I have a real problem with talking about "supersonic jets" occurring in the vacuum of interstellar space. I may indeed be pedantic about this language usage, but in a science publication there should be anther term used for high-speed phenomena.
Scanning for a definition of supersonic renders the following "capable of exceeding speed of sound: relating to, produced by, or capable of reaching a speed that is faster than the speed at which sound travels through the air." Ah well, curmudgeon mode off.
The Astrophysical Journal Letters Volume 758 Number 2
L. E. H. Godfrey et al. 2012 ApJ 758 L27 doi:10.1088/2041-8205/758/2/L27
PERIODIC STRUCTURE IN THE MEGAPARSEC-SCALE JET OF PKS 0637–752
http://iopscience.iop.org/2041-8205/758/2/L27