[vsnet-grb-info 20886] GRB 180720A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22984
SUBJECT: GRB 180720A: Swift-XRT refined Analysis
DATE:    18/07/21 02:17:13 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia
(ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), A. Tohuvavohu
(PSU), S. J. LaPorte (PSU), J.A. Kennea (PSU), P.A. Evans (U.
Leicester) and M.H. Siegel report on behalf of the Swift-XRT team:

We have analysed 7.2 ks of XRT data for GRB 180720A (Siegel et al. GCN
Circ. 22973), from 90 s to 19.4 ks after the  BAT trigger. The data
comprise 3.3 ks in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The refined XRT position is RA, Dec =
0.5286, -2.9189 which is equivalent to:

RA (J2000): 00 02 06.86
Dec(J2000): -02 55 08.1

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

The late-time light curve (from T0+5.6 ks) can be modelled with an
initial power-law decay with an index of alpha=1.18 (+0.16, -0.28),
followed by a break at T+7030 s to an alpha of -0.17 (+0.26, -1.33).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.770 (+/-0.011). The
best-fitting absorption column is  1.42 (+/-0.04) x 10^21 cm^-2, in
excess of the Galactic value of 3.9 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.85 (+/-0.07) and a
best-fitting absorption column of 1.75 (+0.23, -0.22) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.8 x 10^-11 (4.9 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.75 (+0.23, -0.22) x 10^21 cm^-2
Galactic foreground: 3.9 x 10^20 cm^-2
Excess significance: 10.1 sigma
Photon index:	     1.85 (+/-0.07)

If the light curve continues to decay with a power-law decay index of
-0.17, the count rate at T+24 hours will be 21 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.4 x
10^-10 (1.1 x 10^-9) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00848890.

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