Long term X-Ray Observations of Seyfert 1 Galaxy Ark 120: On the origin of soft-excess

TL;DR

This study analyzes 15 years of X-ray data from Ark 120 to understand the origin of its soft-excess, revealing correlations, time delays, and physical parameters changes linked to accretion dynamics.

Contribution

It provides a comprehensive long-term spectral and temporal analysis, proposing that the soft-excess originates from a small number of scatterings in the Compton cloud, supported by simulations.

Findings

Spectral parameters vary over 15 years, linked to accretion changes.

X-ray time delays fluctuate, explained by Comptonization and reflection.

Soft-excess and primary continuum are correlated, sharing a common origin.

Abstract

We present the long-term X-ray spectral and temporal analysis of a 'bare-type AGN' Ark 120. We consider the observations from XMM-Newton, Suzaku, Swift, and NuSTAR from 2003 to 2018. The spectral properties of this source are studied using various phenomenological and physical models present in the literature. We report (a) the variations of several physical parameters, such as the temperature and optical depth of the electron cloud, the size of the Compton cloud, and accretion rate for the last fifteen years. The spectral variations are explained from the change in the accretion dynamics; (b) the X-ray time delay between 0.2-2 keV and 3-10 keV light-curves exhibited zero-delay in 2003, positive delay of 4.71 \pm 2.1 ks in 2013, and negative delay of 4.15 \pm 1.5 ks in 2014. The delays are explained considering Comptonization, reflection, and light-crossing time; (c) the long term intrinsic luminosities obtained using nthcomp, of the soft-excess and the primary continuum show a correlation with a Pearson Correlation Coefficient of 0.922. This indicates that the soft-excess and the primary continuum are originated from the same physical process. From a physical model fitting, we infer that the soft excess for Ark 120 could be due to a small number of scatterings in the Compton cloud. Using Monte-Carlo simulations, we show that indeed the spectra corresponding to fewer scatterings could provide a steeper soft-excess power-law in the 0.2-3 keV range. Simulated luminosities are found to be in agreement with the observed values.