The flux-dependent amplitude of broadband noise variability in X-ray binaries and active galaxies

By P. Uttley, I. McHardy. 2001

280 citations recorded by the source corpus.

Standard shot-noise models, which seek to explain the broadband noise variability that characterizes the X-ray light curves of X-ray binaries and active galaxies, predict that the power spectrum of the X-ray light curve is stationary (i.e. constant amplitude and shape) on short time-scales. We show that the broadband noise power spectra of the black hole candidate Cyg X-1 and the accreting millisecond pulsar SAX J1808.4-3658 are intrinsically non-stationary, in that rms variability scales linearly with flux. Flux-selected power spectra confirm that this effect is due to changes in power-spectral amplitude and not shape. The light curves of three Seyfert galaxies are also consistent with a linear relationship between rms variability and flux, suggesting that it is an intrinsic feature of the broadband noise variability in compact accreting systems over more than six decades of central object mass. The rms variability responds to flux variations on all measured time-scales, raising fundamental difficulties for shot-noise models which seek to explain this result by invoking variations in the shot parameters. We suggest that models should be explored where the longest time-scale variations are fundamental and precede the variations on shorter time-scales. Possible models which can explain the linear rms-flux relation include the fractal break-up of large coronal flares, or the propagation of fluctuations in mass accretion rate through the accretion disc. The linear relationship between rms variability and flux in Cyg X-1 and SAX J1808.4-3658 is offset on the flux axis, suggesting the presence of a second, constant-flux component to the light curve which contributes ~25 per cent of the total flux. The spectrum of this constant component is similar to the total spectrum, suggesting that it may correspond to quiet, non-varying regions in the X-ray emitting corona.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/9f5898ff8a3f6a6fc94a0cdaafb45189445fc5fd

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In Astronomy and Astrophysics

1. Ramesh Narayan
2. J. McKinney
3. T. Johannsen
4. A. C. Fabian
5. S. Vaughan
6. P. Uttley
7. F. Yuan
8. J. E. McClintock

In Instrumentation

1. S. Vaughan
2. Ramesh Narayan
3. A. C. Fabian
4. C. Done
5. D. M. Alexander
6. P. Uttley
7. I. McHardy
8. R. Hickox

Citations in the corpus

• Active galactic nuclei: what’s in a name? (2017, result): “…1989) had difficulties in explaining other X-ray timing properties of both AGN and BHXRBs, such as the observed “rms-flux” relation, whose existence was first shown by Uttley and McHardy (2001) and later confirmed by several studies (e.g. Vaughan et al. 2003; Gaskell 2004; McHardy et al. 2004).”
• Non‐linear X‐ray variability in X‐ray binaries and active galaxies (2005, methodology): “However, the observed X-ray light curves of AGN and XRBs are not merely weakly non-stationary in this sense: the σrms of segments of the light curve varies randomly about a mean value which scales linearly with the flux of the segment (Uttley & McHardy, 2001).”
• Investigating a fluctuating-accretion model for the spectral-timing properties of accreting black hole systems (2005, background): “As noted by Uttley & McHardy (2001) the model of Lyubarskii (1997) is consistent with the observed linear relation between rms-variability amplitude and mean count c© 2005 RAS rate in XRB and AGN X-ray light curves, which implies a coupling between fluctuations on different time-scales, ruling out…”
• Relativistic X-Ray Lines from the Inner Accretion Disks Around Black Holes (2007, background): “In turn, this suggests that only a few flares (or perhaps the base of a jet) may dominate hard X-ray production in actively accreting black holes (Uttley & McHardy 2001, Uttley, McHardy, & Vaughan 2005; for a treatment of jets and coronae, see Markoff, Nowak, & Wilms 2005).”
• On characterizing the variability properties of X-ray light curves from active galaxies (2003, background): “…PSDs of X-ray binaries evolve with time, and the way in which the variability properti s evolve provides a great deal of information on the detailed workings of these systems (see e.g. Belloni & Hasinger 1990; Uttley & McHardy 2001; Belloni, Psaltis & van der Klis 2002; Pottschmidt et al. 2002).”
• X-ray reverberation around accreting black holes (2014, methodology): “Significant support for the propagation model came from the discovery of the linear rms-flux relation in XRB (and AGN) X-ray variability (Uttley & McHardy 2001), which is easily explained by intrinsic accretion flow variability and its propagation through the flow (Lyubarskii 1997).”
• X-Ray Spectral Variability and Rapid Variability of the Soft X-Ray Spectrum Seyfert 1 Galaxies Arakelian 564 and Ton S180 (2001, result): “This result confirms and expands upon the finding of Uttley & McHardy (2001) that found a similar independence of Fvar from flux for three other Seyfert 1s, although those were measured with only two independent flux points.”
• X-ray continuum variability of MCG-6-30-15 (2002, background): “The strong rms-flux correlation first seen in X-ray binaries (Uttley & McHardy 2001) is exhibited by MCG–6-30-15 (see Section 3.2).”
• Combined long and short timescale X-ray variability of NGC 4051 with RXTE and XMM-Newton (2003, methodology): “However randomly timed shots do not produce the linear rms-flux relationship which is seen in GBHs and AGN (Uttley and McHardy 2001), and which is followed closely here by NGC 4051.”
• Measuring the broad-band power spectra of active galactic nuclei with RXTE (2002, methodology): “Conclusions 4 & 5 imply that the power spectra of AGN are consistent with being identical in shape and fractional RMS amplitude (integrated over the whole power spectrum) to those of BHXRBs, with characteristic time-scales scaling linearly with black hole mass. Arguments based on accretion rate seem”
• The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project (2019, background): “This quite striking result (after all, there is a scatter in mean accretion rates itself by a factor of ∼8 in the sample) is a restatement of the rms–flux relationship, which is a ubiquitous feature of BH accretion across all mass ranges (Uttley & McHardy 2001; Heil et al. 2012).”
• Modelling the behaviour of accretion flows in X-ray binaries (2007, background): “The size of the rms fluctuations, σ, is linearly related to the source flux, F , such that that σ/F remains constant (Uttley & McHardy 2001).”
• Generating artificial light curves: revisited and updated (2013, methodology): “Furthermore, the rms-flux relation, i.e. the linear scaling of the fractional root mean square (rms) variability amplitude with the flux, observed in both AGN and X-ray binaries (XRBs) (Uttley & McHardy 2001; Uttley et al. 2005; Gandhi 2009; McHardy 2010), cannot be reproduced by the TK95 algorithm.”