Long-term X-ray observations of seyfert 1 galaxy ark 120: on the origin of soft-excess

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 the optical depth of the electron cloud, the size of the Compton cloud, and the accretion properties for the last fifteen years. The spectral variations are explained from the changes 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 ± 2.1 ks in 2013, and negative delay of 2.90 ± 1.26 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 Co-efficient of 0.90. This indicates that the soft-excess and the primary continuum originate 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.

Survey of Bare Active Galactic Nuclei in the Local Universe (z < 0.2). I. On the Origin of Soft Excess

“…We initially used the powerlaw model to fit the data in the energy range of 3.0−10.0 keV to constrain the primary continuum. Thereafter, the excess counts below 2.0 keV were fitted using another powerlaw component (Walter & Fink 1993;Nandi et al 2021). Along with these components, we used Tbabs and zTBabs (Wilms et al 2000) for Galactic and extragalactic absorptions.…”

“…Moreover, considering the timing aspect of the soft excess with respect to the primary continuum, if a thermal blackbody generates the soft excess, then 0.5-2 keV from the standard accretion disk should lead to the 3-10 keV Comptonized primary continuum in time. However, this was not observed for all cases (Nandi et al 2021). These discoveries provoked the alternate origins of the soft excess.…”

“…A fraction of coronal continuum photons could get reprocessed in the colder circumnuclear matter, like a dusty torus, broad-line region (BLR), and narrowline region, producing several spectral features, like absorption lines and iron lines having various shapes (Jana et al 2020). The X-ray spectra of AGNs are often associated with an excess emission below 2 keV, known as soft excess (Halpern 1984;Arnaud et al 1985;Fabian et al 2009;Done et al 2012;García et al 2014;Nandi et al 2021), and several fluorescent emission lines in the soft and hard X-rays. Among them, the most prominent and ubiquitous is the Fe Kα line at ∼6.4 keV (Ross & Fabian 2005;Fabian et al 2009;García & Kallman 2010).…”

“…In this scenario, the UV photons are Compton upscattered in a warm (kT e ∼ 0.1-1 keV) and optically thick (τ ∼ 10-40) corona that is somewhat sandwiching the inner region of the disk. Recently, Nandi et al (2021) reported a strong correlation (a Pearson coefficient of 0.9) between the soft excess and the primary continuum luminosities in the 0.5-10 keV energy band for Ark 120, a well-known "bare" AGN. Their finding suggests that the origin of both luminosities could be linked to a similar radiation process.…”

“…Further, Bechtold et al (1987) and Vaughan et al (2002) suggested a plausible cause of soft excess emission that depends on the inverse Comptonization in the Compton cloud or hot corona. From the Monte Carlo simulations, it was observed that the fewer scatterings in the corona could provide the steeper power-law spectrum for the soft X-ray regime while the higher number of scatterings could produce the primary continuum (Nandi et al 2021).…”