Pulsar Timing Array
Introduction
A pulsar timing array (PTA) is a set of pulsars which is analyzed to search for correlated signals in the pulse arrival times. There are many potential sources of such signals, but the most interesting is undoubtedly the possibility of detecting the long-period gravitational waves that are predicted by general relativity. These waves would cause small changes in the arrival times of the pulses from the pulsars, and by looking for correlated signals across a wide array of pulsars, it is possible to detect these waves.
Pulsars and their Timing
Pulsars are a type of neutron star that emit beams of radiation out of their magnetic poles. As the star rotates, these beams sweep across the sky, and if the Earth is in the path of the beam, we see a pulse of radiation each time the beam sweeps past us. The rate at which these pulses arrive is extremely regular - in fact, some pulsars are as accurate as atomic clocks.
The timing of these pulses can be measured very accurately, and this forms the basis of the pulsar timing array. By comparing the arrival times of the pulses from a number of different pulsars, it is possible to look for correlations in the timing residuals - the differences between the observed and predicted arrival times of the pulses.
Gravitational Waves and Pulsar Timing Arrays
According to the theory of general relativity, the presence of mass and energy warps the fabric of spacetime, and changes in this distribution of mass and energy should propagate as waves - gravitational waves. These waves have been directly detected by facilities such as LIGO and VIRGO, but these detectors are sensitive to waves with periods of milliseconds to seconds.
Pulsar timing arrays, on the other hand, are sensitive to gravitational waves with periods of years to decades. These long-period waves could be produced by supermassive black hole binaries - pairs of supermassive black holes that are in the process of merging.
Detecting Gravitational Waves with a Pulsar Timing Array
The passage of a gravitational wave would cause small changes in the arrival times of the pulses from a pulsar. These changes would be correlated across the array - that is, the same pattern of changes would be seen in the arrival times from all the pulsars in the array.
Detecting these correlated changes is a significant challenge, not least because there are many other factors that can cause changes in the pulse arrival times. These include changes in the Earth's position and velocity, errors in the clocks used to time the pulses, and intrinsic changes in the pulsars themselves.
However, by using a large array of pulsars, and by carefully modeling and subtracting out all the other potential sources of timing residuals, it is possible to search for the signature of gravitational waves.
Current and Future Pulsar Timing Arrays
There are currently several pulsar timing arrays in operation, including the Parkes Pulsar Timing Array (PPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the European Pulsar Timing Array (EPTA). These arrays use large radio telescopes to observe hundreds of pulsars, and have been collecting data for over a decade.
In the future, the Square Kilometre Array (SKA) will significantly improve our ability to detect gravitational waves with a pulsar timing array. The SKA will be able to observe thousands of pulsars, and its increased sensitivity will allow for more accurate timing measurements.