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 ($\approx10^4\ R_{ \rm G}$).
Categories: astro-ph.HE
