  The explanation of the coronal heating problem potentially lies in the
existence of nanoflares, numerous small-scale heating events occuring across
the whole solar disk. In this paper, we present the first imaging spectroscopy
X-ray observations of three quiet Sun flares during the NuSTAR solar campaigns
on 2016 July 26 and 2017 March 21, concurrent with SDO/AIA observations. Two of
the three events showed time lags of a few minutes between peak X-ray and
extreme ultraviolet (EUV) emissions. Isothermal fits with rather low
temperatures in the range $3.2-4.1$ MK and emission measures of
$(0.6-15)\times10^{44} \textrm{ cm}^{-3}$ describe their spectra well,
resulting in thermal energies in the range $(2-6)\times10^{26}\textrm{ ergs}$.
NuSTAR spectra did not show any signs of a nonthermal or higher temperature
component. However, since the estimated upper limits of (hidden) nonthermal
energy are comparable to the thermal energy estimates, the lack of a nonthermal
component in the observed spectra is not a constraining result. The estimated
GOES classes from the fitted values of temperature and emission measure fall
between $1/1000 \textrm{ and } 1/100$ A class level, making them 8 orders of
magnitude fainter in soft X-ray flux than the largest solar flares.

Solar and Stellar Astrophysics;High Energy Astrophysical Phenomena