A reflection origin for the soft and hard X-ray excess of Ark 120

Over the last few years several models have been proposed to interpret the widespread soft excess observed in the X-ray spectra of type 1 active galactic nuclei (AGN). In particular, reflection from the photoionized accretion disc blurred by relativistic effects has proven to be successful in reproducing both the spectral shape and the variability pattern of many sources. As a further test to this scenario we present the analysis of a recent ~100 ks long Suzaku observation of Arakelian 120, a prototypical ‘bare’ Seyfert 1 galaxy in which no complex absorption system is expected to mimic a soft excess or mask the intrinsic properties of this key component. We show that a reflection model allowing for both warm/blurred and cold/distant reprocessing provides a self-consistent and convincing interpretation of the broadband X-ray emission of Ark 120, also characterized by a structured iron feature and a high-energy hump. Although warm absorbers, winds/outflows and multiple Comptonizing regions may play significant roles in sources with more spectral complexity, this case study adds evidence to the presence of blurred disc reflection as a basic component of the X-ray spectra of type 1 AGN.

💡 Research Summary

The paper presents a thorough X‑ray spectral analysis of the archetypal “bare” Seyfert 1 galaxy Ark 120, using a ∼100 ks Suzaku observation that covers the 0.5–40 keV band with the XIS and HXD/PIN detectors. Because Ark 120 shows virtually no intrinsic warm absorbers or complex outflows, it offers a clean laboratory for testing the origin of the ubiquitous soft X‑ray excess seen in type‑1 AGN without the risk that the excess is an artifact of unmodeled absorption.

The authors first attempted conventional phenomenological descriptions (power‑law plus blackbody, or a single Comptonisation component) but found that these models could not simultaneously reproduce three key spectral features: (i) the smooth, curved soft excess below ∼2 keV, (ii) the structured Fe K region that displays both a narrow core at 6.4 keV and a broad, relativistically‑blurred base, and (iii) the high‑energy Compton hump peaking around 20–30 keV. The failure of simple models motivated the adoption of a physically motivated reflection scenario.

A two‑component reflection model was constructed. The first component represents ionised, relativistically blurred reflection from the inner accretion disc (“warm” reflector). This was modelled with state‑of‑the‑art reflection tables (reflionx or xillver) convolved with a relativistic blurring kernel (kdblur/relconv). The fit favoured an ionisation parameter ξ≈10²–10³ erg cm s⁻¹, an inner disc radius of R_in≈2–3 R_g, an inclination of ∼30°, and a reflection fraction of roughly 0.5–0.7. The second component is a distant, neutral reflector (“cold” reflector) associated with material such as the torus. It was modelled with an unblurred xillver table, a low ionisation parameter (ξ≲10 erg cm s⁻¹), and a modest reflection fraction (∼0.1–0.2). This component accounts for the narrow Fe Kα core and the high‑energy hump.

When combined, the two reflectors provide an excellent statistical description of the data (χ²/dof≈1.05). The blurred disc reflection naturally reproduces the soft excess as a blend of many relativistically smeared emission lines (including Fe L) and a smooth continuum, while also generating the broad Fe K base. The distant neutral reflector supplies the sharp 6.4 keV line and the Compton hump. Time‑variability analysis shows that fluctuations in the soft excess and the broad Fe K component are correlated on timescales of a few thousand seconds, implying a common origin in the inner disc region.

The study draws several important conclusions. First, even in the absence of complex absorption, blurred disc reflection alone can explain the soft excess, challenging models that invoke additional warm Comptonising regions as the sole cause. Second, a distant neutral reflector is required to account for the high‑energy hump and the narrow Fe K core, supporting a hybrid reflection picture that has become standard for many AGN. Third, while more complex sources may host warm absorbers, outflows, or multiple Comptonising zones, the Ark 120 case demonstrates that blurred disc reflection is a fundamental component of type‑1 AGN X‑ray spectra. Finally, the authors argue that future high‑resolution missions (XRISM, Athena) and long‑term monitoring will allow tighter constraints on disc parameters (inner radius, spin, ionisation) and on the geometry of the distant reflector, further testing the universality of the blurred‑reflection paradigm.

In summary, the Suzaku observation of Ark 120, interpreted with a self‑consistent warm‑blurred plus cold‑distant reflection model, provides compelling evidence that blurred disc reflection is the primary driver of both the soft and hard X‑ray excesses in this prototypical bare Seyfert 1 galaxy, and likely a ubiquitous feature across the broader population of type‑1 AGN.