Results from NuSTAR: Dynamics and time evolution in a sub-A class hard X-ray flare

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 flare to be an A0.2 class flare in brightness. This flare, with only ∼5 counts s-1 detector-1 observed by RHESSI, is fainter than any hard X-ray (HXR) flare in the existing literature. The flare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR flare spectra and images. The flare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peaking time with higher energy, similar spatial dimensions to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The flare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with its arising via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the flare to be 1.8×1027 ergs. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as “flarelike” properties.