High-Frequency VLBI Imaging of the Jet Base of M87

VLBA and Global VLBI observations of M87 at 43 GHz, some new and some previously published, are used to study the structural evolution of the jet with a spatial resolution of under 100 Schwarzschild radii. The images, taken between 1999 and 2004, have an angular resolution of 0.00043^''×0.00021^''. An edge-brightened jet structure and an indication of a large opening angle at the jet base are seen in all five epochs. In addition, a probable counterjet is seen in the latter three epochs. A 22 GHz VLBA image also confirms many of the structures seen at the higher frequency, including the counterjet. A comparison of the counterjet flux density at 22 and 43 GHz reveals that it is not free-free absorbed at these frequencies. Attempts to obtain speeds from the proper motions of jet and counterjet components indicate that these observations are undersampled. The closest pair of images gives apparent speeds of (0.25-0.40)c for the jet and 0.17c for the counterjet. These speeds should be treated as lower limits because of possible errors in associating components between epochs. If they are real, they indicate that the jet is oriented 30°-45° from the line of sight and that the component speeds along the jet are (0.3-0.5)c. Using the jet orientation derived from proper motions, the spectral index of the counterjet, and a jet-to-counterjet brightness ratio of 14.4, the inferred bulk flow is (0.6-0.7)c, which, given the considerable uncertainties in how to measure the brightness ratio, is not significantly larger than the component speed.