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Pulsars aren’t perfect

Artist's impression of a pulsar

An artist's impression of a pulsar. Blue lines represent its surrounding magnetic fields, while the purple beams represent the radio waves it emits.

  • Pulsars are small, spinning, magnetised stars
  • Emit regular pulses of radio waves
  • Act like celestial clocks

CSIRO astronomer George Hobbs and colleagues in the UK, Germany and Canada report in the journal Science that they’ve taken a big step towards solving a 30-year-old puzzle—why the “cosmic clocks” called pulsars aren’t perfect.

Pulsars are small, spinning stars that emit a beam of radio waves. When the beam sweeps over the Earth we detect a highly-regular “pulse” of radio waves. The rate at which the pulses repeat, fast or slow, depends on how fast the pulsar spins and therefore how often its radio beam flashes across the Earth.

The work is based on observations of 366 pulsars collected over several decades with the 76m radio telescope at the Jodrell Bank Observatory, run by the University of Manchester, and grew out of work George Hobbs did for his PhD thesis.

Each pulsar generates a cocoon of magnetic fields around itself—its magnetosphere.

The astronomers found that a pulsar’s magnetosphere switches back and forth between two different states.

“We don’t know exactly what happens,” Dr Hobbs said.

“But one idea is that from time to time there is a surge of charged particles—electrons, for instance—whirling through the magnetosphere. Such a surge could apply the brakes a bit to the pulsar spin, and also affect the pulsar’s radio beam.”

The change in a pulsar’s magnetosphere shows up both in the shape of the radio pulses recorded on Earth and the regular pattern of the pulses’ arrival times.

“Pulsars are very stable timekeepers, but not perfect,” said Dr Andrew Lyne of the University of Manchester, lead author of the Science paper and George Hobbs’ PhD supervisor.

“They have what we call ‘pulsar timing noise’, where the spin rate appears to wander around all over the place. This had baffled people for decades.”

Jodrell Bank 76m radio telescope

The Jodrell Bank 76m radio telescope

One of the aims of Dr Hobbs’ PhD thesis was to find an effective way to filter out this ‘timing noise’.

“We worked out how to do this, and along the way we were prompted to think hard about the nature of the timing noise,” Dr Hobbs said.

Haven’t solved all the mysteries yet

The key advance was noticing that when the pulsar timing changed, so did the shape of the radio pulse. “This ran against accepted thinking,” Dr Hobbs said. “Everyone had said they were unrelated. But we’ve shown they are.”

Now astronomers can compensate for ‘timing noise’ by using the pulse shape change to spot when the pulsar magnetosphere has changed its state—this will show when the pulsar spin rate has also changed.

“We now have a more fundamental understanding of how pulsars work,” Dr Hobbs said.

“We’ve shown that many pulsar characteristics are linked, because they have one underlying cause.”

Armed with this understanding, astronomers will find it easier to compensate for errors in their pulsar “clocks” when they use them as tools—for instance, in trying to detect gravitational waves, which is something Dr Hobbs is doing with CSIRO’s Parkes radio telescope.

But Dr Hobbs adds that there is no explanation yet as to why a pulsar’s magnetosphere flips from one state to another.

“The switching seems random in some pulsars and regular in others,” he said.

“We haven’t solved all the mysteries yet.”

Adapted from information issued by CSIRO / Russell Kightley / Jodrell Bank Centre for Astrophysics.