Speaker
Description
Launched on December 9, 2021, the Imaging X-ray Polarimetry Explorer (IXPE) is the first
mission entirely devoted to astronomical X-ray polarimetry in the 2--8 keV energy band.
At the heart of the observatory is a set of three identical, sealed, gas pixel detectors
(GPDs) sensitive to polarization. Alongside their primary function, GPDs offer
simultaneous imaging, timing, and spectroscopic capabilities with moderate resolution.
In this contribution we focus on two time-dependent instrumental phenomena, originally
identified during the development phase of the mission, which necessitate continuous
monitoring throughout the mission's operational phase. Firstly, a secular decrease of the
pressure in the sealed gas cell, due to internal adsorption of the filling dimethyl ether
(DME) with a time scale of months and an asymptotic pressure reduction of 10--20%, has
been observed. Although largely saturated prior to launch, the residual, slow variations in
quantum efficiency, gain and track size are still relevant enough that they need to be
accounted for in the analysis. To this end, we have considered two parallel paths: the
monitoring of a set of sealed control detectors, identical to those currently operating in
space, and the analysis on fluxed detectors in a custom gas filling station, that allows to
study the pressure dependence of the relevant metrics under controlled conditions.
Secondly, we address charge build-up in the dielectric layer of the Gas Electron Multiplier
(GEM), the amplification stage of the detector, with time scales ranging from hours to
days. This charging phenomenon induces a decrease in detector gain under irradiation,
gradually recovering once irradiation ceases, akin to the behaviour of a capacitor. To
mitigate this effect, our data analysis pipeline incorporates a correction algorithm based
on dedicated measurements performed on flight detectors pre-launch and updated
through the continuous gain monitoring performed using onboard calibration sources.
| Role of Submitter | I am the presenter |
|---|
