Studies of blazars at low frequencies with LOFAR

Abstract

Blazars, consisting of flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs), are active galactic nuclei (AGN) defined by a narrow inclination angle between the relativistic jets and our line of sight. While blazars are multiwavelength emitters, their morphological and spectral properties have been poorly explored at ~150 MHz because, generally, low-frequency instruments have lacked the requisite resolution and sensitivity to facilitate studies at this waveband. Such studies are now technically feasible with the Low-Frequency Array (LOFAR). The FSRQ 3C 273 and the BL Lac OJ 287 are notable for their one-sided X-ray jets. LOFAR observations of these blazars are calibrated, and I present the first high-resolution images of these jets at ~150 MHz. The jets consist of compact knots linked by diffuse emission. Studies of blazar samples from the LOFAR Two-Metre Sky Survey are also performed. Flat spectra are typically associated with blazars at gigahertz frequencies, and this was found to be the case at megahertz frequencies too. A tentative link between the flux densities at ~150 MHz and the gamma-ray photon fluxes was identified, suggesting that for a given source, a single population of particles is responsible for some of the emission at both wavebands. It was discovered that the majority of blazars possess complex morphologies at ~150 MHz, which are attributable to beamed jet components and large-scale diffuse emission, with significant variation between sources. The spatial extents of FSRQs are shown to be in concordance with the predictions of the radio-loud AGN unification scheme. Studying the extended low-frequency emission from blazars can improve our understanding of the emission mechanisms that give rise to jet spectra and, ultimately, can help determine the validity of the radio-loud AGN unification scheme.