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 \sim 5 counts s ^ { -1 } detector ^ { -1 } 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 2.4 \times 10 ^ { 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 .