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

Low-frequency radio surveys allow in-depth studies and new analyses of classes of sources that were previously known and characterized only in other bands. In recent years, low radio frequency observations of blazars have become available as a result of new surveys, such as the GaLactic and Extragalactic All-sky Murchison Widefield Array (MWA) survey (GLEAM). We search for gamma-ray blazars in a low-frequency (nu < 240 MHz) survey, to characterize 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 on previous works by using a low-frequency catalogue that is wider and deeper, with a better spectral coverage and the latest and most sensitive gamma-ray source list. Compared with a previous study based on the commissioning survey, the detection rate increased from 35 to 70 per cent. We include data from the Australia Telescope 20-GHz (AT20G) survey in order to extract high-frequency high-angular resolution information about the radio cores of blazars. We find low radio frequency counterparts for 1274 out of 1827 blazars in the range of 72-231 MHz. Blazars have flat spectra at the similar to 100-MHz regime, with a mean spectral index alpha = -0.44 +/- 0.01 (assuming S-nu alpha nu(alpha)). Low synchrotron peaked objects have a flatter spectrum than high synchrotron peaked objects. Low frequency radio and gamma-ray emissions show a significant but scattered correlation. The ratio between lobe and core radio emission in gamma-ray blazars is smaller than previously estimated.