Higher Order Variability Properties of Accreting Black Holes

Abstract

To better constrain the emission mechanism underlying the hard state of galactic black hole candidates, we use high-time resolution RXTE lightcurves for Cyg X-1 and GX 399-4 to compute two higher order variability statistics for these objects, the skewness and the Fourier bispectrum. Similar analyses, in particular using the skewness measure, have been attempted before, but the photon collection area of RXTE allows us to present results of much greater statistical significance. The results for the two objects are qualitatively similar, reinforcing the idea that the same basic mechanisms are at work in both. We find a significantly positive skewness for variability timescales less than $\sim 1$ second, and a {\it negative} skewness for timescale $1-5$ sec. Such a skewness pattern cannot be reproduced by the simplest shot variability models where individual shots have a fixed profile and intensity and are uncorrelated in time. Further evidence against simple shot models comes from the significant detection of a non-zero bicoherence for Fourier periods $\sim 0.1-10$ sec, implying that significant coupling does exist between variations on these timescales. We discuss how current popular models for variability in black hole systems can be modified to match these observations. Using simulated light curves, we suggest that the most likely way to reproduce this observed behavior is to have the variability come in groups of many shots, with the number of shots per unit time fitting an envelope function which has a rapid rise and slow decay, while the individual shots have a slow rise and a rapid decay. Invoking a finite energy reservoir that is depleted by each shot is a natural way of producing the required shot correlations.

Accepted to MNRAS, 10 pages, 7 figures, substantially improved discussion of errors, minor changes to interpretation section

References in corpus (1)

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

Cited by in corpus (27)

• Non-linear X-ray variability in X-ray binaries and active galaxies
• Stingray: A Modern Python Library For Spectral Timing
• The Connection Between Radio Quiet AGN and the High/Soft State of X-Ray Binaries
• An exact analytic treatment of propagating mass accretion rate fluctuations in X-ray binaries
• Long Term Variability of Cyg X-1, II. The rms-Flux Relation
• SAX J1808.4-3658 and the origin of X-ray variability in X-ray binaries and active galactic nuclei
• The chaotic behavior of the black hole system GRS 1915+105
• Random time series in Astronomy
• Coupling between QPOs and broadband noise components in GRS 1915+105
• The biphase explained: understanding the asymmetries in coupled Fourier components of astronomical timeseries
• Simulations of X-ray spectral/timing properties in a propagation model of variability of accreting black holes
• Non-stationary variability in accreting compact objects
• The Bicoherence as a Diagnostic for Models of High Frequency QPOs
• Alternating lags of QPO harmonics : A Generic model and its application to the 67 millihertz QPO of GRS 1915+105
• The puzzling harmonic behavior of the Cathedral QPO in XTE J1859+226
• X-Ray Spectral Variability in Cygnus X-1
• A Likely Inclination Dependence in the Non-linear Variability of Quasi Periodic Oscillations from Black Hole Binaries
• Similar shot profile morphology of fast variability in cataclysmic variable, X-ray binary and blazar; the MV Lyr case
• Parameter Estimation and Confidence Regions in the Method of Light Curve Simulations for the Analysis of Power Density Spectra
• Detecting Dynamical States from Noisy Time Series using Bicoherence
• Reversibility of time series: revealing the hidden messages in X-ray binaries and cataclysmic variables
• Nonlinear Variability of Quasi-Periodic Oscillations in GX 339-4
• Deviations from normal distributions in artificial and real time series: a false positive prescription
• Using the bispectrum to probe radio X-ray correlations in GRS 1915+105
• Time Domain Studies of X-ray Shot Noise in Cygnus X-1
• Fast Spectral Variability from Cygnus X-1
• Investigating Non-linear and Stochastic Hard X-ray Variability of Active Galactic Nuclei using Recurrence Analysis