NuSTAR hard X-ray observation of a sub-A class solar flare

TL;DR

This study reports the first detailed hard X-ray observation of an extremely faint solar microflare, revealing that even very small flares exhibit typical energetic and temporal properties of larger solar flares.

Contribution

It provides the first detailed HXR spectra and imaging of a microflare at the A0.1 class, demonstrating that small flares share key characteristics with larger ones.

Findings

Microflare exhibits typical HXR properties of larger flares.

Flares scale down to very small energies while retaining flare-like features.

Thermal energy increase estimated at 2.4x10^27 ergs.

Abstract

We report a NuSTAR observation of a solar microflare, SOL2015-09-01T04. Although it was too faint to be observed by the GOES X-ray Sensor, we estimate the event to be an A0.1 class flare in brightness. This microflare, with only 5 counts per second per detector observed by RHESSI, is fainter than any hard X-ray (HXR) flare in the existing literature. The microflare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR microflare spectra and images. The microflare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peak time with higher energy, spatial dimensions similar to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The microflare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with an origin via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the microflare to be 2.4x10^27 ergs. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as "flarelike" properties.