Superorbital variability of X-ray and radio emission of Cyg X-1. I. Emission anisotropy of precessing sources

TL;DR

This paper investigates the superorbital 150-day modulation in X-ray and radio emissions of Cyg X-1, modeling emission anisotropy due to precession and fitting these models to observational data to understand the source geometry and jet dynamics.

Contribution

It introduces detailed models of emission anisotropy linked to precession and fits them to multi-wavelength data, providing new insights into the geometry and jet velocity of Cyg X-1.

Findings

Best-fit precession angle ~10-20 degrees

Thermal Comptonization explains variability amplitude decrease

Jet velocity estimated at ~(0.3-0.5)c

Abstract

We study theoretical interpretations of the 150-d (superorbital) modulation observed in X-ray and radio emission of Cyg X-1 in the framework of models connecting this phenomenon to precession. Precession changes the orientation of the emission source (either disc or jet) relative to the observer. This leads to emission modulation due to an anisotropic emission pattern of the source or orientation-dependent amount of absorbing medium along the line of sight or both. We consider, in particular, anisotropy patterns of blackbody-type emission, thermal Comptonization in slab geometry, jet/outflow beaming, and absorption in a coronal-type medium above the disc. We then fit these models to the data from the RXTE/ASM, CGRO/BATSE, and the Ryle and Green Bank radio telescopes, and find relatively small best-fit angles between the precession and orbital planes, ~10-20 degrees. The thermal Comptonization model for the X-ray emission explains well the observed decrease of the variability amplitude from 1 to 300 keV as a result of a reduced anisotropy of the emission due to multiple scatterings. Our modeling also yield the jet bulk velocity of ~(0.3-0.5)c, which is in agreement with the previous constraint from the lack of an observed counterjet and lack of short-term X-ray/radio correlations.