Deconvolution of the X‐Ray Emission and Absorption Components in Centaurus A

Open Access
Publisher Website
Google Scholar
Abstract
We present ROSAT HRI and PSPC plus ASCA X-ray observations of the radio galaxy Centaurus A. The HRI image reveals that the X-ray jet has remained constant in flux since the Einstein observation 15 years ago, while the nuclear flux has decreased by ~60%. The PSPC data allows the first detailed spatially resolved X-ray spectroscopy of the nucleus, jet, and diffuse emission in Cen A. The 0.1-2 keV spectrum of the nucleus is heavily absorbed, as expected. The jet can be described by a power law of photon index Γ ~ 2.3, although thermal models cannot be ruled out. The presence of faint diffuse emission is confirmed, extending out to ~6 kpc from the nucleus. The spectrum of the diffuse emission can be modeled as two "Raymond-Smith" plasmas. A low-temperature component, kT = 0.29 keV, is consistent with emission from gas heated by stars, while the harder component may represent the contribution of galactic X-ray binary systems; the diffuse emission increases in intensity close to the galactic disk. The identification of these extended components makes it possible to separate the nuclear and diffuse contributions to the ASCA spectrum. The 0.6-10 keV nuclear spectrum is well modeled by a power-law continuum of photon index Γ ~ 1.96, in good agreement with the "canonical" value for Seyfert 1 galaxies. Forty percent of the nuclear continuum is absorbed by a column density ~4 × 1023 cm-2, 59% is absorbed by a column ~1 × 1023 cm-2, and 1% is absorbed by a value close to the 21 cm Galactic line-of-sight column. The high degree of nuclear absorption is consistent with the circumnuclear molecular clouds whose existence is suggested by absorption measurements in a number of wave bands. The iron Kα emission line observed appears broad; the best-fit Gaussian width implies an origin in material with velocities ~11,000 km s-1, while the equivalent width is 114 ± 18 eV. Both quantities are in good agreement with an origin in the circumnuclear absorbing material.