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GCN Circular 27594

Subject
GRB 200416A: Swift-XRT refined Analysis
Date
2020-04-16T15:03:26Z (4 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
P. A. Evans (U Leicester) reports on behalf of the Swift-XRT team:

We have analysed 677 s of XRT data for GRB 200416A (Evans et al. GCN
Circ. 27591), from 84 s to 773 s after the  BAT trigger. The data
comprise 43 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 refined XRT position is RA, Dec = 335.6986, -7.5179 which is
equivalent to:

RA (J2000): 22 22 47.66
Dec(J2000): -07 31 04.3

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.9 (+/-0.5), followed by a break at T+167 s to an alpha
of 0.25 (+0.18, -0.25).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.86 (+0.22, -0.20). The
best-fitting absorption column is  9.6 (+6.3, -3.1) x 10^20 cm^-2,
consistent with the Galactic value of 6.5 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 3.6 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.6 (+6.3, -3.1) x 10^20 cm^-2
Galactic foreground: 6.5 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.86 (+0.22, -0.20)

The GRB entered Swift's Moon-proximity observing constraint shortly
after the trigger, and will not be observable until early on Monday
April 20th; observations are planned for that time. The light curve
appears to be in the plateau phase common to X-ray afterglows and is
expected to break to a steeper decay at some point within the next ~24
hours, so we do not attempt to predict the flux at T0+24 hours.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00966554.

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