On the Evolution of and High-Energy Emission from GHz-Peaked-Spectrum Sources

TL;DR

This paper models the evolution and broad-band emission of young radio sources with compact lobes, highlighting free-free absorption as the cause of spectral turnover and predicting high-energy emission detectable by gamma-ray instruments.

Contribution

It introduces a dynamical model for young radio sources' lobes, explaining spectral features and high-energy emission mechanisms, with predictions for gamma-ray detection.

Findings

Spectral turnover due to free-free absorption by interstellar clouds

High-energy emission from inverse-Compton scattering can reach GeV/TeV energies

GPS radio galaxies may be detectable by gamma-ray telescopes like GLAST

Abstract

Here we discuss evolution and broad-band emission of compact (< kpc) lobes in young radio sources. We propose a simple dynamical description for these objects, consisting of a relativistic jet propagating into a uniform gaseous medium in the central parts of an elliptical host. In the framework of the proposed model, we follow the evolution of ultrarelativistic electrons injected from a terminal hotspot of a jet to expanding lobes, taking into account their adiabatic energy losses as well as radiative cooling. This allows us to discuss the broad-band lobe emission of young radio sources. In particular, we argue that the observed spectral turnover in the radio synchrotron spectra of these objects cannot originate from the synchrotron self-absorption process but is most likely due to free-free absorption effects connected with neutral clouds of interstellar medium engulfed by the expanding lobes and photoionized by active centers. We also find a relatively strong and complex high-energy emission component produced by inverse-Compton up-scattering of various surrounding photon fields by the lobes' electrons. We argue that such high energy radiation is strong enough to account for several observed properties of GHz-peaked-spectrum (GPS) radio galaxies at UV and X-ray frequencies. In addition, this emission is expected to extend up to GeV (or possibly even TeV) photon energies and can thus be probed by several modern gamma-ray instruments. In particular, we suggest that GPS radio galaxies should constitute a relatively numerous class of extragalactic sources detected by GLAST.