# A count-based imaging model for the Spectrometer/Telescope for Imaging X-rays (STIX) in Solar Orbiter

## TL;DR

This paper introduces a count-based imaging model for the STIX instrument on Solar Orbiter, demonstrating that reliable image reconstruction is possible using Expectation Maximization, similar to previous methods used for RHESSI.

## Contribution

The paper presents a novel count-based mathematical model for STIX data formation and validates the use of Expectation Maximization for image reconstruction.

## Key findings

- Count-based imaging is feasible for STIX.
- EM algorithm reliably reconstructs images from simulated data.
- Model mimics real photon flux projection into count data.

## Abstract

The Spectrometer/Telescope for Imaging X-rays (STIX) will look at solar flares across the hard X-ray window provided by the Solar Orbiter cluster. Similarly to the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), STIX is a visibility-based imaging instrument, which will ask for Fourier-based image reconstruction methods. However, in this paper we show that, as for RHESSI, also for STIX count-based imaging is possible. Specifically, here we introduce and illustrate a mathematical model that mimics the STIX data formation process as a projection from the incoming photon flux into a vector made of 120 count components. Then we test the reliability of Expectation Maximization for image reconstruction in the case of several simulated configurations typical of flare morphology.