A multi-faceted view of the X-ray spectral variability in Seyfert galaxy Ark 120

TL;DR

This study provides a comprehensive analysis of the complex X-ray spectral variability in Seyfert galaxy Ark 120, revealing timescale-dependent trends, rare behaviors, and the influence of the soft excess component through multiple analytical techniques.

Contribution

It offers a detailed, multi-technique investigation of spectral variability in Ark 120, highlighting the dynamic behaviors of the power-law and soft excess components over different timescales.

Findings

Long-term softer-when-brighter trend in 2-10 keV band

Rare harder-when-brighter trend with low-energy pivoting

Soft excess adds complexity, causing variability pattern transitions

Abstract

Utilizing a range of techniques including multi-band light curves, softness ratio analysis, structure functions, rms spectra, cross-correlation functions, and ratios of spectra from different intervals, we present a comprehensive study of the complex X-ray spectral variability in Seyfert 1 galaxy Ark 120, through re-analyzing its six XMM-Newton observations taken between 2003 and 2014. We find a clear ''softer-when-brighter" trend in the 2--10 keV power-law component over long timescales, with this trend being timescale dependent, as it is much weaker on shorter timescales, similar to that previously detected in NGC 4051. Notably, a rare ''harder-when-brighter" trend is observed during one exposure, indicating dynamic changes in the spectral variability behavior of the power-law component. This exceptional exposure, with the spectral variability indeed marked by a power-law pivoting at an unusually low energy of ~ 2 keV, suggests intricate variations in the thermal Comptonization processes within the corona. Furthermore, when the data below 2 keV are included, we identify that the soft excess component adds significant complexity to the spectral variability, such as evidenced by a transition from ''harder-when-brighter'' to ''softer-when-brighter'' during another single exposure. Such extra complexity arises because the variability of the soft excess sometimes follows and sometimes does not follow the changes in the power-law component. Our findings underscore the necessity of applying multiple analytic techniques to fully capture the multifaceted spectral variability of AGNs.