Spectral fittings of warm coronal radiation with high seed photon temperature: apparent low-temperature and flat soft excess in AGNs

TL;DR

This study uses Monte Carlo simulations to show that high seed photon temperatures in warm corona models can produce apparent low-temperature and flat soft excess features in AGN spectra, affecting spectral fitting accuracy.

Contribution

It demonstrates that high seed photon temperatures significantly influence spectral fitting results, highlighting the need to treat $kT_{s}$ as a free parameter with an upper limit.

Findings

High $kT_{s}$ can cause apparent low-temperature soft excess.

Extremely high $kT_{s}$ may produce unobserved blackbody-like spectra.

Accurate coronal geometry is essential for $kT_{s}>0.01$ keV.

Abstract

A warm corona has been widely proposed to explain the soft X-ray excess (SE) above the 2--10 keV power law extrapolation in AGNs. In actual spectral fittings, the warm coronal seed photon temperature ($T_{\rm s}$) is usually assumed to be far away from the soft X-ray, but $kT_{\rm s}$ can reach close to 0.1 keV in standard accretion disc model. In this study, we used Monte Carlo simulations to obtain radiation spectra from a slab-like warm corona and fitted the spectra using the spherical-geometry-based routine \textsc{thcomp} or a thermal component. Our findings reveal that high $T_{\rm s}$ can influence the fitting results. A moderately high $kT_{\rm s}$ (around 0.03 keV) can result in an apparent low-temperature and flat SE, while an extremely high $kT_{\rm s}$ (around 0.07 keV) can even produce an unobserved blackbody-like SE. Our conclusions indicate that, for spectral fittings of the warm coronal radiation (SE in AGNs), $kT_{\rm s}$ should be treated as a free parameter with an upper limit, and an accurate coronal geometry is necessary when $kT_{\rm s}>0.01$ keV.