Examining the soft excess in a sample of active galactic nuclei

Abstract

In active galactic nuclei (AGN), the soft excess refers to the emission below 2 keV where the flux exceeds the extrapolated high energy continuum. The phenomenon was first observed in the 1980s and there is currently no definitive explanation of its origin. One possible model is blurred reflection, which connects the soft excess to the high energy continuum by combining the contributions of primary X-ray emission with emission reflected off the accretion disk. A second model is a warm corona or warm Comptonization. This deals with two different X-ray sources, the original hot corona is now joined by a so-called warm corona at lower temperatures which is more optically thick and produces a steeper spectrum below 2 keV. In this analysis, both the blurred reflection and warm corona models as well as a combined model are applied to a sample of 64 AGN, including both broad line Seyfert 1s (BLS1s) and narrow line Seyfert 1s (NLS1s), that were observed between 0.6 - 40 keV with Suzaku. The best model for each source is determined using the corrected Akaike Information Criterion (AICc). We find that both populations are best fit with the combined model, while blurred reflection alone produces the worst fit statistics of the three models. We calculate the luminosity ratios for the warm corona and blurred reflection contributions to the soft excess, finding that both populations require significant contributions from both components, while the NLS1s exhibit slightly higher warm corona strengths. We also find correlations between low and high energies in the NLS1s and not in the BLS1s which may indicate that the NLS1s present a more cohesive population or that the two groups host different geometries in their central regions.