The origin of the soft excess in the luminous quasar HE 1029-1401

Abstract

The enigmatic and intriguing phenomenon of the "soft excess" observed in the X-ray spectra of luminous quasars continues to be a subject of considerable interest and debate in the field of high-energy astrophysics. This study focuses on the quasar HE 1029-1401 ($z=0.086$, $\log (L_{\mathrm{bol}}/[\mathrm{erg}{\mathrm{s}}^{-1}])=46.0\pm 0.2$), with a particular emphasis on investigating the properties of the hot corona and the physical origin of the soft excess. In this study, we present the results of a joint \textit{XMM-Newton}/\textit{NuSTAR} monitoring campaign of this quasar conducted in May 2022. The source exhibits a cold and narrow Fe $\mathrm{K}\hat{\mathrm{I}}\pm$ emission line at 6.4 keV, in addition to the detection of a broad component. Our findings suggest that the soft excess observed in HE 1029-1401 can be adequately explained by Comptonized emission originating from a warm corona. Specifically, fitting the spectra with two \nthcomp\, component we found that the warm corona is characterized by a photon index ($\hat{I}{“}^w$) of $2.75\pm 0.05$ and by an electron temperature ($k{T}_e^w$) of $0.39_{-0.04}^{+0.06}$ keV, while the optical depth ($\ddot{I}{„}^w$) is found to be $23\pm 3$. We also test more physical models for the warm corona, corresponding to two scenarios: pure Comptonization and Comptonization plus reflection. Both models provide a good fit to the data, and are in agreement with a radially extended warm corona having a size of a few tens of gravitational radii.

accepted by A&A