Modelling X-ray RMS spectra I: intrinsically variable AGN

TL;DR

This paper introduces simple XSPEC models for analyzing the excess variance spectra of AGN, demonstrating their effectiveness with a case study of IRAS 13224-3809 and highlighting the role of various physical processes in shaping variability.

Contribution

It develops a set of models based on Monte Carlo simulations to interpret AGN variability spectra, incorporating effects like relativistic reflection and ultra fast outflows.

Findings

The models successfully fit the IRAS 13224-3809 spectra.

Reflection dampens variability, while outflows enhance it.

Correlation between flux components varies with frequency.

Abstract

We present simple XSPEC models for fitting excess variance spectra of AGN. Using a simple Monte-Carlo approach, we simulate a range of spectra corresponding to physical parameters varying, then calculate the resulting variance spectra. Starting from a variable power-law, we build up a set of models corresponding to the different physical processes that can affect the final excess variance spectrum. We show that the complex excess variance spectrum of IRAS 13224-3809 can be well described by such an intrinsic variability model, where the power-law variability is damped by relativistic reflection and enhanced by an ultra fast outflow. The reflection flux is correlated with that of the power-law, but not perfectly. We argue that this correlation is stronger at high frequencies, where reverberation lags are detected, while excess variance spectra are typically dominated by low frequency variability.