X-ray power law spectra in active galactic nuclei

TL;DR

This paper models X-ray spectra of active galactic nuclei using clumpy accretion flows, predicting spectral slopes that align with observations and explaining correlations with black hole mass, accretion rate, and variability.

Contribution

It introduces a theoretical framework linking accretion flow properties to X-ray spectral slopes and variability in AGN, consistent with observed trends.

Findings

Harder spectra in more massive AGN

Steeper spectra at higher accretion rates

Spectral-timing correlation with variability timescales

Abstract

X-ray spectra of active galactic nuclei (AGN) are usually described as power law spectra, characterized by the spectral slope $\alpha$ or photon index $\Gamma$. Here we discuss the X-ray spectral properties within the framework of clumpy accretion flows, and estimate the power law slope as a function of the source parameters. We expect harder spectra in massive objects than in less massive sources, and steeper spectra in higher accretion rate systems. The predicted values of the photon index cover the range of spectral slopes typically observed in Seyfert galaxies and quasars. The overall trends are consistent with observations, and may account for the positive correlation of the photon index with Eddington ratio (and the possible anticorrelation with black hole mass) observed in different AGN samples. Spectral properties are also closely related to variability properties. We obtain that shorter characteristic time scales are associated with steeper spectra. This agrees with the observed `spectral-timing' correlation.