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X-ray variability of NGC 3227 and 5506 and the nature of active galactic nucleus ‘states’




We use X-ray monitoring data obtained over a broad range of time-scales to measure the broad-band power spectral density functions (PSDs) of two Seyfert galaxies: the broad-line Seyfert 1 NGC 3227 and the Seyfert 2 NGC 5506, which has recently been identified as an obscured narrow-line Seyfert 1 (NLS 1). Using a Monte Carlo fitting technique, we demonstrate that both PSDs are reminiscent of the PSD of black hole X-ray binaries (BHXRBs) in the high/soft state, and specifically rule out a low/hard-state PSD shape in NGC 3227. This result demonstrates that, at least where variability is concerned, broad-line Seyferts with hard X-ray spectra (photon index Γ∼ 1.6) are not simply the analogues of the low/hard state in BHXRBs, and the dichotomy of NLS 1 and broad-line Seyferts cannot be simply interpreted in terms of the two states. We show that the PSD normalization in NGC 3227 is strongly energy-dependent, with larger variability amplitudes at lower energies, unlike NGC 5506 which shows little energy dependence of variability. We demonstrate that this difference is caused by spectral pivoting of the continuum in NGC 3227 at high energies, which is probably also related to the large amplitude of variability seen in the 2–10 keV band in this active galactic nucleus (AGN). Using the new PSD data and new results in the literature, we replot the PSD break time-scale versus mass plot for all AGNs with PSD breaks measured so far, and demonstrate that higher accretion rate AGNs appear to have relatively shorter break time-scales for their black hole mass than lower accretion rate AGNs.

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