The pulsar, a.k.a. the "Black Widow," is moving through the galaxy at a speed of almost a million kilometers per hour. A bow shock wave due to this motion is visible to optical telescopes, shown in this image as the greenish crescent shape. The pressure behind the bow shock creates a second shock wave that sweeps the cloud of high-energy particles back from the pulsar to form the cocoon.
This composite X-ray (red/white) and optical (green/blue) image reveals an elongated cloud, or cocoon, of high-energy particles flowing behind the rapidly rotating pulsar, B1957+20 (white point-like source).
The pulsar is emitting intense high-energy radiation that appears to be destroying a companion star through evaporation. It is one of a class of extremely rapid rotating neutron stars called millisecond pulsars. Calculations suggest that the "black widow" will evaporate away its companion in about a billion years.http://www.dailygalaxy.com/my_weblog/2012/07/black-widow-pulsar-at-the-top-of-freaky-phenomena-in-the-universe-holiday-weekend-feature.html