The X-ray variability of AGN: power-spectrum and variance analysis of the Swift/BAT light curves

Abstract

We study the X-ray power spectrum of Active Galactic Nuclei (AGN) to investigate whether Seyfert I and II power spectra are similar or not, whether the AGN variability depends on black hole mass and accretion rate, and to compare the AGN power spectra with the Galactic X-ray black hole binaries power-spectra. We used 14-195 keV band light curves from the 157th SWIFT/BAT hard X-ray survey and we computed the mean power spectrum and excess variance of AGN in narrow black-hole mass/luminosity bins. We fitted a power-law model to the AGN power spectra, and we investigated whether the power spectrum parameters and the excess variance depend on black hole mass, luminosity and accretion rate. The Seyfert I and Seyfert II power spectra are identical, in agreement with AGN unification models. The mean AGN X-ray power spectrum has the same, power-law like shape with a slope of -1 in all AGN, irrespective of their luminosity and BH mass. We do not detect any flattening to a slope of zero at frequencies as low as 10e-9 Hz. We detect an anti-correlation between the PSD amplitude and the accretion rate, similar to what has been seen in the past in the 2-10 keV band. This implies that the variability amplitude in AGN decreases with increasing accretion rate. The universal AGN power-spectrum is consistent with the mean, 2-9 keV band Cyg X-1 power spectrum in its soft state. The mean, low frequency AGN X-ray power spectrum is consistent with the extension of the mean, 0.01-25 Hz Cyg X-1 power spectrum to lower frequencies. The agreement between the AGN and the Cyg X-1 power spectrum (either in the soft or the hard state) over many decades in frequency indicates that the X-ray variability process is probably the same in all accreting objects, irrespective of the mass of the compact object. We plan to investigate this issue further in the near future.

arXiv comment13 pages, 10 figures. Accepted for publication in A&A

References in corpus (26)

• X-ray Properties of Black-Hole Binaries
• Active Galactic Nuclei as scaled-up Galactic black holes
• The 70 Month Swift-BAT All-Sky Hard X-Ray Survey
• Measuring the broadband power spectra of active galactic nuclei with RXTE
• Bolometric luminosities and Eddington ratios of X-ray selected Active Galactic Nuclei in the XMM-COSMOS survey
• The 105 month Swift-BAT all-sky hard X-ray survey
• A simple test for periodic signals in red noise
• Combined Long and Short Timescale X-ray Variability of NGC4051 with RXTE and XMM-Newton
• X-ray Fluctuation Power Spectral Densities of Seyfert 1 Galaxies
• CAIXA: a catalogue of AGN in the XMM-Newton archive III. Excess Variance Analysis
• X-ray variability of 104 active galactic nuclei. XMM-Newton power-spectrum density profiles
• Discovery of multiple Lorentzian components in the X-ray timing properties of the Narrow Line Seyfert 1 Ark 564
• BASS XXII: The BASS DR2 AGN Catalog and Data
• The Second INTEGRAL AGN Catalogue
• Long-term variability of AGN at hard X-rays
• Tracing the accretion history of supermassive Black Holes through X-ray variability: results from the Chandra Deep Field-South
• Measuring X-ray variability in faint/sparsely-sampled AGN
• Detection of a high frequency break in the X-ray power spectrum of Ark 564
• The Energy-dependent X-ray Timing Characteristics of the Narrow Line Seyfert 1 Mkn 766
• Power-Colours: Simple X-ray Binary Variability Comparison
• Hard X-ray Variability of AGN
• Probing the temporal variability of Cygnus X-1 into the soft state
• Hard X-ray spectral variability of the brightest Seyfert AGN in the Swift/BAT sample
• The X-ray Power Spectral Density Function of the Seyfert Active Galactic Nucleus NGC 7469
• The First Hard X-Ray Power Spectral Density Functions of AGN
• The universal shape of the X-ray variability power spectrum of AGN up to $z\sim 3$

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• Investigating the interplay between the coronal properties and the hard X-ray variability of active galactic nuclei with NuSTAR
• Continuum optical-UV and X-ray variability of AGN: current results and future challenges
• The ensemble broad-frequency power spectrum of Stripe-82 quasars from multiple surveys
• Energy dependence of the X-ray power spectrum in NGC4051 and NGC4395