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Pulsar Timing Irregularities: Detection and Interpretation

Most pulsars are observed to have remarkable rotational stability, mainly due to their isolated environments and large, stable moments of inertia. Therefore, the detection of timing irregularities in their pulsed emission requires dedicated observation of many pulsars to increase the possibility that irregular rotational behaviour is found. There are two types of timing irregularities in pulsar timing: "timing-noise" which is characterised by a random phase wandering in pulses relative to the general slow-down model, and "glitches" which are discontinuous changes in the rotation period, accompanied by quasi-exponential recoveries with timescales of hours up to years. Such variation in TOAs can be interpreted as changes in the pulsar environment and/or neutron star interior. This irregular behaviour is not isolated to changes in rotational frequency and pulse phase, but also extends to the intensity and timescale of observed emission, as well as variation in the shapes of pulse profiles.


Integrated pulse profiles for a sample of nine pulsars showing the variability of pulsar emission.
Copyright: Handbook of Pulsar Astronomy

The Jodrell Bank timing database provides one of the most useful archives for studying this behaviour in pulsars, with a total of over 6000 years of pulsar rotational history. Further inspection of this dataset over the past few years has revealed the peculiar rotational activity of some particularly interesting pulsars. These pulsars will be studied extensively in order to determine the physical processes which lead to the irregularities seen in their radio emission. Investigation into the possible correlation between pulse shape variation and spin-down rate is also of great interest.

The lighthouse model of pulsars. When the beam of radio-waves is pointing towards the Earth,
a `pulse' can be detected. Credits: Michael Kramer

This is motivated by the application of pulsar timing arrays to General Relativity and the detection of gravitational waves. This research should provide a better understanding of pulsars and their environments, subsequently leading to increased accuracy in timing models. This will enable improved probes of GR and will also help to find the best candidates for high precision timing.

By: Neil Young (JBCA)