Pulsar timing array

A pulsar timing array monitors pulse arrival times from widely separated millisecond pulsars to detect correlated variations caused by very low-frequency gravitational waves.

Pulsar timing: The measurement of pulse arrival times to study a pulsar’s rotation, orbit, and surrounding spacetime. Repeated arrival-time measurements form the raw data for gravitational-wave searches.
Neutron star: A compact stellar remnant composed mostly of densely packed neutrons. Pulsars are rotating neutron stars whose beams produce the timed pulses.
Supermassive black hole binary: A pair of gravitationally bound black holes, each with millions or billions of solar masses. Their slow orbital evolution is a leading source of nanohertz gravitational waves.
Joseph Hooton Taylor Jr.: An American astrophysicist who shared the 1993 Nobel Prize in Physics for work on a binary pulsar. Binary-pulsar timing established gravitational-wave tests that helped motivate later timing-array searches.
Hellings–Downs curve: The predicted angular correlation between timing residuals from pulsar pairs exposed to an isotropic gravitational-wave background. Its distinctive correlation pattern is a key test for a background signal.
Radio astronomy: The study of astronomical objects through their emission at radio wavelengths. Most array observations measure pulsar pulses with radio telescopes.
Nanohertz gravitational wave: A gravitational wave oscillating at frequencies around a billionth of a hertz. This frequency range produces timing variations detectable over years or decades.
Hellings and Downs: Ronald Hellings and George Downs, who derived the angular correlation expected from an isotropic gravitational-wave background in pulsar timing. Their 1983 result supplied the defining spatial-correlation prediction.
Timing residual: The difference between an observed pulse arrival time and the arrival time predicted by a pulsar timing model. Gravitational waves appear as structured deviations in these residuals.
Dispersion measure: The integrated column density of free electrons along the line of sight to a radio source. Changing interstellar dispersion shifts pulse arrival times and must be corrected.

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