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

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 ($T_{\mathrm{s}}$) is usually assumed to be far away from the soft X-ray, but $k{T}_{\mathrm{s}}$ 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 $\text{textsc}thcomp$ or a thermal component. Our findings reveal that high $T_{\mathrm{s}}$ can influence the fitting results. A moderately high $k{T}_{\mathrm{s}}$ (around 0.03 keV) can result in an apparent low-temperature and flat SE, while an extremely high $k{T}_{\mathrm{s}}$ (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), $k{T}_{\mathrm{s}}$ should be treated as a free parameter with an upper limit, and an accurate coronal geometry is necessary when $k{T}_{\mathrm{s}}>0.01$ keV.