Observational Appearance of Black Hole Winds

TL;DR

This paper investigates how relativistic black hole winds appear observationally, focusing on the photosphere's temperature distribution, limb-darkening effects, and luminosity variations with wind speed.

Contribution

It provides a detailed calculation of the apparent photosphere and luminosity of relativistic black hole winds, highlighting the effects of relativistic speeds on observations.

Findings

Apparent photosphere depends on flow velocity and direction.

Limb-darkening effect is significantly enhanced relativistically.

Observed luminosity increases with wind speed, independent of mass-outflow rate.

Abstract

We examine the observational appearance of an optically thick, spherically symmetric, relativistic wind (a black hole wind). In a relativistic flow, the apparent optical depth becomes small in the downstream direction, while it is large in the upstream direction due to the relativistic effect. Hence, the apparent photosphere of the relativistic flow depends on the flow velocity and direction as well as the density distribution. We calculate the temperature distribution of the apparent photosphere of the optically thick black hole wind, where the wind speed is assumed to be constant and radiation dominates matter, for various values of the wind speed and mass-outflow rate. We found that the limb-darkening effect is strongly enhanced in the relativistic regime. We also found that the observed luminosities of the black hole wind become large as the wind speed increases, but do not depend on the mass-outflow rate.