  We report the detection of emission from a non-thermal electron distribution
in a small solar microflare (GOES class A5.7) observed by the Nuclear
Spectroscopic Telescope Array (NuSTAR), with supporting observation by the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The flaring
plasma is well accounted for by a thick-target model of accelerated electrons
collisionally thermalizing within the loop, akin to the "coronal thick target"
behavior occasionally observed in larger flares. This is the first positive
detection of non-thermal hard X-rays from the Sun using a direct imager (as
opposed to indirectly imaging instruments). The accelerated electron
distribution has a spectral index of 6.3 +/- 0.7, extends down to at least 6.5
keV, and deposits energy at a rate of ~2x1027 erg/s, heating the flare loop to
at least 10 MK. The existence of dominant non-thermal emission in X-rays down
to <5 keV means that RHESSI emission is almost entirely non-thermal, contrary
to what is usually assumed in RHESSI spectroscopy. The ratio of non-thermal to
thermal energies is similar to that of large flares, in contrast to what has
been found in previous studies of small RHESSI flares. We suggest that a
coronal thick target may be a common property of many small microflares based
on the average electron energy and collisional mean free path. Future
observations of this kind will enable understanding of how flare particle
acceleration changes across energy scales, and will aid the push toward the
observational regime of nanoflares, which are a possible source of significant
coronal heating.
