  We present the first joint observation of a small microflare in X-rays with
the Nuclear Spectroscopic Telescope ARray (NuSTAR), UV with the Interface
Region Imaging Spectrograph (IRIS) and EUV with the Solar Dynamics
Observatory/Atmospheric Imaging Assembly (SDO/AIA). These combined observations
allows us to study the microflare's hot coronal and cooler
chromospheric/transition region emission. This small microflare peaks from
SOL2016-07-26T23:35 to 23:36UT, in both NuSTAR, SDO/AIA and IRIS. Spatially
this corresponds to a small loop visible in the SDO/AIA Fe XVIII emission,
which matches a similar structure lower in the solar atmosphere seen by IRIS in
SJI1330{\AA} and 1400\AA. The NuSTAR emission in both 2.5-4 keV and 4-6 keV, is
located in a small source at this loop location. The IRIS slit was over the
microflaring loop, and fits show little change in Mg II but do show intensity
increases, slight width enhancements and redshifts in Si IV andO IV, indicating
that this microflare had most significance in and above the upper chromosphere.
The NuSTAR microflare spectrum is well fitted by a thermal component of 5.8MK
and $2.3\times10^{44}$ cm$^{-3}$, which corresponds to a thermal energy of
$10^{26}$ erg, making it considerably smaller than previously studied X-ray
microflares. No non-thermal emission was detected but this could be due to the
limited effective exposure time of the observation. This observation shows that
even ordinary features seen in UV and EUV, can remarkably have a higher energy
component that is clear in X-rays.

Solar and Stellar Astrophysics