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 \sim 3 -day length of the NuSTAR observations .
The broad-band 0.5-60 k1eV 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 _ { 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 \approx 3 \pm 1 ) or slab ( \tau \approx 1.2 \pm 0.6 ) 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 .