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GCN Circular 22685

Subject
GRB 180504A: Swift-XRT refined Analysis
Date
2018-05-05T08:09:42Z (6 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), S. J. LaPorte (PSU), J.A.
Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), K.L. Page (U. Leicester),
A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), A. Melandri
(INAF-OAB) and A.Y. Lien report on behalf of the Swift-XRT team:

We have analysed 12 ks of XRT data for GRB 180504A (Lien et al. GCN
Circ. 22682), from 98 s to 30.5 ks after the  BAT trigger. The data
comprise 163 s in Windowed Timing (WT) mode (the first 8 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The best available XRT position  (using the promptly downlinked
event data, the XRT-UVOT alignment and matching UVOT field sources to
the USNO-B1 catalogue) is RA, Dec = 331.1442, -14.6583 which is
equivalent to:

RA (J2000): 22 04 34.61
Dec(J2000): -14 39 30.0

with an uncertainty of 2.0 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.7 (+nan, -nan).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.80 (+/-0.11). The
best-fitting absorption column is  6.9 (+3.2, -2.8) x 10^20 cm^-2, in
excess of the Galactic value of 3.5 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.86 (+0.25, -0.23)
and a best-fitting absorption column of 1.5 (+0.8, -0.7) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.7 x 10^-11 (4.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.5 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 3.5 x 10^20 cm^-2
Excess significance: 2.7 sigma
Photon index:	     1.86 (+0.25, -0.23)

If the light curve continues to decay with a power-law decay index of
0.7, the count rate at T+24 hours will be 7.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x
10^-13 (3.4 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/00830822.

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