Chandra X-ray Observations of Pictor A: High Energy Cosmic Rays in a Radio Galaxy?

Abstract

We report X-ray observations of the nearby, powerful radio galaxy Pictor A with the Chandra Observatory and optical and near uv observations of its western radio hot spot with the Hubble Space Telescope. X-ray emission is detected from the nucleus, a 1.9 arcmin (110 kpc) long jet to the west of the nucleus, the western radio hot spot some 4.2 arcmin (240 kpc) from the nucleus, and the eastern radio lobe. The morphology of the western hot spot is remarkably similar to that seen at radio and optical wavelengths, where the emission is known to be synchrotron radiation. The X-ray spectrum of the hot spot is well described by an absorbed power law with photon index Γ= 2.07 (+/- 0.11). The X-ray jet coincides with a weak radio jet and is laterally extended by \simeq 2.0 arcsec (1.9 kpc). The observed jet is up to \simeq 15 times brighter in X-rays than any counter jet, a difference ascribed to relativistic boosting as the western radio lobe is probably the closer. The jet's spectrum is well modelled by an absorbed power law with Γ= 1.94 (+0.43/-0.49) and poorly fitted by a Raymond-Smith thermal plasma model. (Abstract truncated).