XMM-Newton spectrum of the radio-loud quasar 3C 215: Slim accretion disk or SMBH binary

¹ Gran Sasso Science Institute (GSSI), Viale Francesco Crispi 7, 67100 L’Aquila, AQ, Italy
e-mail: alessio.mei@gssi.it
² Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy
³ INFN-Laboratori Nazionali del Gran Sasso, 67100 L’Aquila, AQ, Italy
⁴ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁵ NASA/Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
⁶ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
⁷ INFN-Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy

Received: 1 February 2021
Accepted: 28 July 2021

Abstract

Context. Radio-loud active galactic nuclei (RL AGN) exhibit very powerful jet emission in the radio band, while the radio-quiet (RQ) AGN do not. This RL-RQ dichotomy would imply a sharp difference existing among these two classes, however, modern theoretical models and observations suggest a common nuclear environment that is possibly characterized by different working regimes.

Aims. We explore the geometrical structure and mutual interactions of the innermost components of the broad line radio galaxy 3C 215, with a particular focus on the accretion and ejection mechanisms involving the central supermassive black hole (SMBH). We compare these observational features with those of the RQ Seyfert 1 galaxies. Investigating their differences is aimed at improving our understanding of the jet launching mechanisms and devising an explanation for why this phenomenon is efficient only in a small fraction of all the AGNs.

Methods. Using high-quality data from a ∼60 ks observation with XMM-Newton, we carried out a detailed X-ray spectral analysis of 3C 215 in the broad energy range of 0.5−10 keV. We modeled the spectrum with an absorbed double power-law model for the primary continuum, reprocessed by reflection from ionized and cold neutral material and modified by relativistic blurring. We also compared our results with those of earlier multi-wavelength observations.

Results. We obtained a primary continuum photon index from the corona, namely, Γ₁ = 1.97 ± 0.06, along with evidence of a jet contribution, modeled as a power law with photon index of Γ₂ ≃ 1.29. The reflector, which is possibly attributed to the accretion disk and portions of the broad-line region (BLR), is ionized (logξ = 2.31_−0.27^+0.37 erg s⁻¹ cm) and relatively distant from the SMBH (R_in > 38 R_g), where R_g = GM_BH/c² is the gravitational radius. The obscuring torus seems patchy, dust-poor, and inefficient, while the jet emission shows a twisted and knotted geometry. We propose three scenarios to describe the following characteristics: 1. An ADAF state in the inner disk; 2. A slim accretion disk; and 3. A sub-pc SMBH binary system (SMBHB).

Conclusions. While the first scenario is not in agreement with the SMBH accretion regime, the slim disk scenario is consistent with the observational features of this radio galaxy, showing that 3C 215 is similar to non-jetted AGNs, accreting at a high rate. Nonetheless, the first two scenarios are unable to account for the particular shape of 3C 215 jet emission. The SMBHB scenario seems to be in agreement with almost all 3C 215 observational features, but we are not able to unequivocally determine this source as a strong SMBHB candidate. A final determination will require further analysis.