Exploring the disk-jet connection in NGC 315

Abstract

Aims. Hot accretion flows are thought to be able to power the relativistic jets observed in active galactic nuclei. They can be presented as standard and normal evolution (SANE) disks or magnetically arrested disks (MAD) – two states that imply profound differences in the physical properties of the disks themselves and the outflows they produce. Methods. We used a multifrequency and multiepoch data set to study the giant radio galaxy NGC 315, with the goal of exploring the properties of its accretion disk and sub-parsec jet. We analyzed the source maps with a pixel-based analysis and we used theoretical models to link the observational properties of the jet to the physical state of the accretion disk. Results. We propose that the bulk flow in NGC 315 accelerates on sub-pc scales, concurrently with the parabolic expansion. We show that this fast acceleration can be theoretically reconciled with a magnetically driven acceleration. Along the acceleration and collimation zone, we observe an unexpected spectral behavior, with very steep spectral index values α ∼ −1.5 (Sν ∝ να) between 22 GHz and 43 GHz. Based on the properties of this region, we predict the black hole of NGC 315 to be rotating fast and the magnetic flux threading the accretion disk to be in excellent agreement with that expected in the case of a MAD. Using a new formalism based on the core shift effect, we modeled the magnetic field downstream a quasi-parabolic accelerating jet and we reconstructed it up to the event horizon radius. In the MAD scenario, we compared it with the expected magnetic saturation strengths in the disk, finding a good agreement.