2005ApJ...625L..39M

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)x10⁷ M_☉. 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.