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The All Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE) hascontinuously monitored a number of Active galactic nuclei (AGNs) with similarsampling rates for 14 years from 1996 January to 2009 December. Utilizing thearchival ASM data of 27 AGNs, we calculate the normalized excess variances ofthe 300-day binned X-ray light curves on the longest timescale (between 300days and 14 years) explored so far. The observed variance appears to beindependent of AGN black hole mass and bolometric luminosity, respectively.According to the scaling relation with black hole mass (and bolometricluminosity) from Galactic black hole X-ray binaries (GBHs) to AGNs, the breaktimescales which correspond to the break frequencies detected in the powerspectral density (PSD) of our AGNs are larger than binsize (300 days) of theASM light curves. As a result, the singly broken power-law (soft-state) PSDpredicts the variance to be independent of mass and luminosity, respectively.Nevertheless, the doubly-broken power-law (hard-state) PSD predicts, with thewidely accepted ratio of the two break frequencies, that the variance increaseswith increasing mass and decreases with increasing luminosity, respectively.Therefore, the independence of the observed variance on mass and luminositysuggests that AGNs should have the soft-state PSDs. If taking into account thescaling of breaking timescale with mass and luminosity synchronously, theobserved variances are also more consistent with the soft-state than thehard-state PSD predictions. With the averaged variance of AGNs and thesoft-state PSD assumption, we obtain a universal PSD amplitude of$0.030\pm0.022$. By analogy with the GBH PSDs in the high/soft state, thelongest timescale variability supports the standpoint that AGNs are scaled-upGBHs in the high accretion state, as already implied by the direct PSDanalysis.

The longest timescale X-ray variability reveals an evidence for active galactic nuclei in the high accretion state