Detecting non-thermal emission in a solar microflare using nested sampling

Abstract

ABSTRACT Microflares are energetically smaller versions of solar flares, demonstrating the same processes of plasma heating and particle acceleration. However, it remains unclear down to what energy scales this impulsive energy release continues, which has implications for how the solar atmosphere is heated. The heating and particle acceleration in microflares can be studied through their X-ray emission, finding predominantly thermal emission at lower energies; however, at higher energies it can be difficult to distinguish whether the emission is due to hotter plasma and/or accelerated electrons. We present the first application of nested sampling to solar flare X-ray spectra, an approach that provides a quantitative degree of confidence for one model over another. We analyse Nuclear Spectroscopic Telescope Array X-ray observations of a small active region microflare (A0.02 GOES/XRS class equivalent) that occurred on 2021 November 17, with a new python package for spectral fitting, sunkit-spex, to compute the parameter posterior distributions and the evidence of different models representing the higher energy emission as due to thermal or non-thermal sources. Calculating the Bayes factor, we show that there is significantly stronger evidence for the higher energy microflare emission to be produced by non-thermal emission from flare-accelerated electrons than by an additional hot thermal source. Qualitative confirmation of this non-thermal source is provided by the lack of hotter (10 MK) emission in Solar Dynamic Observatory’s Atmospheric Imaging Assembly’s extreme ultraviolet data. The nested sampling approach used in this paper has provided clear support for non-thermal emission at the level of 3 × 1024 erg s−1 in this tiny microflare.