A radio telescope once used to track ballistic missiles has helped astronomers determine how the magnetic field structure and rotation of the young and rapidly rotating Crab pulsar evolve with time.
The Crab pulsar is a neutron star that formed in a massive cosmic explosion seen in both Europe and China in AD 1054 as a bright star in the daytime sky. Now rotating 30 times per second, this highly compact star emits beams of radio waves that, like a lighthouse, produce flashes each time it rotates. The star itself is only about 16 miles (25 kilometers) across but contains the mass of nearly 1 million Earths.
Andrew Lyne and his colleagues from The University of Manchester, United Kingdom, reported on a steady change in these flashes during a 22-year experiment watching the star. The observations told them about the pulsar’s strong magnetic field and helped scientists learn about the otherwise inaccessible interior of the star.
The flashes, or pulses, come in pairs. The new observations show that the spacing of these pairs of pulses is increasing by 0.6° per century, an unexpectedly large rate of evolution. The scientists have shown that this means that the magnetic pole is moving toward the equator.
The astronomers employed a 42-foot (13-meter) telescope that was used to track the Blue Streak missile at the Woomera Rocket Test Range in Australia until 1981, when it was dismantled, transported, and re-erected at the Jodrell Bank Observatory in Cheshire, England. This relatively modest telescope has been used to observe the Crab pulsar almost daily for 31 years, during which time the pulsar has rotated 30 billion times. Jodrell Bank has kept count of every rotation. The most accurate observations, made since 1991, show the small gradual change in the pulse spacing.
Lyne said that the most surprising aspect is that this change is happening so rapidly, when the interior of the star is superconducting and the magnetic field should be frozen in position.
“This pulsar is just 960 years old, so while 22 years gives only a small sample of its lifetime, it is a much larger fraction of a stellar lifetime than astronomers usually get to study,” said Francis Graham Smith from the University of Manchester.
“It is amazing to think that this relatively small missile-tracking telescope, installed in Australia in 1974 by Giuglielmo Marconi and donated to the Jodrell Bank Observatory in 1981, where it was converted to observe pulsars, has proved to be such a boon to astronomers,” said Christine Jordan from Jodrell Bank Observatory. “This is a real sword to ploughshare concept in action.”
Patrick Weltevrede, also of the University of Manchester, believes that this result will have important implications for our understanding of the evolution of pulsars and how they emit radiation. He said, “The Crab pulsar is iconic; it is seen across the entire electromagnetic spectrum and is an exemplar, and so this result provides vital clues about how these cosmic lighthouses shine and explains a longstanding mystery about the way pulsars slow down over time.”