The high energy probability distribution of accretion disc luminosity fluctuations

Abstract

The probability density function of accretion disc luminosity fluctuations at high observed energies (i.e., energies larger than the peak temperature scale of the disc) is derived, under the assumption that the temperature fluctuations are log-normally distributed. Thin disc theory is used throughout. While log-normal temperature fluctuations would imply that the disc's bolometric luminosity is also log-normal, the observed Wien-like luminosity behaves very differently. For example, in contrast to a log-normal distribution, the standard deviation of the derived distribution is not linearly proportional to its mean. This means that these systems do not follow a linear rms-flux relationship. Instead they exhibit very high intrinsic variance, and undergo what amounts to a phase transition, in which the mode of the distribution (in the statistical sense) ceases to exist, even for physically reasonable values of the underlying temperature variance. The moments of this distribution are derived using asymptotic expansion techniques. A result that is important for interpreting observations is that the theory predicts that the fractional variability of these disc systems should increase as the observed frequency is increased. The derived distribution will be of practical utility in quantitatively understanding the variability of disc systems observed at energies above their peak temperature scale, including X-ray observations of tidal disruption events.

arXiv comment10 pages, 13 figures, accepted for publication in MNRAS

References in corpus (12)

• Non-linear X-ray variability in X-ray binaries and active galaxies
• The flux-dependent amplitude of broadband noise variability in X-ray binaries and active galaxies
• Cold gas stripping in satellite galaxies: from pairs to clusters
• Long Term Variability of Cyg X-1, II. The rms-Flux Relation
• Accretion-induced variability links young stellar objects, white dwarfs, and black holes
• The Universal Nature of Accretion-induced Variability: The RMS-Flux Relation in an Accreting White Dwarf
• The rapid X-ray variability of NGC 4051
• Testing the Propagating Fluctuations Model with a Long, Global Accretion Disk Simulation
• The flux-dependent rms variability of X-ray binaries in the optical
• Energy-dependent variability from accretion flows
• An upper observable black hole mass scale for tidal disruption events with thermal X-ray spectra
• Investigating the Theory of Propagating Fluctuations with Numerical Models of Stochastic Accretion Discs

Cited by in corpus (5)

• Long term variability of Cygnus X-1. VIII. A spectral-timing look at low energies with NICER
• eRASSt J074426.3+291606: prompt accretion disc formation in a 'faint and slow' tidal disruption event
• A new 2D stochastic methodology for simulating variable accretion discs: propagating fluctuations and epicyclic motion
• The turbulent variability of accretion discs observed at high energies
• The X-ray variability of tidal disruption events