TITLE:   GCN CIRCULAR
NUMBER:  40173
SUBJECT: GRB 250419A: Swift-XRT refined Analysis
DATE:    25/04/19 08:09:20 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

K.L. Page (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio
(INAF-OAB), M. Ferro (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU),
D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

We have analysed 2.0 ks of XRT data for the SVOM/ECLAIRs-detected burst
GRB 250419A (SVOM burst-id sb25041901), from 3.8 ks to 9.6 ks after the
 SVOM/ECLAIRs trigger. The data are entirely in Photon Counting (PC)
mode.  The light curve is consistent with a constant source of mean
count rate 1.1e-01 ct/sec. A power-law fit gives an index of 0.3 (+0.6,
-0.5). Despite a constant count rate, we are likely detecting the GRB
afterglow. The source is consistent with the position of the optical
afterglow detected by the SVOM/COLIBRÍ (FM-GFT) telescope (Lopez et
al., GCN 40169), SVOM/VT (Xin et al., GCN 40170), KAIT (Zheng et al.,
GCN 40171), GOTO (Kumar et al., GCN 40172).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+0.4, -0.3). The
best-fitting absorption column is  7.7 (+9.9, -5.5) x 10^20 cm^-2,
consistent with the Galactic value of 2.2 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 3.5 x 10^-11 (4.1 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.7 (+9.9, -5.5) x 10^20 cm^-2
Galactic foreground: 2.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.9 (+0.4, -0.3)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00019724.

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