It is possible to produce a vibrato with the bow, much in the manner that one would produce with the left hand.
The vibrato is produced by varying the pressure of the bow with the right hand over a sustained pitch. These variations can be small or great and can be performed at great speed.
The difference between this and a traditional left hand vibrato is that bow vibrato is a rhythmic effect produced by small changes in dynamic as opposed to micro-fluctuations in pitch. The vibrato can be rhythmically measured or free and it is possible to create polyrhythms which can compliment or conflict with the actions of the left hand.
Dynamic Range: pp-ff
Brian Ferneyhough Trittico Per G.S (measured vibrato)