Broad-band X-ray observation of broad-line radio galaxy 3C 109

Abstract

We present a study of the central engine in the broad-line radio galaxy 3C 109. To investigate the immediate surrounding of this accreting, supermassive black hole, we perform a multi-epoch broad-band spectral analysis of a joint NuSTAR/XMM observation (2017), an archival xmm observation (2005) and the 105-month averaged Swift-BAT data. We are able to clearly separate the spectrum into a primary continuum, neutral and ionized absorption, and a reflection component. The photon index of the primary continuum has changed since 2005 ($\Gamma =1.61\begin{array}{c}+0.02\\ -0.01\end{array}\to 1.54\pm 0.02$), while other components remain unchanged, indicative of minimal geometric changes to the central engine. We constrain the high-energy cutoff of 3C 109 (E${}_{\text{cut}}=49\begin{array}{c}+7\\ -5\end{array}$\,keV ) for the first time. The reflector is found to be ionized (log $\xi$ = $2.3\begin{array}{c}+0.1\\ -0.2\end{array}$) but no relativistic blurring is required by the data. SED analysis confirms the super-Eddington nature of 3C 109 initially (${\lambda }_{Edd}>$ 2.09). However, we do not find any evidence for strong reflection (R = $0.18\begin{array}{c}+0.04\\ -0.03\end{array}$) or a steep power law index, as expected from a super-Eddington source. This puts the existing virial mass estimate of 2 $\times 10^8$M${}_{\odot }$ into question. We explore additional ways of estimating the Eddington ratio, some of which we find to be inconsistent with our initial SED estimate. We obtain a new black hole mass estimate of 9.3 $\times 10^8$M${}_{\odot }$, which brings all Eddington ratio estimates into agreement and does not require 3C 109 to be super-Eddington.

13 pages, 8 figures

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