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The rapid X-ray variability of NGC 4051

The rapid X-ray variability of NGC 4051
The rapid X-ray variability of NGC 4051
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 to 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 to 0.1 Hz range and ~5–10 per cent in the 10-4 to 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.
1365-2966
Vaughan, S.
f8cf1168-b49a-4376-9d2c-c990a21e865e
Uttley, P.
db770bd7-d97e-43f5-99d4-a585bccd352a
Pounds, K.A.
dd998d04-7bad-4882-81ef-4241a79dc620
Nandra, K.
b0d1151f-c055-4142-bec1-8de21ee72745
Strohmayer, T.E.
7e764e70-cef1-4eda-adbd-65ab771224d8
Vaughan, S.
f8cf1168-b49a-4376-9d2c-c990a21e865e
Uttley, P.
db770bd7-d97e-43f5-99d4-a585bccd352a
Pounds, K.A.
dd998d04-7bad-4882-81ef-4241a79dc620
Nandra, K.
b0d1151f-c055-4142-bec1-8de21ee72745
Strohmayer, T.E.
7e764e70-cef1-4eda-adbd-65ab771224d8

Vaughan, S., Uttley, P., Pounds, K.A., Nandra, K. and Strohmayer, T.E. (2011) The rapid X-ray variability of NGC 4051. Monthly Notices of the Royal Astronomical Society. (doi:10.1111/j.1365-2966.2011.18319.x).

Record type: Article

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 to 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 to 0.1 Hz range and ~5–10 per cent in the 10-4 to 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.

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Published date: 21 March 2011

Identifiers

Local EPrints ID: 184355
URI: http://eprints.soton.ac.uk/id/eprint/184355
ISSN: 1365-2966
PURE UUID: af16d23a-bb34-465a-92f5-0eb887d9e18e

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Date deposited: 13 May 2011 15:23
Last modified: 14 Mar 2024 03:07

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Contributors

Author: S. Vaughan
Author: P. Uttley
Author: K.A. Pounds
Author: K. Nandra
Author: T.E. Strohmayer

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