What Is the Accretion Rate in NGC 4258?

Abstract

We estimate the accretion rate M onto the black hole in the nucleus of NGC 4258. The result is model dependent. We find that a thin disk that is flat out to ≳2 × 10^3GM/c^2 (M ≡ black hole mass) and that has inclination equal to that of the masing disk must have M ≃ 0.01 M☉ yr^(-1) for consistency with near-infrared data. A thin disk that is warped at ~103GM/c2 requires M ≳ 1.5 × 10^(-4) M☉ yr^(-1) for consistency with near-infrared data. Reasonable warp models and the near-infrared data are not consistent with M as low as the 7 × 10^(-5) α M☉ yr^(-1) proposed by Neufeld & Maloney. An advection-dominated flow model combined with the X-ray data implies M ≃ 0.01 M☉ yr^(-1). Consistency with a tight 22 GHz upper limit on the flux from the central source requires a transition radius of ~(10-100)GM/^c2, although the 22 GHz upper limit may be relaxed by appeal to free-free absorption. Physical conditions in the observed VLBI jet features can also be related to conditions in the inner accretion flow. We conclude with a list of observations that might further constrain M.