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PPTA

Parkes Pulsar Timing Array project

News

Overview

Recent and on-going searches for pulsars have dramatically increased the known sample of millisecond pulsars (MSPs). This makes feasible implementation of a pulsar timing array based on observations of a large number (say 20 - 50) of MSPs, spread over the celestial sphere using one or more radio telescopes. A timing array is in effect a gravitational wave detector using the Earth as a test mass. It is sensitive to perturbations on timescales ranging from days to decades. By making precision timing observations of an array of such pulsars distributed on the sky, we can investigate the stability of terrestrial clocks and improve our understanding of Solar-system dynamics. We also have the exciting possibility of making the first detection of gravity waves.

Only MSPs are useful for such an array. First, their short period and narrow pulse width means that measured times of arrival are more accurate. Secondly, and more importantly, MSPs appear not to suffer any of the intrinsic period irregularities that affect longer-period and especially young pulsars. So far, only upper limits have been placed on intrinsic irregularities in MSPs and these limits are orders of magnitude smaller than values for young pulsars.

Only by combining data from pulsars spread over the celestial sphere can the various perturbations be separated. For example, a clock error will affect all pulsars in the same way, that is, all will appear to speed up or slow down in unison. In contrast, an error in the solar-system ephemeris will cause pulsars in one direction to apparently speed up, but those in the opposite direction to apparently slow down. From the direction and possible periodicity of the effect, it should be possible to identify the source of the perturbation. A gravitational wave on the other hand has a quadrupolar signature, with expansion and contraction axes separated by 90 degrees. Of course, the pulsars as well as the Earth are affected by passing gravitational waves. However, the effect on the pulsars will be uncorrelated since they are widely separated in space, whereas those on the Earth will be correlated (or anti-correlated) between the different pulsars and hence detectable.

Radio interference is an especially important problem for pulsar astronomy because of the wide receiver bandwidths employed to get the necessary sensitivity. Interference is inevitable within these very wide bands and, unless removed, will almost certainly limit the precision of the pulse arrival time measurements. Up to now, most interference suppression has been by brute force, just rejecting contaminated data. This is effective provided the interference is not too widespread and is also strong enough to recognise easily. This is just not good enough for the timing array project which requires exquisite precision in the derived pulse arrival times for a large sample of MSPs. Development of innovative and robust algorithms working on high-speed processors is required to cope with the real-world situation.

To achieve the objectives of the timing array project we need to observe about 20 millisecond pulsars at weekly intervals for about four years. The project is a collaborative effort between the ATNF, Swinburne University of Technology, the University of Brownsville, Texas and various other international collaborators.

An introduction to pulsars can be found here. We also provide a catalogue of pulsar parameters. A page dedicated to the current or possible scientific outcomes of the PPTA project is given here.