HEAD 2003 Meeting
Session 4. Galactic Center and Intermediate Mass Black Holes II
Oral, Sunday, March 23, 2003, 11:30am-12:10pm

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[4.03] A XMM-Newton and Chandra Survey of Very Low Accretion Rate AGN

A. Ptak (Johns Hopkins University), Y. Terashima (ISAS), L. C. Ho (OCIW), E. Quataert (U.C. Berkeley)

We present the results of an ongoing Chandra and XMM-Newton survey of galactic nuclei with known black hole masses and well-sampled spectral energy distributions, with emphasis here on our best-studied case, NGC 3998, a ``type 1'' LINER galaxy (i.e., with significant broad H\alpha emission). Our goal is to test the extent to which radiatively inefficient accretion flow (RIAF) models and/or scaled-down AGN models are consistent with the observed properties of these galaxies. We find that the XMM-Newton data, when simultaneously fitted with archival ASCA~ and BeppoSAX data, show that the 0.1--100 keV X-ray spectrum of NGC 3998 is consistent with a simple power-law model with a photon index of ~1.9. This power law also extrapolates to the UV flux determined with the OM. The upper-limit for narrow Fe-K emission is ~ 24 eV, which is one of the strictest limits to date for any AGN. This significantly rules out Fe-K emission as is expected to be observed in typical Seyfert 1 galaxies. The hard X-ray flux of NGC 3998 has not been observed to vary significantly (at >30% level) either between or within the X-ray observations, 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. RIAF models fit the optical to X-ray spectral energy distribution of NGC 3998 well. In these models the mid-IR flux also constrains the emission from any outer thin disk component that might be present. The optical to X-ray SED is also consistent with a Comptonized thin disk with a very low accretion rate (\dot{M} < 10-5\dot{M}\rm Edd). Accretion disk models in general do not account for the observed radio flux of NGC 3998. We will also present preliminary results for the other galaxies in our sample.

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Bulletin of the American Astronomical Society, 35#2
© 2003. The American Astronomical Soceity.