A FLASH on Blazars: Capturing the Radio Realm of 4FGL Blazars with SKAO Pathfinders

TL;DR

This study cross-matches 165 4FGL blazars with the FLASH continuum survey to build extensive cm–mm radio spectral energy distributions using multi-frequency data from FLASH, RACS, GLEAM-X, AT20G, and ALMACAL, complemented by ALMA. Through Bayesian radio SED fitting, it finds that roughly half the blazars exhibit re-triggered peaked spectra dominated by a single emission region, with a median rest-frame peak near $4.6$ GHz, and reveals a persistent, steep low-frequency component indicating extended emission. The work uncovers a robust radio–gamma correlation across cm–GHz bands, strongest at ~855 MHz, and a class-dependent radio–X-ray relation, with FSRQs showing tighter coupling than BL Lacs, consistent with accretion-driven jet physics. By leveraging spectroscopic redshifts, the authors discuss evolution and jet–environment interactions and outline a path for future studies with SKA pathfinders and the Cherenkov Telescope Array. These results establish the radio properties of FLASH-targeted blazars as valuable probes of jet physics, AGN evolution, and neutral gas environments at cosmological scales.

Abstract

This work investigates the multi-wavelength properties of 165 4FGL blazars from the Fermi-LAT fourth source catalogue, looking for with counterparts in the Australian SKA Pathfinder (ASKAP) First Large Absorption Survey in HI (FLASH) continuum. Using high-resolution data from FLASH and complementary radio datasets, combined with archival Atacama Large Millimeter Array (ALMA) observations, we perform detailed spectral energy distribution (SED) analyses across cm-to-mm wavelengths. Our findings reveal that most blazars exhibit re-triggered peaked spectra, indicative of emission dominated by a single emitting region. Additionally, we identify strong correlations between radio and gamma-ray luminosities, highlighting the significant role of relativistic jets in these active galactic nuclei. The inclusion of spectroscopic redshifts from Sloan Digital Sky Survey (SDSS) and Gaia enables a comprehensive analysis of the evolutionary trends and physical characteristics of the sources. Furthermore, we report a tight Radio-X-ray Correlation for Flat Spectrum Radio Quasars, contrasting with the more scattered behaviour observed in BL-Lacs, reflecting their distinct accretion and jet-driving mechanisms. These results provide critical insights into the physics of blazars and their environments, paving the way for future studies with next-generation facilities like the SKA Observatory (SKAO) for radio observations and Cherenkov Telescope Array for gamma-ray studies.

Figures (9)

• Figure 1: Map showing the coverage of Fermi-LAT 4FGL. The light pink regions represent the Multi-Order Coverage of the FLASH fields utilised in this work.
• Figure 2: Illustrations showing three sources categorised as 'IN' (Left),'OUT' (Center), and 'UNCERTAIN' (Right), respectively. These images are based on FLASH maps. The large white circle is aligned with the Fermi-LAT location, sharing the same radius as the Fermi-LAT's resolution (0.1 degrees). The pink rectangle is positioned at the FLASH continuum island's location, with dimensions matching those of the island as determined by Selavyselavy. The dark green ellipse indicates the size and position of the corresponding RACS-LOW island, and the small white circle marks the GLEAM-X matched source's location, with a size equivalent to the (median) GLEAM-X resolution (in arcseconds). The pink 'X' and light green diamond denote the best-corresponding positions of the AT20G source to the 4FGL position and the FLASH continuum position, respectively.
• Figure 3: Distribution of spectroscopic redshifts from SDSS for sources marked as 'IN' with a spectroscopic counterpart.
• Figure 4: Spectral energy distribution fitting for one of the selected blazars in our sample. The models incorporate GLEAM-X, RACS, AT20G, and ALMACAL data. The dark red line is the best fit from the posterior of the selected model type, while individual photometric points are marked. The light red lines are random draws from the posterior representing the range of best-fitting parameters.
• Figure 5: Relation between the trough frequency, $\nu_{\text{trough}}$, and the spectral index at lower frequencies, $\alpha_{\text{trough}}$. The colour scale represents the peak frequency, $\nu_{\text{peak}}$, which can be used as a proxy for the cooling time of the source. The colours of the markers' edges represent the three blazar populations: FSRQs, BL Lacs, and BCUs.