GCN Circular 23736

A. P. Beardmore (U. Leicester) reports on behalf of the Swift-XRT Team:

The Swift-XRT Windowed Timing (WT) mode spectrum of GRB 190114C shows
an apparent strong excess of emission below ~0.8 keV when compared to
a simple absorbed powerlaw model. The excess is not thought to be
intrinsic to the GRB (e.g. an additional spectral component), but is
instead caused by a well-known instrumental effect whereby charge from
deep charge traps (formed by radiation damage to the CCD) is released
on a timescale comparable to the WT readout time (1.78ms). The
released charge is subsequently detected in trailing readout rows
as additional low energy events, causing excess emission to be seen at
low energies (see http://www.swift.ac.uk/analysis/xrt/digest_cal.php#trail).
The excess is more apparent in strongly absorbed sources such as this.

The XRT Team is developing software to identify and remove occurences
of such trailing charge from WT data. In the meantime, the WT spectra
can be reliably modelled above ~0.8 keV.