Wide-Band Spectral Variability of Peaked Spectrum Sources

TL;DR

This study investigates broadband spectral variability of 15 peaked-spectrum radio sources across 0.072--10 GHz, revealing that most variability at megahertz frequencies is due to interstellar scintillation, with some sources showing spectral shape changes linked to absorption or jet activity.

Contribution

The paper provides the first broadband spectral variability analysis of peaked-spectrum sources, linking variability to interstellar scintillation and absorption effects, and demonstrating spectral variability as a tool for understanding source physics.

Findings

Most variability at megahertz frequencies is due to interstellar scintillation.

Some sources show spectral shape changes due to absorption or jet ejections.

Spectral variability helps distinguish physical origins of radio source changes.

Abstract

Characterising spectral variability of radio sources is a technique that offers the ability to determine the astrophysics of the intervening media, source structure, emission and absorption processes. We present broadband (0.072--10 GHz) spectral variability of 15 peaked-spectrum (PS) sources with the Australia Telescope Compact Array (ATCA) and the Murchison Widefield Array (MWA). These 15 PS sources were observed quasi-contemporaneously with ATCA and the MWA four to six times during 2020 with approximately a monthly cadence. Variability was not detected at 1--10GHz frequencies but 13 of the 15 targets show significant variability with the MWA at megahertz frequencies. We conclude the majority of variability seen at megahertz frequencies is due to refractive interstellar scintillation of a compact component ~25 mas across. We also identify four PS sources that show a change in their spectral shape at megahertz frequencies. Three of these sources are consistent with a variable optical depth from an inhomogeneous free-free absorbing cloud around the source. One PS source with a variable spectral shape at megahertz frequencies is consistent with an ejection travelling along the jet. We present spectral variability as a method for determining the physical origins of observed variability and for providing further evidence to support absorption models for PS sources where spectral modelling alone is insufficient.