AChandraX‐Ray Study of Cygnus A. II. The Nucleus

Abstract

We report Chandra Advanced CCD Imaging Spectrometer and quasi-simultaneous Rossi X-Ray Timing Explorer (RXTE) observations of the nearby, powerful radio galaxy Cygnus A, with the present paper focusing on the properties of the active nucleus. In the Chandra observation, the hard (>a few keV) X-ray emission is spatially unresolved with a size 1'' (1.5 kpc, H0 = 50 km s-1 Mpc-1) and coincides with the radio and near-infrared nuclei. In contrast, the soft ( 1 ergs cm s-1 and the electron density ne 6 cm-3 given the observed distance of the soft X-ray emission from the nucleus. The electron column density inferred from the X-ray observations is much too low to account for the extended optical scattered light, strongly suggesting that the polarized optical light is scattered by dust. The presence of highly ionized Ne lines in the soft X-ray spectrum requires 20 ergs cm s-1 ξ 300 ergs cm s-1; these lines may originate closer to the nucleus than the extended soft continuum or in a lower density gas. A collisionally ionized thermal model of the extended soft X-rays cannot be ruled out but is unattractive in view of the low metal abundance required (Z = 0.03 Z☉). The hard X-ray to far-infrared ratio for the nucleus of Cygnus A is similar to that seen in Seyfert 1 and unobscured radio galaxies. By means of the correlation between hard X-ray luminosity and nuclear optical absolute magnitude for these classes of object, we estimate MB = -22.4 for Cygnus A, near the borderline between Seyfert galaxies and QSOs.