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

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

Published 2015-11-05Version 1

We present simultaneous XMM-Newton and NuSTAR observations spanning 3-78 keV of the nearest radio galaxy, Centaurus A (Cen A), performed during a very high flux state. The accretion geometry around the central engine in Cen A is still debated, and we investigate possible configurations using detailed X-ray spectral modeling. NuSTAR imaged the central region of Cen A with subarcminute resolution at X-ray energies above 10 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 with a photon index {\Gamma} = 1.815 +/- 0.005 and a fluorescent Fe K{\alpha} line in good agreement with literature values. The spectrum does not require a high-energy exponential rollover, with a constraint of E_fold > 1MeV. A thermal Comptonization continuum describes the data well, with parameters that agree with values measured by INTEGRAL, in particular an electron temperature of kT_e ~ 220 keV, assuming a 10 eV seed photon input temperature. We do not find evidence for reflection or a broad iron line and put stringent upper limits of R < 0.01 on the reflection fraction and accretion disk illumination. We use archival Chandra data to estimate the 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 for the physical origin of the observed X-ray spectrum, and conclude that the inner disk is replaced by an advection-dominated accretion flow or that the hard X-rays are dominated by synchrotron self-Compton emission from the inner regions of the radio jet or a combination thereof.