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GCN Circular 23125

Subject
GRB 180809B: Swift-XRT refined Analysis
Date
2018-08-10T20:40:03Z (6 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
Z. Liu (NAOC / U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC), A. Tohuvavohu (PSU), S. J. LaPorte (PSU),
J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester) and M.J. Moss report on behalf of the Swift-XRT team:

We have analysed 3.9 ks of XRT data for GRB 180809B (Moss et al. GCN
Circ. 23105), from 74 s to 12.6 ks after the  BAT trigger. The data
comprise 1.7 ks in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 1724 s of PC mode data and 4 UVOT
images, we find an enhanced XRT position (using the XRT-UVOT alignment
and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
299.69997, -15.29893 which is equivalent to:

RA (J2000): 19h 58m 47.99s
Dec(J2000): -15d 17' 56.1"

with an uncertainty of 1.4 arcsec (radius, 90% confidence).

The late-time light curve (from T0+4.7 ks) can be modelled with a
power-law decay with a decay index of alpha=1.37 (+/-0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.609 (+/-0.017). The
best-fitting absorption column is  2.29 (+0.09, -0.08) x 10^21 cm^-2,
in excess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.63 (+/-0.11) and a
best-fitting absorption column of 2.4 (+/-0.5) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.6 x 10^-11 (5.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 4.0 sigma
Photon index:	     1.63 (+/-0.11)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00852553.

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