Introduction to the IPTA Concept
The International Pulsar Timing Array (IPTA) is a consortium of consortia , comprised of the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA). The principal goal of the IPTA is to detect gravitational waves using an array of approximately 30 pulsars. This goal is shared by each of the participating consortia individually, but they have all recognized that their goal will be achieved more quickly in collaboration, and by combining their respective resources. Sharing resources will also help to reach other IPTA goals, for example, establishing a pulsar-based reference timescale.
The basic experiment exploits the predictability the pulses from millisecond pulsars (MSPs) and uses them as a system of Galactic clocks. Disturbances in the clocks will be measurable at Earth. A disturbance from a passing gravitational wave will have a particular signature across the ensemble of pulsars, and will be thus detected.
Participating Pulsar Timing Arrays
The experiment is analogous to ground-based interferometric detectors such as LIGO and VIRGO, where the time-of-flight of a laser beam is measured along a particular path and compared to the time-of-flight along an orthogonally oriented path. Instead of the time-of-flight of a laser beam the IPTA is measuring the time- of-flight of an electromagnetic pulse from the pulse. Instead of 4 km arms (as in the case of LIGO) the ‘arms’ of the IPTA are thousands of light-years (the distance between the pulsars and the earth.) Each of the PTAs times approximately 20 millisecond pulsars (MSPs) each month. With significant overlap between the collaborations the total number of MSPs timed by the IPTA (and thus the number of ‘arms’ in the detector) is approximately 30.
These differences between the IPTA and the ground-based interferometers allow them to probe a completely different range in gravitational-wave frequency and thus a different category of sources. Whereas ground-based detectors are sensitive to 10’s-1000’s of Hz, the IPTA is sensitive to 10’s-100’s of microHertz. Their primary source of gravitational waves is supermassive black-hole binaries (billions of solar masses), presumed to exist in plenty in the universe at the centers of galaxies, resulting from previous mergers of those galaxies.
Pulsar timing was tied for top ranking in the “medium size” category for priorities from the Particle Astrophysics and Gravitational Panel of the Astro2010 Decadal Review sponsored by the National Academy. The table is in Table B.1 of the report.