Skip to main content
New Announcement Feature, Code of Conduct, Circular Revisions. See news and announcements

GCN Circular 23083

GRB 180805B: Swift-XRT refined Analysis
2018-08-06T02:03:06Z (6 years ago)
Phil Evans at U of Leicester <>
S. J. LaPorte (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), P.A. Evans (U. Leicester), A. Melandri
(INAF-OAB), V. D'Elia (ASDC), B. Sbarufatti (INAF-OAB/PSU) and P.
D'Avanzo report on behalf of the Swift-XRT team:

We have analysed 6.6 ks of XRT data for GRB 180805B (D'Avanzo et al.
GCN Circ. 23076), from 82 s to 36.7 ks after the  BAT trigger. The data
comprise 197 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN Circ. 23077).

The light curve can be modelled with a series of power-law decays. The
light curve initially rises, with an index alpha=-1.5 (+0.0, -0.5). At
T+102 s it breaks to an alpha of 1.61 (+0.15, -0.65). The light curve
breaks again at T+241 s to a decay with alpha=3.1 (+/-0.4),  before a
final break at T+891 s s after which the decay index is 0.88 (+0.21,

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 0.93 (+0.09, -0.06). The
best-fitting absorption column is  2.3 (+4.7, -0.7) x 10^20 cm^-2,
consistent with the Galactic value of 1.6 x 10^20 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 1.90 (+0.25,
-0.23) and a best-fitting absorption column of 7.3 (+6.6, -5.4) x 10^20
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.5 x 10^-11 (4.0 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.3 (+6.6, -5.4) x 10^20 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: 1.7 sigma
Photon index:	     1.90 (+0.25, -0.23)

If the light curve continues to decay with a power-law decay index of
0.88, the count rate at T+24 hours will be 2.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.8 x
10^-14 (7.8 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at

This circular is an official product of the Swift-XRT team.
Looking for U.S. government information and services? Visit