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

F. Fürst, C. Müller, K. K. Madsen, L. Lanz, E. Rivers, M. Brightman, P. Arevalo, M. Baloković, T. Beuchert, S. E. Boggs, Finn Erland Christensen, W. W. Craig, T. Dauser, D. Farrah, C. Graefe, C. J. Hailey, F. A. Harrison, M. Kadler, A. King, F. KraussG. Madejski, G. Matt, A. Marinucci, A. Markowitz, P. Ogle, R. Ojha, R. Rothschild, D. Stern, D. J. Walton, J. Wilms, W. W. Zhang

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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 Efold > 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.
Original languageEnglish
Article number150
JournalThe Astrophysical Journal
Issue number2
Number of pages13
Publication statusPublished - 2016


  • Galaxies: active
  • Galaxies: individual (Centaurus A)
  • X-rays: galaxies

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