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Pulsar Glitches Explained

A step has been taken to explaining curious changes in signals from pulsars.

A step has been taken to explaining curious changes in signals from pulsars. Credit: Russell Kightley

By Stephen Luntz

Dr George Hobbs of CSIRO has found a pattern to odd shifts in the timing of pulsars. His work may contribute to a greater understanding of the behaviour of these important astronomical objects, and could make pulsars even more powerful tools for testing the fundamental laws of the universe.

The radio signals that pulsars release as they spin form remarkably accurate timing devices, but they gradually slow down as the electromagnetic emissions drain energy from the stars’ rotation.

Nevertheless, the timing of these signals is not perfectly consistent. While some variations can be explained through external forces, such as large nearby objects altering a pulsar’s orbit, others have been harder to explain.

Working with colleagues around the world, Hobbs noticed that variations in the timing related to changes in the shape of the pulsar’s signal. “This ran against accepted thinking,” says Hobbs. “Everyone had said they were unrelated. But we’ve shown they are.”

The explanation appears to lie in the pulsar’s magneto-sphere. Hobbs concludes that the magnetosphere has two states. The nature of the difference between these states remains unclear, but in each state the pulsar’s flash has a different-shaped light curve while the pulsar’s spin slows down at a different rate.

Pulsars will stay in one state for periods of months or years before suddenly switching state. The most extreme example is a pulsar that Hobbs says behaves “absolutely normally” for about 30 days and then the signal disappears for around

20 days before returning.

In other cases, the changes to the light-curve are barely large enough to be detectable with the world’s most powerful telescopes. However, it appears that wider, flatter pulses coincide with a greater slowing in the pulsar’s rotation. Large changes in signal shape also appear to correlate with larger changes in the rate of spin between the two modes.

The effect has been observed in only a minority of pulsars, but Hobbs believes this is because in most cases we have not taken sufficiently detailed observations. “We can see it in all 20 we have looked at where the signal-to-noise ratio is clear enough to observe it.”

Astronomers are keen to use miniscule changes in the timing of pulsar signals for the first direct detection of gravitational waves, but this is only possible if other sources of variation can be accounted for. Hobbs hopes this work will make such detection possible.