Confronting Comptonization and Reflection Models to Explain the Soft-Excess in AGN : The Case of Mrk 509
Abstract
The observed X-ray spectra from active galactic nuclei (AGN) in Seyfert (Sy) galaxies is typically dominated by a power-law continuum with a flat distribution in energy density extending to high energies. In a large fraction of these sources (as high as 50%), a strong and featureless component is observed in excess to this continuum, peaking at soft energies at ~1-2 keV. The origin of this soft-excess has been matter of debate for the last three decades. Presently, the two leading explanations are: (1) the presence of an optically thick (τ~10—20) and warm corona (kT~0.5 keV) that Comptonizes disk photons; or (2) reprocessing of the power-law photons into a very dense inner accretion disk which produces a reflection spectrum smeared by relativistic effects near the black hole. We present a spectral analysis of recent NuSTAR and Suzaku observations of the Sy 1 AGN Mrk 509 implementing both of these models to describe the broad band X-ray spectrum. These two scenarios yield equivalent fit statistics. However, through detailed photoionization calculations, we show that even in the most favorable conditions the warm corona will also produce very strong absorption features4 which are inconsistent with the observed data. Meanwhile, the relativistic reflection scenario requires both extreme blurring and high density, a configuration favored by enhancement of X-ray flux at soft energies due to the additional free-free heating in a dense reflector. Therefore, based on our analysis, we favor the high-density relativistic reflection scenario (2) to explain the soft excess in Mrk 509. We further discuss the implications of these new high density models in mitigating the very large iron abundances typically derived from reflection spectroscopy studies.
- Publication:
-
AAS/High Energy Astrophysics Division
- Pub Date:
- March 2019
- Bibcode:
- 2019HEAD...1710637G