| Presentation Title: | Detecting thermal and non-thermal emission in solar microflares using nested sampling |
| Author(s): | Kristopher Cooper, Iain Hannah, Lindsay Glesener, Brian Grefenstette |
Abstract:
We use the Nuclear Spectroscopic Telescope Array (NuSTAR) to investigate very weak and highly frequent X-ray microflares that occur in active regions (ARs). NuSTAR is an astrophysical direct imaging X-ray telescope capable of observing the Sun with a unique sensitivity to emission >2.2 keV. Determining the emission mechanisms present in flares at weak scales can be difficult. We commonly find a weak but significant emission excess at higher energies after fitting an isothermal model to the spectrum, and it is sometimes unclear whether this excess is due to accelerated particles or flare-heated plasma. We employ a nested sampling algorithm to compute the parameter posterior distributions and the evidence of one model representing the excess with thermal emission and another with non-thermal emission. Calculating Bayes factor from these evidences, we are able to assess which emission mechanism is more likely to be responsible for the high-energy count excess visible in the isothermal spectral fit. We present spectral, spatial, and temporal analysis performed on several sub-A GOES class equivalent microflares that were observed with NuSTAR in November 2021. Along with EUV observations from the Solar Dynamics Observatory's Atmospheric Imaging Assembly, we utilise a Python spectral fitting tool, Sunkit-spex, to perform nested sampling analysis on the X-ray microflare spectra. In at least one example, we show there is significantly stronger evidence for the high energy count excess to be represented by non-thermal emission from flare accelerated electrons than by an additional hot thermal source (Cooper et al. 2024 MNRAS 529 1). Other 2021 November NuSTAR microflares at similar small scales either indicate the presence of particle acceleration as well or plasma temperatures up to ~10 MK.