ChandraX‐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 19 (110 kpc) long jet to the west of the nucleus, the western radio hot spot some 42 (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 20 (1.9 kpc). The observed jet is up to 15 times brighter in X-rays than any counterjet, a difference ascribed to relativistic boosting since the western radio lobe is probably the closer. The jet's spectrum is well modeled by an absorbed power law with Γ = 1.94 and poorly fitted by a Raymond-Smith thermal plasma model. The emission processes responsible for the X-rays are discussed in detail. The radio-to-optical spectrum of the hot spot breaks or turns down at 1013-1014 Hz, and its X-ray spectrum is not a simple extension of the radio-to-optical spectrum to higher frequencies. Thermal models for the hot spot's X-ray emission are ruled out. Synchrotron self-Compton models involving scattering from the known population of electrons give the wrong spectral index for the hot spot's X-ray emission and are also excluded. A composite synchrotron plus synchrotron self-Compton model can match the X-ray observations but requires similar contributions from the two components in the Chandra band. We show that the hot spot's X-ray emission could be synchrotron self-Compton emission from a hitherto unobserved population of electrons emitting at low radio frequencies but do not favor this model in view of the very weak magnetic field required. An inverse Compton model of the jet, in which it scatters microwave background photons but moves nonrelativistically, requires a magnetic field a factor of 30 below equipartition and ad hoc conditions to explain why the radio lobes are fainter than the jet in X-rays but brighter in the radio. These problems are alleviated if the jet moves relativistically, but models with an equipartition field require an implausibly small angle (θ) between the jet and the line of sight. A model with θ 23° and a field a factor of 6 below equipartition seems viable. Synchrotron radiation is an alternative process for the X-ray emission. The expected synchrotron spectrum from relativistic electrons accelerated by strong shocks and subject to synchrotron radiation losses is in very good agreement with that observed for both the hot spot and jet. The possibility that the relativistic electrons result via photopion production by high-energy protons accelerated in shocks (a proton-induced cascade) is briefly discussed.