GCN Circular 31827

A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR),
A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), D.N. Burrows (PSU),
J.P. Osborne (U. Leicester) and N.J. Klingler report on behalf of the
Swift-XRT team:

We have analysed 6.3 ks of XRT data for GRB 220403B (Klingler et al.
GCN Circ. 31820
Loading...
 
 ), from 86 s to 28.9 ks after the  BAT trigger. The data
comprise 50 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The refined XRT position is RA, Dec = 191.4734, +89.1846 which is
equivalent to:

RA (J2000): 12 45 53.62
Dec(J2000): +89 11 04.6

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

The light curve can be modelled with an initial power-law decay with an
index of alpha=4.4 (+/-0.4), followed by a break at T+229 s to an alpha
of 0.35 (+0.04, -0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.86 (+0.16, -0.14). The
best-fitting absorption column is  1.47 (+0.51, -0.27) x 10^21 cm^-2,
consistent with the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.7 x 10^-11 (4.6 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.47 (+0.51, -0.27) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.86 (+0.16, -0.14)

If the light curve continues to decay with a power-law decay index of
0.35, the count rate at T+24 hours will be 0.057 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.1 x
10^-12 (2.6 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/01101053.

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