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, 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, and W. Zhang,The Astrophysical Journal, Volume 819, Number 2,https://iopscience.iop.org/article/10.3847/0004-637X/819/2/150

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Abstract

We present simultaneous XMM-Newton and Nuclear Spectroscopic Telescope Array (NuSTAR) observations spanning 3–78 keV of the nearest radio galaxy, Centaurus A (Cen A). 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 found 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 Γ = 1.815 ± 0.005 and a fluorescent Fe K⍺ line in good agreement with literature values. The spectrum does not require a high-energy exponential rollover, with a constraint of Efold > 1 MeV. A thermal Comptonization continuum describes the data well, with parameters that agree with values measured by INTEGRAL, in particular an electron temperature kTₑ between ≈100–300 keV and seed photon input temperatures between 5 and 50 eV. 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 outer 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 hard X-ray spectrum and conclude that the inner disk is replaced by an advection-dominated accretion flow or that the X-rays are dominated by synchrotron self-Compton emission from the inner regions of the radio jet or a combination thereof.

NuSTAR AND XMM-NEWTON OBSERVATIONS OF THE HARD X-RAY SPECTRUM OF CENTAURUS A

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