High noon for pulsar timings

Pulsars are rapidly rotating neutron stars with strong magnetic fields that charged particles shoot along. The motion of the charged particles along the curvature creates a beam of radio waves that blasts out so strongly it can be detected by radio receivers on Earth. As the star spins, the radio noisy magnetic poles point toward and then away from us in turn, creating a light house effect that registers here as pulses.

They tend to pulse away at extremely regular intervals, however over time the rate at which they pulse declines very slowly. As such they have been used in all sorts of applications, even a cosmic GPS signal capable of narrowing down any position in the galaxy to a few metres. One use of the regular signal could be to search for gravitational waves, undulations in the fabric of space-time created by large shifts of gravitational sources such as the merger of two neutron stars, via changes induced by gravitational time dilation as the waves pass over the star. But there is a problem. The precision required is such that the actual slowdown of the pulsars becomes a significant factor. If we’re to measure minute changes in the pulses, we have to understand these relatively major alterations in their profiles first. In response to this, observations of a number of pulsars have been taken using the 76m Lovell Telescope at Jodrell Bank Radio Observatory at unprecedented temporal resolution.

The results show that although the rate of pulsing is relatively constant, the magnitude of pulses, the rate of slowing and the shape of pulses can change. One moment, they register as a simple sharp increase and decrease in the signal amplitude, but another mode of operation sees a much messier signal with multiple merged peaks apparently blended in together. Furthermore the different pulses coincide with different slow down rates. The braking is faster for the smoother pulse than the messier pulse. Physically, the profile results from changes in the flow of material to and from the magnetic pole, with a faster rate of passage in the clean pulse and a slower one in the messier pulse.

The good news is that since the two spin down rates are linked to profile shapes, it should be possible to create a correction factor that irons out the associated irregularities with the spin rate, which in turn should make pulsars that little bit more accurate as celestial time keepers and so a little bit more helpful in the search for gravitational waves.

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