NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has observed several small microflares in its\ninitial solar pointings. In this paper, we present the first joint observation of a microflare with NuSTAR and Hinode/\nXRT on 2015 April 29 at âË?¼11:29 UT. This microflare shows the heating of material to several million Kelvin, observed\nin soft X-rays with Hinode/XRT, and was faintly visible in the extreme ultraviolet with SDO/AIA. For three of the four\nNuSTAR observations of this region (pre-flare, decay, and post-flare phases), the spectrum is well fitted by a single\nthermal model of 3.2ââ?¬â??3.5 MK, but the spectrum during the impulsive phase shows additional emission up to 10 MK,\nemission equivalent to the A0.1 GOES class. We recover the differential emission measure (DEM) using SDO/AIA,\nHinode/XRT, and NuSTAR, giving unprecedented coverage in temperature. We find that the pre-flare DEM peaks at\nâË?¼3 MK and falls off sharply by 5 MK; but during the microflareââ?¬â?¢s impulsive phase, the emission above 3 MK is\nbrighter and extends to 10 MK, giving a heating rate of about 2.5 1025 erg sâË?â??1. As the NuSTAR spectrum is purely\nthermal, we determined upper limits on the possible non-thermal bremsstrahlung emission. We find that for the\naccelerated electrons to be the source of heating, a power-law spectrum of 7 with a low-energy cutoff Ec 7 keV\nis required. In summary, this first NuSTAR microflare strongly resembles much more powerful flares.
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