NuSTAR and XMM-Newton Observations of the Hard X-Ray Spectrum of Centaurus A

We present simultaneous XMM-Newton and NuSTAR observations spanning 3–78 keV of the nearest radiogalaxy, Centaurus A (Cen A), performed during a very high flux state. The accretion geometry around thecentral engine in Cen A is still debated, and we investigate possible configurations using detailed X-ray spectralmodeling. NuSTAR imaged the central region of Cen A with subarcminute resolution at X-ray energies above10 keV for the first time, but finds no evidence for an extended source or other off-nuclear point-sources.The XMM-Newton and NuSTAR spectra agree well and can be described with an absorbed power-law witha photon index Γ = 1.815 ± 0.005 and a fluorescent Fe Kα line in good agreement with literature values.The spectrum does not require a high-energy exponential rollover, with a constraint of E_fold > 1 MeV. Athermal Comptonization continuum describes the data well, with parameters that agree with values measuredby INTEGRAL, in particular an electron temperature of kTe ≈ 220 keV, assuming a 10 eV seed photon inputtemperature. We do not find evidence for reflection or a broad iron line and put stringent upper limits ofR <0.01 on the reflection fraction and accretion disk illumination. We use archival Chandra data to estimatethe contribution from diffuse emission, extra-nuclear point-sources, and the X-ray jet to the observed NuSTAR and XMM-Newton X-ray spectra and find the contribution to be negligible. We discuss different scenarios forthe physical origin of the observed X-ray spectrum, and conclude that the inner disk is replaced by an advection-dominatedaccretion flow or that the hard X-rays are dominated by synchrotron self-Compton emission fromthe inner regions of the radio jet or a combination thereof.