Non‐linear X‐ray variability in X‐ray binaries and active galaxies
By P. Uttley, I. McHardy, S. Nasagsfc, U. Southampton, U. Leicester. 2005
355 citations recorded by the source corpus.
We show that the rms-flux relation recently discovered in the X-ray light curves of Active Galactic Nuclei (AGN) and X-ray binaries (XRBs) implies that the light curves have a formally non-linear, exponential form, provided the rms-flux relation applies to variations on all time-scales (as it appears to). This phenomenological model implies that stationary data will have a log normal flux distribution. We confirm this result using an observation of Cyg X-1, and further demonstrate that our model predicts the existence of the powerful millisecond flares observed in Cyg X-1 in the low/hardstate, and explains the general shape and amplitude of the bicoherence spectrum in that source. Our model predicts that the most variable light curves will show the most extreme non-linearity. This result can naturally explain the apparent non-linear variability observed in some highly variable Narrow Line Seyfert 1 (NLS1) galaxies, as well as the low states observed on long time-scales in the NLS1 NGC 4051, as being nothing more than extreme manifestations of the same variability process that is observed in XRBs and less variable AGN. That variability process must be multiplicative (with variations coupled together on all time-scales) and cannot be additive (such as shot-noise), or related to self-organised criticality, or result from completely independent variations in many separate emitting regions. Successful models for variability must reproduce the observed rms-flux relation and non-linear behaviour, which are more fundamental characteristics of the variability process than the power spectrum or spectral-timing properties. Models where X-ray variability is driven by accretion rate variations produced at different radii remain the most promising.
Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics
View Paper: https://www.semanticscholar.org/paper/49db937075f7a38d273602fd74712fb506ea70df
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Citations in the corpus
- Generating artificial light curves: revisited and updated (2013, methodology): “B1 in Uttley et al. (2005).”
- ARE THE VARIATIONS IN QUASAR OPTICAL FLUX DRIVEN BY THERMAL FLUCTUATIONS? (2009, background): “Uttley et al. (2005) have also argued for describing accreting black holes as a Gaussian stochastic process in the logarithm of the flux.”
- A Bayesian test for periodic signals in red noise (2009, background): “It is fairly well established that Xray light curves from Seyfert galaxies are stationary once allow nce has been made for their red noise character and the linear “rmsflux” relation (see Vaughan et al. 2003; Uttley et al. 2005).”
- X-ray reverberation around accreting black holes (2014, methodology): “The observed non-linear, lognormal flux distribution in BHXRBs (and apparent non-linear variability in AGN) can also be simply explained by the propagation of variations through the flow, which multiply together as they move inwards (Uttley et al. 2005).”
- Measuring the kinetic power of active galactic nuclei in the radio mode (2007, background): “Therefore the most likely value of the ratio of the mean to theobserved luminosity is given by (Nipoti & Binney 2005; Uttley et al. 2005): 〈L〉 Lobs = e 3 2 σ2 l = (σ2rms + 1) 3/2, (14) where we have introduced the rms (fractional) variability of the observed lightcurveσ2rms = expσ 2 l − 1, an…”
- A review of quasi-periodic oscillations from black hole X-ray binaries: Observation and theory (2019, background): “This is expected, since the broadband noise is not Gaussian distributed, but log-normal distributed, which naturally arises from propagation of fluctuations in mass accretion rate [231].”
- Rapid optical and X-ray timing observations of GX 339-4: multi-component optical variability in the low/hard state (2010, background): “…and optical studies have now shown that perturbations in the outer parts of the accretion flow propagate towards thecentre, seeding and coupling fluctuations in all inner regions, and resulting in a linear relation between rms and flux (e.g. Uttley et al. 2005; Gandhi 2009, and references therein).”
- Active galactic nuclei: what’s in a name? (2017, background): “The existence of an rms-flux relation implies that the underlying physical process is multiplicative and that the long-term variations should modulate the short-term variations (Gaskell 2004; Uttley et al. 2005).”
- TURBULENT, EXTREME MULTI-ZONE MODEL FOR SIMULATING FLUX AND POLARIZATION VARIABILITY IN BLAZARS (2013, methodology): “Following a suggestion by Uttley et al. (2005), the TEMZ code exponentiates each number produced by the subroutine in order to derive the multiplicative factor that is applied to the input value of the energy density.”
- Modelling the behaviour of accretion flows in X-ray binaries (2007, background): “There is no way to do this in any model of variability which takes a sum of independent events, so all shot noise models are ruled out (Uttley et al. 2005).”
- What drives the growth of black holes (2011, background): “…in response to changes to the BH 3The X-ray variability of AGN seen on intra-day timescales is related to the mass accretion but is unrelated to the accretion-disc state (e.g., Ulrich et al., 1997; Vaughan et al., 2003; Uttley et al., 2005). accretion rate (e.g., Nowak, 1995; Done et al., 2007).”