Pulsar timing array

A pulsar timing array is an observational technique for measuring the propagation properties of electromagnetic signals in the nearby Universe, by very precise measurements of the arrival times of pulses from a collection of millisecond pulsars along different lines of sight.

Millisecond pulsars are used because they appear not to be prone to the starquakes and accretion events which can affect the period of classical pulsars.

The most interesting influence on these propagation properties is low-frequency gravitational waves, with a frequency of 10⁻⁹ to 10⁻⁶ Hertz; the expected astrophysical sources of such gravitational waves are massive black hole binaries in the centres of merging galaxies, where tens of millions of solar masses are in orbit with a period between months and a few years.

The gravitational waves cause the time of arrival of the pulses to vary by a few tens of nanoseconds over their wavelength (so, for a frequency of 3 x 10 ⁻⁸ Hz, one cycle per year, you would find that pulses arrive 20 ns early in July and 20 ns late in January). This is an extremely delicate experiment, although millisecond pulsars are stable enough clocks that the time of arrival of the pulses can be predicted to the required accuracy; the experiments use collections of 20 to 50 pulsars to account for dispersion effects in the atmosphere and in the space between us and the pulsar. It is necessary to monitor each pulsar roughly once a week; a higher cadence of observation would allow the detection of higher-frequency gravitational waves, but it is unclear whether there would be loud enough astrophysical sources at such frequencies.

It is not possible to get particularly accurate sky locations for the sources by this method - analysing timings for twenty pulsars would produce a region of uncertainty of 100 square degrees, a patch of sky about the size of the constellation Scutum which would contain at least thousands of merging galaxies.

Active and proposed PTAs

Globally there are three active pulsar timing array projects. The Parkes Pulsar Timing Array at the Parkes radio-telescope has been collecting data since March 2005. The European Pulsar Timing Array uses data from the four largest telescopes in Europe: the Lovell Telescope, the Westerbork Synthesis Radio Telescope, the Effelsberg Telescope and the Nancay Radio Telescope. Upon completion the Sardinia Radio Telescope will be added to the EPTA also. The North American Nanohertz Gravitational Wave Observatory uses data collected by the Arecibo and Green Bank Radio telescopes. These three projects have begun collaborating under the title of the International Pulsar Timing Array project.

References

G Hobbs et al. "The International Pulsar Timing Array project: using pulsars as a gravitational wave detector". Classical and Quantum Gravity, 27:8 (2010). DOI : 10.1088/0264-9381/27/8/084013. arXiv:0911.5206.