RIS: Regularized imaging spectroscopy for STIX on board Solar Orbiter

¹ MIDA, Dipartimento di Matematica, Università di Genova, via Dodecaneso 35, 16146 Genova, Italy
² Department of Mathematics, TUM School of Computation, Information and Technology, Technische Universität München, Boltzmannstraße 3, 85748 Garching b. München, Germany
³ Bioengineering Center, Institute of Biological and Medical Imaging, Helmholtz Munich, Ingolstädter Landstraße 1, 85764 Neuherberg, Germany
⁴ Dipartimento di Scienze Matematiche Giuseppe Luigi Lagrange, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
⁵ Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy

^⋆ Corresponding authors; volpara@dima.unige.it, piana@dima.unige.it, massone@dima.unige.it

Received: 1 July 2024
Accepted: 19 December 2024

Abstract

Context. The generation of spatially resolved count spectra and of cubes of count maps at different energies via imaging spectroscopy is one of the main goals of solar hard X-ray missions based on Fourier imaging. Thus, so far, for these telescopes, this goal has been realized via the generation of either count maps that are independently reconstructed in the different energy channels or electron flux maps reconstructed via a deconvolution of the approximate forms for the bremsstrahlung cross-section.

Aims. Our aim is to introduce a regularized imaging spectroscopy method (RIS), whereby the regularization implemented in the count space imposes a smoothing constraint across contiguous energy channels, without the need for computing any deconvolution of the bremsstrahlung effect.

Methods. STIX records imaging data, while computing the visibilities in the spatial frequency domain. Our RIS is a sequential scheme in which part of the information coded in the image is reconstructed at a specific energy channel and transferred to the reconstruction process at a contiguous channel via a visibility interpolation computed by means of variably scaled kernels (VSKs).

Results. In the case of STIX visibilities recorded during the November 11, 2022 flaring event, we show that RIS is able to generate hard X-ray maps, whose morphology is seen to smoothly evolve from one energy channel to the contiguous one; accordingly, from these maps, it is possible to infer spatially resolved count spectra characterized by a notable numerical stability. We also show that the performance of this approach is robust with respect to both the image reconstruction method and the count energy channel utilized to trigger the sequential process.

Conclusions. We conclude that RIS is not only an appropriate, but also an effective and necessary approach to constructing image cubes from STIX visibilities that are characterized by smooth behavior across count energies. Thus, it allows for the generation of numerically stable (and, thus, physically reliable) local count spectra.