  Observational measurements of active region emission measures contain clues
to the time-dependence of the underlying heating mechanism. A strongly
non-linear scaling of the emission measure with temperature indicates a large
amount of hot plasma relative to warm plasma. A weakly non-linear (or linear)
scaling of the emission measure indicates a relatively large amount of warm
plasma, suggesting that the hot active region plasma is allowed to cool and so
the heating is impulsive with a long repeat time. This case is called {\it
low-frequency} nanoflare heating and we investigate its feasibility as an
active region heating scenario here. We explore a parameter space of heating
and coronal loop properties with a hydrodynamic model. For each model run, we
calculate the slope $\alpha$ of the emission measure distribution $EM(T)
\propto T^\alpha$. Our conclusions are: (1) low-frequency nanoflare heating is
consistent with about 36% of observed active region cores when uncertainties in
the atomic data are not accounted for; (2) proper consideration of
uncertainties yields a range in which as many as 77% of observed active regions
are consistent with low-frequency nanoflare heating and as few as zero; (3)
low-frequency nanoflare heating cannot explain observed slopes greater than 3;
(4) the upper limit to the volumetric energy release is in the region of 50 erg
cm$^{-3}$ to avoid unphysical magnetic field strengths; (5) the heating
timescale may be short for loops of total length less than 40 Mm to be
consistent with the observed range of slopes; (6) predicted slopes are
consistently steeper for longer loops.
