High‐Frequency VLBI Imaging of the Jet Base of M87

Abstract

VLBA and Global VLBI observations of M87 at 43 GHz, some new and some\npreviously published, are used to study the structural evolution of the jet\nwith a spatial resolution of under 100 Schwarzschild radii. The images, taken\nbetween 1999 and 2004, have an angular resolution of 0.00043" x 0.00021". An\nedge-brightened jet structure and an indication of a large opening angle at the\njet base are seen in all five epochs. In addition, a probable counter-jet is\nseen in the latter three epochs. A 22 GHz VLBA image also confirms many of the\nstructures seen at the higher frequency, including the counter-jet. A\ncomparison of the counter-jet flux density at 22 and 43 GHz reveals that it is\nnot free-free absorbed at these frequencies.\n Attempts to obtain speeds from the proper motions of jet and counter-jet\ncomponents indicate that these observations are undersampled. The closest pair\nof images gives apparent speeds of 0.25 to 0.40c for the jet and 0.17c for the\ncounter-jet. These speeds should be treated as lower limits because of possible\nerrors in associating components between epochs. If they are real, they\nindicate that the jet is oriented 30-45 degrees from the line-of-sight and that\nthe component speeds along the jet are 0.3-0.5c. Using the jet orientation\nderived from proper motions, the spectral index of the the counter-jet, and a\njet-to-counter-jet brightness ratio of 14.4, the inferred bulk flow is\n0.6-0.7c, which, given the considerable uncertainties in how to measure the\nbrightness ratio, is not significantly larger than the component speed.\n