Accelerated Electrons Observed Down to <7 keV in a NuSTAR Solar Microflare

We report the detection of emission from a nonthermal electron distribution in a small solar microflare (GOES class A5.7) observed by the Nuclear Spectroscopic Telescope Array, 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 nonthermal 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 ∼2 × 1027 erg s−1, heating the flare loop to at least 10 MK. The existence of dominant nonthermal emission in X-rays down to