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+Nuclear Spectroscopic Telescope Array (NuSTAR) observations of two low-luminosity active galactic nuclei (LLAGNs), 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 broadband 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 3σ reflection fraction upper limits of 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 be fit with either a narrow Fe K line at 6.4 keV and a moderately broad Fe XXV line or with three relatively narrow lines, which include contribution from Fe XXVI; (3) the 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) the NGC 3998 spectrum is consistent with a Comptonization model with either a sphere (τ ≈ 3 ± 1) or slab (τ ≈ 1.2 ± 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 AGNs, other LLAGNs, and hot accretion flow models.