Non-linear X-ray variability in X-ray binaries and active galaxies

Abstract

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 lognormal 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/hard state, 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.

20 pages, 9 figures, accepted for publication in MNRAS

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• 20 years of monitoring: PKS 2155-304 and PKS 1510-089 in the eyes of Swift and Fermi. II. PKS 1510-089 and comparison
• NICER Perspective on TeV Blazar Mrk~421: X-ray Variability and Particle Acceleration
• Broadband variability study of MAXI J1631-479 in its Hard-Intermediate State observed with Insight-HXMT
• Statistical analysis of archival Vela X-1 data
• The Origin of X-ray Emission in the Gamma-ray emitting Narrow-Line Seyfert 1 1H 0323+342
• Suzaku Observations of Cyg X-1
• Optimal strategies for observation of active galactic nuclei variability with Imaging Atmospheric Cherenkov Telescopes
• Modeling of High-Frequency Variability in X-ray Binaries with Black Holes
• Intrinsic electromagnetic variability in celestial objects containing rapidly spinning black holes
• iid2022: A Workshop on Statistical Methods for Event Data in Astronomy