Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes

TL;DR

This paper develops a general relativistic model for X-ray irradiation of black hole accretion discs by various source geometries, impacting the accuracy of black hole spin measurements from reflection features.

Contribution

It introduces a comprehensive relativistic approach to model different X-ray source geometries and their effects on reflection line shapes, influencing spin measurement interpretations.

Findings

Compact sources produce broad lines enabling spin measurement.

Elongated sources result in narrower lines, complicating spin determination.

No unique spin value can be derived in most geometries, only lower limits.

Abstract

X-ray irradiation of the accretion disc leads to strong reflection features, which are then broadened and distorted by relativistic effects. We present a detailed, general relativistic approach to model this irradiation for different geometries of the primary X-ray source. These geometries include the standard point source on the rotational axis as well as more jet-like sources, which are radially elongated and accelerating. Incorporating this code in the relline model for relativistic line emission, the line shape for any configuration can be predicted. We study how different irradiation geometries affect the determination of the spin of the black hole. Broad emission lines are produced only for compact irradiating sources situated close to the black hole. This is the only case where the black hole spin can be unambiguously determined. In all other cases the line shape is narrower, which could either be explained by a low spin or an elongated source. We conclude that for all those cases and independent of the quality of the data, no unique solution for the spin exists and therefore only a lower limit of the spin value can be given.