The limb-brightened jet of M87 down to the 7 Schwarzschild radii scale

J.-Y. Kim, T. P. Krichbaum, R.-S. Lu, E. Ros, U. Bach, M. Bremer, P. de Vicente, M. Lindqvist, J. A. Zensus
2018 Astronomy and Astrophysics  
M87 is one of the nearest radio galaxies with a prominent jet extending from sub-pc to kpc-scales. Because of its proximity and large mass of the central black hole, it is one of the best radio sources to study jet formation. We aim at studying the physical conditions near the jet base at projected separations from the BH of ∼7-100 Schwarzschild radii (R_ sch). Global mm-VLBI Array (GMVA) observations at 86 GHz (λ=3.5 mm) provide an angular resolution of ∼50μas, which corresponds to a spatial
more » ... solution of only 7 R_ sch and reach the small spatial scale. We use five GMVA data sets of M87 obtained during 2004--2015 and present new high angular resolution VLBI maps at 86GHz. In particular, we focus on the analysis of the brightness temperature, the jet ridge lines, and the jet to counter-jet ratio. The imaging reveals a parabolically expanding limb-brightened jet which emanates from a resolved VLBI core of ∼(8-13) R_ sch size. The observed brightness temperature of the core at any epoch is ∼(1-3)×10^10 K, which is below the equipartition brightness temperature and suggests magnetic energy dominance at the jet base. We estimate the diameter of the jet at its base to be ∼5 R_ sch assuming a self-similar jet structure. This suggests that the sheath of the jet may be anchored in the very inner portion of the accretion disk. The image stacking reveals faint emission at the center of the edge-brightened jet on sub-pc scales. We discuss its physical implication within the context of the spine-sheath structure of the jet.
doi:10.1051/0004-6361/201832921 fatcat:gboivf4cbjgflanwa6mbak7r6q