Discrete Knot Ejection from the Jet in a Nearby Low Luminosity Active Galactic Nucleus, M81*

TL;DR

This study observes a discrete, mildly relativistic knot ejection in the low-luminosity AGN M81*, revealing radial jet motions at smaller scales and lower velocities than in powerful jets, with implications for jet production theories.

Contribution

First detection of radial jet motion in a low-luminosity AGN, linking radio flares to discrete knot ejections and measuring jet velocity and magnetic field strength.

Findings

Detected a moving knot at 0.51c in M81*

Measured magnetic field strength between 1.9 and 9.2 G

Observed radial jet motion at smaller scales

Abstract

Observational constraints of relativistic jets from black holes has largely come from the most powerful and extended jets\cite{Jorstad05,Asada14}, leaving the nature of the low luminosity jets a mystery\cite{Falcke04}. M81* is one of the nearest low-luminosity jets, which underwent an extremely large radio flare in 2011, allowing us to study compact core emission with unprecedented sensitivity and linear resolution. Utilizing a multi-wavelength campaign, we were able to track the flare as it re-brightened and became optically thick. Simultaneous X-ray observations indicated the radio re-brightening was preceded by a low energy X-ray flare at least $t_{\rm delay}>12\ {\rm days}$ prior. Associating the time delay between the two bands as the cooling time in a synchrotron flare\cite{Urry97,Bai03}, we find the magnetic field strength was $1.9<B<9.2\ {\rm G}$, which is consistent with magnetic field estimate from spectral-energy distribution modeling\cite{Kellerman81}, $B<10.2\ {\rm G}$. In addition, VLBA observations at 23 GHz clearly illustrate a discrete knot moving mildly relativistically at $v_{\rm app}/c=0.51\pm0.17$ associated with the initial radio flare. The observations indicate radial jet motions for the first time in M81*. This has profound implications for jet production, as it means radial motion can be observed in even the lowest-luminosity AGN, but at slower velocities and smaller radial extents ($\approx10^4\ R_{\rm G}$).