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GCN Circular 33185

Subject
GRB 230116D: Swift-XRT refined Analysis
Date
2023-01-17T10:52:45Z (a year ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
D.N. Burrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester),
V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), A. D'Ai
(INAF-IASFPA) and E. Sonbas report on behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 230116D (Sonbas et al. GCN
Circ. 33176), from 113 s to 40.1 ks after the  BAT trigger. The data
comprise 47 s in Windowed Timing (WT) mode (the first 10 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 Osborne et
al. (GCN Circ. 33180).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.87 (+0.28, -0.19). At T+559 s  the decay
flattens to an alpha of 0.72 (+0.10, -0.28) before breaking again at
T+11.3 ks to a final decay with index alpha=2.4 (+2.5, -0.8).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.75 (+0.20, -0.09). The
best-fitting absorption column is  consistent with the Galactic value
of 1.6 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 3.9 x 10^-11 (4.8 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 1.6 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.75 (+0.20, -0.09)

If the light curve continues to decay with a power-law decay index of
2.4, the count rate at T+24 hours will be 5.1 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.0 x
10^-14 (2.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01149293.

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