The resolved jet of 3C 273 at 150 MHz : Sub-arcsecond imaging with the LOFAR international baselines

Abstract

Context. Since its discovery in 1963, 3C 273 has become one of the most widely studied quasars with investigations spanning the electromagnetic spectrum. While much has therefore been discovered about this historically notable source, its low-frequency emission is far less well understood. Observations in the megahertz (MHz) regime have traditionally lacked the resolution required to explore small-scale structures, such as knots and diffuse jet emission, that are crucial to understanding the processes that result in the observed emission. Advances in the processing of LOFAR international baseline data have now removed this limitation, providing the opportunity to explore this key area for the first time. Aims. In this paper we use the first sub-arcsecond images of 3C 273 at MHz frequencies to investigate the morphology of the compact jet structures and the processes that result in the observed spectrum.We determine the jet's kinetic power, place constraints on the bulk speed and inclination angle of the jets, and look for evidence of the elusive counterjet at 150 MHz. Methods. Using the full complement of the LOFAR international stations (German, Poland, France, UK, Sweden), we produce 0.31 × 0.21 arcsec images of 3C 273 at 150 MHz. Using ancillary data at gigahertz frequencies, we fit free-free absorption (FFA) and synchrotron self-absorption (SSA) models to each region in order to determine their validity in explaining the observed spectra. Results. The images presented display for the first time that robust high-fidelity imaging of low-declination complex sources is now possible with the LOFAR international baselines. We show that the main small-scale structures of 3C 273 match those seen at higher frequencies, with a tenuous detection of an extension to the outer lobe. We find that FFA and SSA models are able to describe the spectrum of the knots and, while differentiating between model types requires further observations, we conclude that absorption is present in the observed emission. We determine the kinetic power of the jet to be in the range of 3.5 × 1043-1.5 × 1044 erg s-1, which agrees with estimates made using higher frequency observations. We derive lower limits for the bulk speed and Lorentz factor of β ≥ 0.55 and Γ ≥ 1.2, respectively. The counterjet remains undetected at 150 MHz, placing a limit on the peak brightness of Scj-150 < 40 mJy beam-1.