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[32.05] Modelling the Broad-Band Power Density Spectra of AGN to Include Relativistic Effects in Highly Inclined Systems

M. Mueller, G.M. Madejski (Stanford Linear Accelerator Center), P.T. Zycki (CAMK, Warsaw), C. Done (U. Durham)

Timing studies of stellar-mass and supermassive black hole systems seem to suggest that the associated accretion flows close to the BH are simply scaled versions of each other, with the characteristic time scales proportional to the mass of the BH. We present a thorough analysis of the broad-band X-ray power density spectrum (PDS) of the Seyfert 2 galaxy NGC 4945, where spectroscopic and imaging studies indicate an inclination angle close to 90 degrees. This is in contrast to the rest of the AGN for which the X-ray variability has been extensively studied, which are all Seyfert 1-1.5 and thus viewed closer to the disk axis. We model the PDS in NGC 4945 with a broken power law, where the power law index changes at the so-called break frequency. By performing a best-fit procedure, we obtain a break frequency of about 10^-6 Hz; this is the break that corresponds to the Cyg X-1 high-frequency break, which is present in both the soft and hard states of Cyg X-1. The best-fit model has a power law index above the break of 1.5, which is significantly smaller (corresponding to a "harder" PDS) than the canonical value of 2 found in Seyfert 1s and in Cyg X-1. Relativistic effects on the time series, in particular light-bending of transient emission on the far side of the accretion disk and Doppler boosting due to Keplerian rotation, offer a possible explanation for this observational result. As part of this work, we apply our modified version of the light curve simulation algorithm to the AGN time series for which X-ray variability studies have been published. Our results are largely consistent with these earlier reports; however, the changes introduced to the analysis tools yield more reliable best fits to the PDS by explicitely incorporating the statistical properties of the periodogram.

This work is supported by the Department of Energy contract to the Stanford Linear Accelerator Center and by NASA grants.

The author(s) of this abstract have provided an email address for comments about the abstract: mmueller@slac.stanford.edu

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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.