Simple interpretation of the seemingly complicated X-ray spectral variation of NGC 5548

TL;DR

This study presents a simplified model of X-ray spectral variation in NGC 5548 using double-layer clumpy absorbers, clarifying the physical mechanisms behind observed spectral changes over 16 years.

Contribution

It introduces a physically consistent double partial covering model to explain spectral variability, avoiding unphysical correlations from previous models.

Findings

Spectral variations are mainly due to changes in soft excess, power-law normalization, and covering fraction.

Photon index remains largely constant despite spectral changes.

The model successfully explains long-term spectral variability without unphysical assumptions.

Abstract

NGC 5548 is a very well-studied Seyfert 1 galaxy in broad wavelengths. Previous multiwavelength observation campaigns have indicated that its multiple absorbers are highly variable and complex. A previous study applied a two-zone partial covering model with different covering fractions to explain the complex X-ray spectral variation and reported a correlation between one of the covering fractions and the photon index of the power-law continuum. However, it is not straightforward to physically understand such a correlation. In this paper, we propose a model to avoid this unphysical situation; the central X-ray emission region is partially covered by clumpy absorbers composed of double layers. These "double partial coverings" have precisely the same covering fraction. Based on our model, we have conducted an extensive spectral study using the data taken by XMM-Newton, Suzaku, and NuSTAR in the range of 0.3-78 keV for 16 years. Consequently, we have found that the X-ray spectral variations are mainly explained by independent changes of the following three components; (1) the soft excess spectral component below ~1 keV, (2) the cut-off power-law normalization, and (3) the partial covering fraction of the clumpy absorbers. In particular, spectral variations above ~1 keV are mostly explained only by the changes of the partial covering fraction and the power-law normalization. In contrast, the photon index and all the other spectral parameters are not significantly variable.