[vsnet-grb-info 20719] GRB 180620B: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22817
SUBJECT: GRB 180620B: Swift-XRT refined Analysis
DATE:    18/06/21 10:04:12 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

A. Melandri (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea (PSU), B.
Sbarufatti (INAF-OAB/PSU), Z. Liu (NAOC / U. Leicester), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), V.
D'Elia (ASDC) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.6 ks of XRT data for GRB 180620B (Evans et al. GCN
Circ. 22807), from 89 s to 56.0 ks after the  BAT trigger. The data
comprise 237 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Beardmore et al. (GCN Circ. 22814).

The late-time light curve (from T0+4.3 ks) can be modelled with a
power-law decay with a decay index of alpha=0.61 (+/-0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.34 (+/-0.05). The
best-fitting absorption column is  2.30 (+/-0.15) x 10^21 cm^-2, in
excess of the Galactic value of 1.4 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.99 (+0.11, -0.10)
and a best-fitting absorption column of 1.6 (+/-0.3) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.5 x 10^-11 (4.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+/-0.3) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: 7.9 sigma
Photon index:	     1.99 (+0.11, -0.10)

If the light curve continues to decay with a power-law decay index of
0.61, the count rate at T+24 hours will be 0.12 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.1 x
10^-12 (5.5 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00843211.

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