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

¹ Scuola Universitaria Superiore IUSS Pavia, Piazza della Vittoria 15, 27100 Pavia, Italy
² Department of Physics, University of Trento, Via Sommarive 14, 38123 Povo (TN), Italy
³ Istituto Nazionale di Astrofisica, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via A. Corti 12, 20133 Milano, Italy
⁴ Dipartimento di Matematica e Fisica, Universitá degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁵ Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, 837 State Street, Atlanta, GA 30332-0430, USA
⁶ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁷ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy
⁸ Space Science Data Center, SSDC, ASI, Via del Politecnico snc, 00133 Roma, Italy
⁹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

Received: 7 February 2024
Accepted: 16 May 2024

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_bol/[erg s⁻¹]) = 46.0 ± 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 XMM-Newton/NuSTAR monitoring campaign of this quasar conducted in May 2022. The source exhibits a cold and narrow Fe Kα 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 (Γ^w) of 2.75 ± 0.05 and by an electron temperature (kT_e^w) of 0.39_−0.04^+0.06 keV, while the optical depth (τ^w) is found to be 23 ± 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.