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We report the analysis of simultaneous XMM-Newton+NuSTAR observations of twolow-luminosity Active Galactic Nuclei (LLAGN), NGC 3998 and NGC 4579. We do notdetect any significant variability in either source over the ~3 day length ofthe NuSTAR observations. The broad-band 0.5-60 keV spectrum of NGC 3998 is bestfit with a cutoff power-law, while the one for NGC 4579 is best fit with acombination of a hot thermal plasma model, a power-law, and a blend ofGaussians to fit an Fe complex observed between 6 and 7 keV. Our main spectralresults 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 3998and NGC 4579, respectively; (2) the 6-7 keV line complex in NGC 4579 couldeither be fit with a narrow Fe K line at 6.4 keV and a moderately broad Fe XXVline, 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 andbest constrained high-energy cutoff ever measured for an LLAGN; (5) NGC 3998spectrum is consistent with a Comptonization model with either a sphere($\tau\approx3\pm1$) or slab ($\tau\approx1.2\pm0.6$) geometry, correspondingto plasma temperatures between 20 and 150 keV. We discuss these results in thecontext of hard X-ray emission from bright AGN, other LLAGN, and hot accretionflow models.

NuSTAR Hard X-ray View of Low-luminosity Active Galactic Nuclei: High-energy Cutoff and Truncated Thin Disk