First joint X-ray solar microflare observations with NuSTAR and Solar Orbiter/STIX

Abstract

We present the first joint spectral and imaging analysis of hard X-ray (HXR) emission from 3 microflares observed by the Nuclear Spectroscopic Telescope ARray (NuSTAR) and Solar Orbiter/Spectrometer/Telescope for Imaging X-rays (STIX). We studied 5 joint spectra from GOES A7, B1 and B6 class microflares from active region AR12765 on 2020 June 6 and 7. As these events are very bright for NuSTAR, resulting in extremely low (<1%) livetime, we introduce a pile-up correction method. All five joint spectra were fitted with an isothermal model finding temperatures in the 9-11 MK range. Furthermore, three joint spectra required an additional non-thermal thick-target model finding non-thermal powers of $10^{25}$-$10^{26}$ erg s${}^{-1}$. All the fit parameters were within the ranges expected for HXR microflares. The fit results give a relative scaling of STIX and NuSTAR mostly between 6-28% (one outlier at 52%) suggesting each instrument are well calibrated. In addition to spectral analysis, we performed joint HXR imaging of the June 6 and one of the June 7 microflares. In NuSTAR's field of view (FOV), we observed two separate non-thermal sources connected by an elongated thermal source during the June 6 microflares. In STIX's FOV (44 degrees W with respect to NuSTAR), we imaged thermal emission from the hot flare loops which when reprojected to an Earth viewpoint matches the thermal sources seen with NuSTAR and in the hotter EUV channels with the Solar Dynamic Observatory's Atmospheric Imaging Assembly.

References in corpus (18)

• The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission
• An Observational Overview of Solar Flares
• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• Microflares and the Statistics of X-ray Flares
• Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
• Accelerated electrons observed down to <7 keV in a NuSTAR solar microflare
• MEM_GE: a new maximum entropy method for image reconstruction from solar X-ray visibilities
• The First Focused Hard X-ray Images of the Sun with NuSTAR
• The first X-ray imaging spectroscopy of quiescent solar active regions with NuSTAR
• NuSTAR observations of a repeatedly microflaring active region
• Joint X-ray, EUV and UV Observations of a Small Microflare
• Solar flare hard X-rays from the anchor points of an eruptive filament
• NuSTAR Observation of a Minuscule Microflare in a Solar Active Region
• NuSTAR Observation of Energy Release in Eleven Solar Microflares
• FOXSI-2 Solar Microflares II: Hard X-ray Imaging Spectroscopy and Flare Energetics
• Multi-instrument STIX microflare study
• Study of Time Evolution of Thermal and Non-Thermal Emission from an M-Class Solar Flare
• The First Survey of Quiet Sun Features Observed in Hard X-Rays With NuSTAR

Cited by in corpus (5)

• Using the STIX background detector as a proxy for GOES
• Spectral component imaging of solar X-ray flares
• Quiet Sun impulsive events observed with NuSTAR during solar minimum
• Superhot (> 30 MK) flare observations with STIX: Joint spectral fitting
• Observations of a Faint Nonthermal Onset before a GOES C-class Flare