
Date
2018-10-01T13:02:03Z (7 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), P.A. Evans (U. Leicester), V. D'Elia (ASDC), A. D'Ai
(INAF-IASFPA), A. Melandri (INAF-OAB), S. J. LaPorte (PSU) and A.
Tohuvavohu report on behalf of the Swift-XRT team:
We have analysed 2.6 ks of XRT data for GRB 180930A (Tohuvavohu et al.
GCN Circ. 23279), from 212 s to 6.0 ks after the BAT trigger. The data
comprise 47 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Osborne et al. (GCN Circ. 23280).
The light curve can be modelled with a power-law decay with a decay
index of alpha=2.04 (+/-0.09).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.01 (+/-0.15). The
best-fitting absorption column is 7.2 (+3.7, -3.3) x 10^20 cm^-2, in
excess of the Galactic value of 2.1 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 3.3 x 10^-11 (3.9 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 7.2 (+3.7, -3.3) x 10^20 cm^-2
Galactic foreground: 2.1 x 10^20 cm^-2
Excess significance: 2.6 sigma
Photon index: 2.01 (+/-0.15)
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 3.5 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-15 (1.4 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/00864584.
This circular is an official product of the Swift-XRT team.