Faraday Rotation in the Jet of M87 inside the Bondi Radius: Indication of Winds from Hot Accretion Flows Confining the Relativistic Jet

Abstract

We study Faraday rotation in the jet of M87 inside the Bondi radius using eight Very Long Baseline Array data sets, one at 8 GHz, four at 5 GHz, and three at 2 GHz. We obtain Faraday rotation measures (RMs) measured across the bandwidth of each data set. We find that the magnitude of RM systematically decreases with increasing distance from the black hole from 5,000 to 200,000 Schwarzschild radii. The data, showing predominantly negative RM sign without significant difference of the RMs on the northern and southern jet edges, suggest that the spatial extent of the Faraday screen is much larger than the jet. We apply models of hot accretion flows, thought to be prevalent in active galactic nuclei having relatively low luminosity such as M87, and find that the decrease of RM is described well by a gas density profile ρ ∝ r −1 . This behavior matches the theoretically expected signature of substantial winds, nonrelativistic un-collimated gas outflows from hot accretion flows, which is consistent with the results of various numerical simulations. The pressure profile inferred from the density profile is flat enough to collimate the jet, which can result in gradual acceleration of the jet in a magneto-hydrodynamical process. This picture is in good agreement with the observed gradual collimation and acceleration of the M87 jet inside the Bondi radius. The dominance of negative RMs suggests that jet and wind axis are misaligned such that the jet emission exposes only one side of the toroidal magnetic fields permeating the winds.

M87 black hole mass and spin estimate through the position of the jet boundary shape break

“…We are able to fit the the observed jet boundary shape with the theoretical curve in the parabolic domain with b ≈ 2.05 − 2.07, which is consistent with Bondi accretion flow models Quataert & Narayan (2000); Narayan & Fabian (2011). However, the direct measurements of a particle number density in ISM either by X-ray observations (Russell et al 2015), or by modelling the Faraday rotation measure on the ambient medium (Park et al 2019), provide n ∝ r −1 , which corresponds to smaller b for adiabatic flow. This caveat may be solved if the temperature rises closer to the central source, as was predicted by Quataert & Narayan (2000) and discussed in Russell et al (2015).…”

Kinematics of the M87 Jet in the Collimation Zone: Gradual Acceleration and Velocity Stratification

“…We note that our recent study of Faraday rotation in the jet at distances 2 × 10 5 R S suggests κ 2(Park et al 2019), which allows a parabolic jet shape without a transition to a conical shape(Komissarov et al 2009), as observed in this region.4 We note, however, that GRMHD simulations consistently found that gas outflows launched from the disk cannot reach relativistic speeds due to high mass-loading (e.g.,Sadowski et al 2013;Yuan et al 2015;Nakamura et al 2018;Qian et al 2018). …”

A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG 1553+113

“…Given that the jet position angle wobbles across the different observing epochs, we can argue that the RM variations are produced by variations of the path length (d l ) through the external Faraday screen and/or variations in the electron density distribution n e (Stirling et al 2003). The persistent negative sign in the observed RM across the different epochs, within the scenario in which the Faraday screen is mostly represented by the hot accretion flow or wind outflow, could be explained by a misalignment of the jet axis with respect to the wind axis (Park et al 2019). If compared to Mrk 421 (e.g.…”