High‐Frequency VLBI Imaging of the Jet Base of M87

• DOI: 10.1086/512846
• Publication Date: 2007-04-20T20:38:52Z
• Authors: Chun Ly; R. Craig Walker; William Junor
• Keywords: 13. Climate action; Astrophysics (astro-ph); 0103 physical sciences; FOS: Physical sciences; Astrophysics; 01 natural sciences

16 pages, 1 table, 3 color figures, accepted to the Astrophysical Journal<br/>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" x 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 counter-jet 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 counter-jet. A comparison of the counter-jet 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 counter-jet components indicate that these observations are undersampled. The closest pair of images gives apparent speeds of 0.25 to 0.40c for the jet and 0.17c for the counter-jet. 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 degrees from the line-of-sight and that the component speeds along the jet are 0.3-0.5c. Using the jet orientation derived from proper motions, the spectral index of the the counter-jet, and a jet-to-counter-jet brightness ratio of 14.4, the inferred bulk flow is 0.6-0.7c, which, given the considerable uncertainties in how to measure the brightness ratio, is not significantly larger than the component speed.<br/>