Variation in the primary and reprocessed radiation from an orbiting spot around a black hole

TL;DR

This paper investigates how relativistic effects influence the observed X-ray reflection spectra from a spot on an accretion disc around a rotating black hole, revealing significant variability that can inform black hole spin estimates.

Contribution

It provides a detailed analysis of relativistic effects on reflected X-ray spectra from orbiting spots, including flux ratios and spectral variations, to better understand black hole spin.

Findings

Reflected flux can surpass primary flux under certain conditions.

Equivalent width variations can reach 30% at 30° inclination.

Reflected flux and hardness ratio vary with spot phase, especially at small orbital radii.

Abstract

We study light curves and spectra (equivalent widths of the iron line and some other spectral characteristics) which arise by reflection on the surface of an accretion disc, following its illumination by a primary off-axis source - an X-ray 'flare', assumed to be a point-like source just above the accretion disc resulting in a spot with radius dr/r<1. We consider General Relativity effects (energy shifts, light bending, time delays) near a rotating black hole, and we find them all important, including the light bending and delay amplification due to the spot motion. For some sets of parameters the reflected flux exceeds the flux from the primary component. We show that the orbit-induced variations of the equivalent width with respect to its mean value can be as high as 30% for the observer's inclination of 30 degrees, and much more at higher inclinations. We calculate the ratio of the reflected flux to the primary flux and the hardness ratio which we find to vary significantly with the spot phase mainly for small orbital radii. This offers the chance to estimate the lower limit of the black hole spin if the flare arises close to the black hole.