Clumpy wind accretion in Cygnus X-1

TL;DR

This paper models the short-term X-ray variability in Cygnus X-1 using 2D relativistic hydrodynamic simulations of clumpy wind accretion, revealing oscillatory shocks that match observed power spectra.

Contribution

It introduces a 2D relativistic hydrodynamic simulation of wind-fed accretion with clumpy winds to explain rapid X-ray variability in Cygnus X-1.

Findings

Oscillatory shock solutions explain variability.

Simulation matches observed power density spectra.

Clumpy wind accretion causes short-term X-ray fluctuations.

Abstract

Cygnus X-1 is one of the brightest X-ray sources observed and shows the X-ray intensity variations on time scales from milliseconds to months in both the soft and hard X-rays. The accretion onto the black hole is believed to be wind fed due to focused stellar wind from the binary companion HDE-226868. We aim to understand the physical mechanism responsible for the short timescale X-ray variability ($<$100 s) of the source in its Hard/Low state. We compute the 2D relativistic hydrodynamic simulation of the low angular momentum accretion flow with a time dependent outer boundary condition that reflects the focused, clumpy wind from the super-giant in this X-ray binary system. We follow the dynamical evolution of our model for about 100 s and present the results showing an oscillatory shock, being a potential explanation of variability observed in hard X-rays. The simulated model with shock solutions is in good agreement with the observed power density spectra of the source.