Effects of Kerr Strong Gravity on Quasar X-ray Microlensing

TL;DR

This paper demonstrates that Kerr strong gravity significantly influences quasar X-ray microlensing light curves, affecting size estimates of emission regions and enabling potential constraints on black hole parameters.

Contribution

It introduces a combined Kerr lensing and microlensing scheme, revealing how Kerr gravity alters light curves and impacts size and parameter estimations of quasar X-ray emission regions.

Findings

Kerr strong gravity can alter microlensing fluctuation amplitudes by ~0.65-0.75 mag.

Size estimates of X-ray emission regions can be biased by up to 50%.

Including Kerr lensing improves accuracy of quasar size and black hole parameter measurements.

Abstract

Recent quasar microlensing observations have constrained the sizes of X-ray emission regions to be within about 10 gravitational radii of the central supermassive black hole. Therefore, the X-ray emission from lensed quasars is first strongly lensed by the black hole before it is lensed by the foreground galaxy and star fields. We present a scheme that combines the initial strong lensing of a Kerr black hole with standard linearized microlensing by intervening stars. We find that X-ray microlensed light curves incorporating Kerr strong gravity can differ significantly from standard curves. The amplitude of the fluctuations in the light curves can increase or decrease by ~0.65-0.75 mag by including Kerr strong gravity. Larger inclination angles give larger amplitude fluctuations in the microlensing light curves. Consequently, current X-ray microlensing observations might have under or overestimated the sizes of the X-ray emission regions. We estimate this bias using a simple metric based on the amplitude of magnitude fluctuations. The half light radius of the X-ray emission region can be underestimated up to ~50% or overestimated up to ~20%. Underestimates are found in most situations we have investigated. The only exception is for a disk with large spin, radially flat emission profile, and observed nearly face on, where an overestimate is found. Thus, more accurate microlensing size constraints should be obtainable by including Kerr lensing. The caustic crossing time can differ by months after including Kerr strong gravity. A simultaneous monitoring of gravitational lensed quasars in both X-ray and optical bands with densely sampled X-ray light curves might reveal this feature. We conclude that it should be possible to constrain important parameters such as inclination angles and black hole spins from combined Kerr and microlensing effects.