TITLE:   GCN CIRCULAR
NUMBER:  36181
SUBJECT: GRB 240418A: Swift-XRT refined Analysis
DATE:    24/04/19 15:17:47 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

A. Tohuvavohu (U. Toronto), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR),
V. D'Elia (SSDC & INAF-OAR), D.N. Burrows (PSU), J. D. Gropp (PSU) and
P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.1 ks of XRT data for GRB 240418A, from 107 s to 44.9
ks after the  BAT trigger. The data comprise 7 s in Windowed Timing
(WT) mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.9 (+0.3, -0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.0 (+0.6, -0.4). The
best-fitting absorption column is  consistent with the Galactic value
of 5.3 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.2 x 10^-11 (3.5 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5 (+/-12) x 10^20 cm^-2
Galactic foreground: 5.3 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.0 (+0.6, -0.4)

If the light curve continues to decay with a power-law decay index of
2.9, the count rate at T+24 hours will be 1.2 x 10^-8 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x
10^-19 (4.1 x 10^-19) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01222885.

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