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

TL;DR

This study investigates the soft excess in the X-ray spectrum of the luminous quasar HE 1029-1401, proposing that it originates from Comptonized emission in a warm corona, based on joint XMM-Newton and NuSTAR observations.

Contribution

It provides the first detailed analysis of the soft excess in HE 1029-1401, demonstrating that warm Comptonization explains the phenomenon and exploring physical models of the corona.

Findings

Soft excess explained by warm corona Comptonization.

Warm corona characterized by specific photon index and temperature.

Both physical models fit the data well, indicating an extended corona.

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 ($z=0.086$, $\log(L_{\rm{bol}}/[\rm{erg\,s^{-1}}])= 46.0 \pm 0.2$), 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 $\rm{K}\alpha$ 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 ($\Gamma^{w}$) of $2.75\pm0.05$ and by an electron temperature ($kT_{e}^{w}$) of $0.39^{+0.06}_{-0.04}$ keV, while the optical depth ($\tau^{w}$) is found to be $23\pm3$. 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.