Radio spectral properties of cores and extended regions in blazars in the MHz regime

TL;DR

This study analyzes the low-frequency radio spectral properties of blazars using GLEAM survey data, revealing their spectral behavior, core-lobe emission ratios, and correlations with gamma-ray emission, thus enhancing understanding of blazar radio characteristics.

Contribution

It provides a comprehensive low-frequency spectral analysis of blazars with improved detection rates and integrates high-frequency data, offering new insights into their emission components and correlations.

Findings

1274 out of 1827 blazars have low-frequency counterparts.

Blazars exhibit a mean spectral index of -0.44 at ~100MHz.

The ratio of lobe to core emission is smaller than previously thought.

Abstract

Low-frequency radio surveys allow in-depth studies and new analyses of classes of sources previously known and characterised only in other bands. In recent years, low radio frequency observations of blazars have been available thanks to new surveys, such as the GaLactic and Extragalactic All-sky MWA Survey (GLEAM). We search for gamma-ray blazars in a low frequency (${\nu}$ < 240MHz) survey, to characterise the spectral properties of the spatial components. We cross-correlate GLEAM with the fourth catalogue of active galactic nuclei (4LAC) detected by the Fermi satellite. This improves over previous works using a low frequency catalogue that is wider, deeper, with a better spectral coverage and the latest and most sensitive gamma-ray source list. In comparison to the previous study based on the commissioning survey, the detection rate increased from 35% to 70%. We include Australia Telescope 20GHz (AT20G) Survey data to extract high-frequency high-angular resolution information on the radio cores of blazars. We find low radio frequency counterparts for 1274 out of 1827 blazars in the 72-231 MHz range. Blazars have at spectrum at $\sim$ 100MHz regime, with a mean spectral index ${\alpha}$ = -0.44 +-0.01 (assuming S $\propto$ ${\nu}^ {\alpha}$ ). Low synchrotron peaked objects have a scatter spectrum than high synchrotron peaked objects. Low frequency radio and gamma-ray emission show a significant but scattered correlation. The ratio between lobe and core radio emission in gamma-ray blazars is smaller than previously estimated.