Low-luminosity active galactic nuclei as analogs of galactic black holes in the low/hard state: Evidence from X-ray timing of NGC 4258

We present a broadband power spectral density function (PSD) measured from extensive RXTE monitoring data of the low-luminosity active galactic nucleus (LLAGN) NGC 4258, which has an accurate, maser-determined black hole mass of (3.9 +/- 0.1) X 10(7) M-circle dot. We constrain the PSD break timescale to be greater than 4.5 days at > 90% confidence, which appears to rule out the possibility that NGC 4258 is an analog of black hole X-ray binaries (BHXRBs) in the high/soft state. In this sense, the PSD of NGC 4258 is different from those of some more luminous Seyfert galaxies, which appear similar to the PSDs of high/soft state X-ray binaries. This result supports previous analogies between LLAGNs and X-ray binaries in the low/hard state based on spectral energy distributions, indicating that the AGN/BHXRB analogy is valid across a broad range of accretion rates.