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GCN Circular 22560

Subject
GRB 180329B: Swift-XRT refined Analysis
Date
2018-03-29T21:14:48Z (6 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), S. J.
LaPorte (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), A.P.
Beardmore (U. Leicester), P.A. Evans (U. Leicester), S.L. Gibson (U.
Leicester) and J.L. Racusin report on behalf of the Swift-XRT team:

We have analysed 7.4 ks of XRT data for GRB 180329B (Racusin et al. GCN
Circ. 22558), from 109 s to 17.7 ks after the  BAT trigger. The data
comprise 260 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Beardmore et al. (GCN Circ. 22559).

The late-time light curve (from T0+4.6 ks) can be modelled with a
power-law decay with a decay index of alpha=1.14 (+/-0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.54 (+/-0.04). The
best-fitting absorption column is  7.5 (+1.4, -1.3) x 10^20 cm^-2, in
excess of the Galactic value of 2.5 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.86 (+/-0.11) and a
best-fitting absorption column of 6.5 (+2.8, -2.6) x 10^20 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.5 (+2.8, -2.6) x 10^20 cm^-2
Galactic foreground: 2.5 x 10^20 cm^-2
Excess significance: 2.6 sigma
Photon index:	     1.86 (+/-0.11)

If the light curve continues to decay with a power-law decay index of
1.14, the count rate at T+24 hours will be 0.012 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.4 x
10^-13 (5.0 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/00819490.

This circular is an official product of the Swift-XRT team.
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