An X-ray movie of the Vela pulsar, made from a series of observations by
NASA’s Chandra X-ray Observatory, reveals a spectacularly erratic jet that
varies in a way never seen before. The jet of high-energy particles whips
about like an untended firehose at about half the speed of light. This
behavior gives scientists new insight into the nature of jets from pulsars
and black holes.
Chandra observed the Vela pulsar, a rotating neutron star, 13 times between
January 2000 and August 2002. These observations, which were designed to
study the nature of the outflow of matter and antimatter from the pulsar led
to the discovery that an outer jet of particles was bending and moving
sideways at phenomenal speeds.
“This jet is half a light year in length, and is shooting out ahead of the
moving pulsar,” said George Pavlov of Pennsylvania State University in
University Park, lead author of a paper in the July 10th issue of The
Astrophysical Journal. “The most striking thing about this jet is how
rapidly it changes both its shape and brightness. Such strong, fast
variability has never been observed in astrophysical jets.”
The time-lapse movie shows that in a matter of weeks the jet changes from
being straight to hook-shaped, while bright blobs move along the jet at
about half the speed of light. The jet is composed of extremely high-energy
electrons or positrons (an antimatter form of electrons) that are spiraling
around a magnetic field. The particles in the jet are created and
accelerated by voltages 100 million times that of a lightning bolt. These
voltages produced by the combined action of the fast rotation of the neutron
star and its intense magnetic field.
Over its entire length, the width of the jet remains approximately constant.
This suggests that the jet is confined by magnetic fields generated by
electrons flowing along the axis of the jet. Laboratory studies of jets or
beams have shown that they can change rapidly due to an effect called the
“firehose instability.”
“Imagine a firehose lying on the ground,” said Marcus Teter, also of Penn
State and co-author on the paper. “After you turn on the water, you will see
different parts of the hose kinking up, and moving rapidly in different
directions, pushed by the increased pressure at the bends in the hose. The
Vela jet resembles a hose made of magnetic fields, which confines the
electrically charged particles.”
The instability could be triggered by the strong head-wind created as the
pulsar moves through the surrounding gas at a speed of 300,000 kilometer per
hour (about 200,000 miles per hour). The bright blobs in the jet are thought
to be a manifestation of the increased magnetic field and particle pressure
at the kinks in the jet.
The observed brightness of the outer jet and the rapid motions of the jet
and blobs in it indicate that the bright arcs around the pulsar may not be
rings circling its equator, as previously thought. Instead, they may
represent shock waves caused by the motion of the inner jet through the
cloud of particles around the pulsar.
“The study of pulsar jets is important not only in itself,” said Oleg
Kargaltsev, a Penn State graduate student and co-investigator, “but it could
also help to understand the nature of the enormous jets coming from
supermassive black holes. Those jets may also vary, but on time scales of
millions of years, instead of weeks as in the Vela pulsar jet.”
NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra
program for the Office of Space Science, NASA Headquarters, Washington.
Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime
development contractor for the observatory. The Smithsonian Astrophysical
Observatory controls science and flight operations from the Chandra X-ray
Center in Cambridge, Mass.
The image and additional information are available at:
