Abstract
We present an analysis of the high-frequency X-ray variability of NGC 4051 (MBH∼ 1.7 × 106 M⊙) based on a series of XMM–Newton observations taken in 2009 with a total exposure of ∼570 ks (EPIC pn). These data reveal the form of the power spectrum over frequencies from 10−4 Hz, below the previously detected power spectral break, to ≳10−2 Hz, above the frequency of the innermost stable circular orbit (ISCO) around the black hole (νISCO∼ 10−3–10−2 Hz, depending on the black hole spin parameter j). This is equivalent to probing frequencies of ≳1 kHz in a stellar mass (MBH∼ 10 M⊙) black hole binary system. The power spectrum is a featureless power law over the region of the expected ISCO frequency, suggesting no strong enhancement or change in the variability at the fastest orbital period in the system. Despite the huge amplitude of the flux variations between the observations (peak-to-peak factor of ≳50), the power spectrum appears to be stationary in shape and varies in amplitude at all observed frequencies following the previously established linear rms–flux relation. The rms–flux relation is offset in flux by a small and energy-dependent amount. The simplest interpretation of the offset is in terms of a very soft spectral component that is practically constant (compared to the primary source of variability). One possible origin for this emission is a circumnuclear shock energized by a radiatively driven outflow from the central regions and emitting via inverse-Compton scattering of the central engine’s optical–UV continuum (as inferred from a separate analysis of the energy spectrum). A comparison with the power spectrum of a long XMM–Newton observation taken in 2001 gives only weak evidence for non-stationarity in power spectral shape or amplitude. Despite being among the most precisely estimated power spectra for any active galaxy, we find no strong evidence for quasi-periodic oscillations (QPOs) and determine an upper limit on the strength of a plausible QPO of ≲2 per cent rms in the 3 × 10−3 -0.1 Hz range and ∼5–10 per cent in the 10−4 -3 × 10−3 Hz range. We compare these results to the known properties of accreting stellar mass black holes in X-ray binaries, with the further aim of developing a ‘black hole unification’ scheme.
Highlights
If strong gravity dominates the dynamics of the inner accretion flows around black holes, an elementary consequence is scale invariance: many important aspects of accretion on to supermassive black holes (MBH 106 M ) in active galactic nuclei (AGN) should be fundamentally the same as for stellar mass black holes (MBH ∼ 10 M ) in Galactic black hole systems (GBHs). (See e.g. ShakuraC 2011 The Authors Monthly Notices of the Royal Astronomical Society C 2011 RAS& Sunyaev 1976; Mushotzky, Done & Pounds 1993.) Over the past few years, several similarities have been observed in the X-ray variability of nearby AGN and GBHs, supporting the idea of black hole unification (Fender et al 2006)
Relativistic jets must be launched from the region dominated by strong-field gravity, and so scale invariance implies that this accretion state/jet unification scheme should extend to AGN (Heinz & Sunyaev 2003), a hypothesis supported by observations (Merloni, Heinz & di Matteo 2003; Falcke, Kording & Markoff 2004; Kording, Jester & Fender 2006)
We have presented an analysis of the rapid X-ray flux variability of the low-mass Seyfert 1 galaxy NGC 4051 covering the range from 10−4 to ∼2 × 10−2 Hz, based on a series of 15 XMM–Newton observations taken during 2009
Summary
If strong gravity dominates the dynamics of the inner accretion flows around black holes, an elementary consequence is scale invariance: many important aspects of accretion on to supermassive black holes (MBH 106 M ) in active galactic nuclei (AGN) should be fundamentally the same as for stellar mass black holes (MBH ∼ 10 M ) in Galactic black hole systems (GBHs). Relativistic jets must be launched from the region dominated by strong-field gravity, and so scale invariance implies that this accretion state/jet unification scheme should extend to AGN (Heinz & Sunyaev 2003), a hypothesis supported by observations (Merloni, Heinz & di Matteo 2003; Falcke, Kording & Markoff 2004; Kording, Jester & Fender 2006) From these results a new paradigm is emerging in which the accretion mode, X-ray spectrum, high-frequency timing properties and jet production for both GBH ‘states’ and AGN ‘types’ may be unified into a single framework for the activity cycles of accreting black holes. We discuss the results obtained from a series of XMM– Newton observations of the bright, highly variable Seyfert 1 galaxy NGC 4051 These observations were designed to provide some of the best constraints to date on the high-frequency X-ray variability properties of any AGN and improve our understanding of black hole unification generally.
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