Resolving the innermost parsec of Centaurus A at mid-infrared wavelengths

Context.To reveal the origin of mid-infrared radiation from the core of Centaurus A, we carried out interferometric observations with the MID-infrared Interferometer (MIDI) at ESO's VLTI telescope array. Aims.Observations were obtained with four baselines between unit telescopes of the VLTI, two of them roughly along the radio axis and two orthogonal to it. The interferometric measurements are spectrally resolved with $\lambda/\Delta \lambda = 30$ in the wavelength range 8 to 13 μm. Their resolution reaches 15 mas at the shortest wavelengths. Supplementary observations were obtained in the near-infrared with the adaptive optics instrument NACO, and at mm wavelengths with SEST and JCMT. Methods.The mid-infrared emission from the core of Centaurus A is dominated by an unresolved point source (<10 mas). Observations with baselines orientated perpendicular to the radio jet reveal an extended component which can be interpreted as a geometrically thin, dusty disk, the axis of which is aligned with the radio jet. Its diameter is about 0.6 pc. It contributes between 20% (at $\lambda \simeq 8~\mu$m) and 40% (at $\lambda \simeq 13~\mu$m) to the nuclear flux from Centaurus A and contains dust at about 240 K. We argue, that the unresolved emission is dominated by a synchrotron source. Its overall spectrum is characterized by an $F_\nu \sim \nu^{-0.36}$ power-law which cuts off exponentially towards high frequencies at $\nu_{\rm c} = 8\times10^{13}$ Hz and becomes optically thick at $\nu < \nu_1 \simeq 45$ GHz. Results.Based on a Synchrotron Self Compton (SSC) interpretation for the γ-ray emission, we find a magnetic field strength of 26 μT and a maximum energy of relativistic electrons of $\gamma_{\rm c} = E_{\rm c}/m_{\rm e}c^2 = 8500$. Near $\gamma_{\rm c}$, the acceleration time scale is $\tau_{\rm acc} = 4$ days, in good agreement with the fastest flux variations, observed at X-ray frequencies. Our SSC model argues for a Doppler factor $\delta \simeq 1$ which – together with the jet-counter jet ratio of the radio jets on parsec scale – results in an upper limit for the bulk Lorentz factor $\Gamma_{\rm jet} < 2.5$, at variance with the concept of a “mis-directed BL Lac object”. Conclusions.We estimate a thermal luminosity of the core, $P_{\rm th} \simeq 1.3\times 10^{34}\,{\rm W} = 1.5\times 10^{-4}\times L_{\rm Edd}$, intermediate between the values for highly efficiently accreting AGN (e.g. Seyfert galaxies) and those of typical FR I radio galaxies. This luminosity, which is predominantly released in X-rays, is most likely generated in an Advection Dominated Accretion Flow (ADAF) and seems just sufficient to heat the dusty disk