Origin of the Broad Iron Line Feature and the Soft X-ray Variation in Seyfert Galaxies

TL;DR

This study demonstrates that the variable partial covering model explains both spectral variations and broad iron line features in Seyfert galaxies, attributing observed changes mainly to fluctuations in absorbing clouds rather than intrinsic luminosity shifts.

Contribution

It extends the application of the variable partial covering model to 20 Seyfert galaxies, providing a unified explanation for spectral features and variability.

Findings

The model accounts for broad iron line features via iron K-edge absorption.

Spectral variations are driven by changes in cloud covering fraction, not intrinsic luminosity.

Soft X-ray luminosity remains relatively stable despite spectral changes.

Abstract

Many Seyfert galaxies are known to exhibit significant X-ray spectral variations and seemingly broad iron K-emission line features. In this paper, we show that the "variable partial covering model", which has been successfully proposed for MCG-6.30-15 (Miyakawa, Ebisawa & Inoue 2012) and 1H0707.495 (Mizumoto, Ebisawa & Sameshima 2014), can also explain the spectral variations in 2-10 keV as well as the broad iron line features in 20 other Seyfert galaxies observed with Suzaku. In this model, the absorbed spectral component through the optically-thick absorbing clouds has a significant iron K-edge, which primarily accounts for the observed seemingly broad iron line feature. Fluctuation of the absorbing clouds in the line of sight of the extended X-ray source results in variation of the partial covering fraction, which causes an anti-correlation between the direct (not covered) spectral component and the absorbed (covered) spectral component below ~10 keV. Observed spectral variation in 2-10 keV in a timescale of less than ~day is primarily explained by such variations of the partial covering fraction, while the intrinsic soft X-ray luminosity is hardly variable.