GCN Circular 31888

A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto),
B. Sbarufatti (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U.
Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAB), T.
Sbarrato (INAF-OAB) and N.J. Klingler report on behalf of the Swift-XRT
team:

We have analysed 8.6 ks of XRT data for GRB 220412A (Klingler et al.
GCN Circ. 31881
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 ), from 106 s to 68.7 ks after the  BAT trigger. The
data comprise 158 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 enhanced XRT position for this burst was given by Evans
et al. (GCN Circ. 31887
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.10 (+0.08, -0.07), followed by a break at T+4017 s to
an alpha of 0.0 (+0.0, -1.1).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.02 (+0.18, -0.09). The
best-fitting absorption column is  consistent with the Galactic value
of 3.7 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.68 (+0.22, -0.18) and a best-fitting absorption
column of 5.7 (+5.9, -2.0) x 10^20 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.9 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.7 (+5.9, -2.0) x 10^20 cm^-2
Galactic foreground: 3.7 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.68 (+0.22, -0.18)

If the light curve continues to decay with a power-law decay index of
0.0, the count rate at T+24 hours will be 0.011 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.4 x
10^-13 (4.7 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01102281.

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