Variation of the primary and reprocessed radiation in the flare-spot model

TL;DR

This paper investigates how light curves and spectra from X-ray flares near rotating black holes are affected by general relativity, revealing significant flux variations and potential methods to estimate black hole spin.

Contribution

It provides a detailed analysis of relativistic effects on reprocessed X-ray emission from accretion disks, including flux ratios and spectral variations, to infer black hole properties.

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 flare orbital phase, especially at small radii.

Abstract

We study light curves and spectra (equivalent widths of the iron line and some other spectral characteristics) which arise by reprocessing 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. We consider all general relativity effects (energy shifts, light bending, time delays, delay amplification due to the spot motion) near a rotating black hole. 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 an 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. We show the results for different values of the flare orbital radius.