Simulations of the Fe K-alpha Energy Spectra from Gravitationally Microlensed Quasars

TL;DR

This paper uses detailed simulations to show how gravitational microlensing can produce complex, multiply peaked Fe K-alpha energy spectra in quasars, helping to constrain black hole and accretion disk properties.

Contribution

It combines emission and microlensing simulations to demonstrate the formation of multiply peaked spectra and explores parameter dependencies for better understanding of quasar environments.

Findings

Microlensing can produce multiply peaked Fe K-alpha spectra.

Observed spectral peaks imply high black hole inclination (>70°).

Corona height influences spectral peak energies.

Abstract

The analysis of the Chandra X-ray observations of the gravitationally lensed quasar RX J1131-1231 revealed the detection of multiple and energy-variable spectral peaks. The spectral variability is thought to result from the microlensing of the Fe K-alpha emission, selectively amplifying the emission from certain regions of the accretion disk with certain effective frequency shifts of the Fe K-alpha line emission. In this paper, we combine detailed simulations of the emission of Fe K-alpha photons from the accretion disk of a Kerr black hole with calculations of the effect of gravitational microlensing on the observed energy spectra. The simulations show that microlensing can indeed produce multiply peaked energy spectra. We explore the dependence of the spectral characteristics on black hole spin, accretion disk inclination, corona height, and microlensing amplification factor, and show that the measurements can be used to constrain these parameters. We find that the range of observed spectral peak energies of QSO RX J1131-1231 can only be reproduced for black hole inclinations exceeding 70 degree and for lamppost corona heights of less than 30 gravitational radii above the black hole. We conclude by emphasizing the scientific potential of studies of the microlensed Fe K$\alpha$ quasar emission and the need for more detailed modeling that explores how the results change for more realistic accretion disk and corona geometries and microlensing magnification patterns. A full analysis should furthermore model the signal-to-noise ratio of the observations and the resulting detection biases.