Testing Radiatively Inefficient Accretion Flow Theory: An XMM-Newton Observation of NGC 3998

We present the results of a 10 ks XMM-Newton observation of NGC 3998, a "type I" LINER galaxy (i.e., with significant broad Hα emission). Our goal is to test the extent to which radiatively inefficient accretion flow (RIAF) models and/or scaled-down active galactic nuclei (AGNs) models are consistent with the observed properties of NGC 3998. A power-law fit to the XMM-Newton spectra results in a power-law slope of Γ = 1.9 and 2-10 keV flux of 1.1 × 10-11 ergs cm-2 s-1, in excellent agreement with previous hard X-ray observations. The OM UV flux at 2120 Å appears to be marginally resolved, with ~50% of the flux extended beyond 2''. The nuclear component of the 2120 Å flux is consistent with an extrapolation of the X-ray power law, although ~50% of the flux may be absorbed. The OM U flux lies significantly above the X-ray power-law extrapolation and contains a significant contribution from extragalactic emission. The upper limit for narrow Fe K emission derived from the XMM-Newton spectra is 33 eV (for Δχ2 = 2.7). The upper limit for narrow Fe K emission derived from a combined fit of the XMM-Newton and BeppoSAX spectra is 25 eV, which is one of the strictest limits to date for any AGN. This significantly rules out Fe K emission, which is expected to be observed in typical Seyfert 1 galaxies. The X-ray flux of NGC 3998 has not been observed to vary significantly (at >30% level) within the X-ray observations, and only between observations at a level of ~50%, which is also in contrast to typical Seyfert 1 galaxies. The lack of any reflection features suggests that any optically thick, geometrically thin accretion disk must be truncated, probably at a radius of order 100-300 (in Schwarzschild units). RIAF models fit the UV to X-ray spectral energy distribution of NGC 3998 reasonably well. In these models the mid-IR flux also constrains the emission from any outer thin disk component that might be present. The UV to X-ray spectral energy distribution (SED) is also consistent with a Comptonized thin disk with a very low accretion rate ( < 10-5Edd), in which case the lack of Fe K emission may be due to an ionized accretion disk. Accretion models in general do not account for the observed radio flux of NGC 3998, and the radio flux may be due to a jet. Recent jet models may also be consistent with the nuclear fluxes of NGC 3998 in general, including the X-ray, optical/UV, and mid-IR bands. The (ground-based) near-IR to optical photometric data for the nuclear region of NGC 3998 contain large contributions from extranuclear emission. We also derive nuclear fluxes using archival Hubble Space Telescope WFPC2 data, resulting in meaningful constraints to the nuclear SED of NGC 3998 in the optical band.