The role of the reflection fraction in constraining black hole spin

TL;DR

This paper investigates how the reflection fraction in X-ray spectra constrains black hole spin, demonstrating that high reflection fractions indicate rapidly rotating black holes and improve spin measurements.

Contribution

It provides a method to calculate the maximum reflection fraction for a given black hole spin in the lamp post model and incorporates this constraint into spin estimation.

Findings

High reflection fractions (>2) only occur in rapidly spinning black holes.

Incorporating reflection fraction constraints improves spin measurement accuracy.

Software routines implementing these constraints are made publicly available.

Abstract

In many active galaxies, the X-ray reflection features from the innermost regions of the accretion disc are relativistically distorted. This distortion allows us to measure parameters of the black hole such as its spin. The ratio in flux between the direct and the reflected radiation, the so-called reflection fraction, is determined directly from the geometry and location of primary source of radiation. We calculate the reflection fraction in the lamp post geometry in order to determine its maximal possible value for a given value of black hole spin. We show that high reflection fractions in excess of 2 are only possible for rapidly rotating black holes, suggesting that the high spin sources produce the strongest relativistic reflection features. Using simulations we show that taking this constraint into account does significantly improve the determination of the spin values. We make software routines for the most popular X-ray data analysis packages available that incorporate these additional constraints.