NuSTAR Hard X-ray View of Low-luminosity Active Galactic Nuclei:
  High-energy Cutoff and Truncated Thin Disk
  We report the analysis of simultaneous XMM-Newton+NuSTAR observations of two
low-luminosity Active Galactic Nuclei (LLAGN), NGC 3998 and NGC 4579. We do not
detect any significant variability in either source over the ~3 day length of
the NuSTAR observations. The broad-band 0.5-60 keV spectrum of NGC 3998 is best
fit with a cutoff power-law, while the one for NGC 4579 is best fit with a
combination of a hot thermal plasma model, a power-law, and a blend of
Gaussians to fit an Fe complex observed between 6 and 7 keV. Our main spectral
results are the following: (1) neither source shows any reflection hump with a
$3\sigma$ reflection fraction upper-limits $R<0.3$ and $R<0.18$ for NGC 3998
and NGC 4579, respectively; (2) the 6-7 keV line complex in NGC 4579 could
either be fit with a narrow Fe K line at 6.4 keV and a moderately broad Fe XXV
line, or 3 relatively narrow lines, which includes contribution from Fe XXVI;
(3) NGC 4579 flux is 60% brighter than previously detected with XMM-Newton,
accompanied by a hardening in the spectrum; (4) we measure a cutoff energy
$E_{\rm cut}=107_{-18}^{+27}$ keV in NGC 3998, which represents the lowest and
best constrained high-energy cutoff ever measured for an LLAGN; (5) NGC 3998
spectrum is consistent with a Comptonization model with either a sphere
($\tau\approx3\pm1$) or slab ($\tau\approx1.2\pm0.6$) geometry, corresponding
to plasma temperatures between 20 and 150 keV. We discuss these results in the
context of hard X-ray emission from bright AGN, other LLAGN, and hot accretion
flow models.
