The X-Ray-faint Emission of the Supermassive Nuclear Black Hole of IC 1459

Abstract

Chandra observations of the supermassive black hole in the nucleus of IC 1459 show a weak (L_X=8×10⁴⁰ ergs s^-1, 0.3-8 keV), unabsorbed nuclear X-ray source, with a slope Γ=1.88+/-0.09, and no strong Fe K line at 6.4 keV (EW<382 eV). This describes a normal active galactic nucleus (AGN) X-ray spectrum but lies at 3×10^-7 below the Eddington limit. The spectral energy distribution of the IC 1459 nucleus is extremely radio-loud compared to normal radio-loud quasars. The nucleus is surrounded by a hot interstellar medium (kT~0.5-0.6 keV) with an average density of 0.3 cm^-3, within the central ~180 pc radius, which is comparable to the gravitational capture radius, r_A~140 pc. We estimate that for a standard AGN efficiency of 10%, the Bondi accretion would correspond to a luminosity of ~6×10⁴⁴ ergs s^-1, nearly 4 orders of magnitude higher than L_X. ADAF solutions can explain the X-ray spectrum, but not the high radio/X-ray ratio. A jet model fits the radio-100 μm and X-ray spectra well. The total power in this jet is ~10% of L_Bondi, implying that accretion close to the Bondi rate is needed.