Pulsars

Pulsars are rapidly rotating neutron stars that emit beams of electromagnetic radiation from their magnetic poles. The beams sweep across the sky as the star spins, producing regular pulses when the beam crosses Earth. The first pulsars were discovered in 1967 by Jocelyn Bell Burnell and Antony Hewish during radio observations. While most known pulsars are detected in radio wavelengths, some emit detectable radiation in the optical, X-ray, or gamma-ray bands.

Pulsars have rotation periods ranging from about 1.4 milliseconds to several seconds. A subset known as millisecond pulsars rotate with periods shorter than about 20 milliseconds and are typically found in binary systems, having been spun up by accretion of matter from a companion. The radio and high-energy emission is produced by particle acceleration in the magnetosphere of a compact, highly magnetized neutron star, with radiation beamed along magnetic field lines.

Pulsars are used as precise cosmic clocks. Their regular pulsing enables high-precision timing experiments, tests of theories of gravity, studies of the interstellar medium via dispersion and scattering, and constraints on the neutron-star equation of state. The binary pulsar PSR B1913+16 provided indirect evidence for gravitational waves, a result that helped confirm general relativity. Pulsar timing arrays aim to detect low-frequency gravitational waves from supermassive black hole binaries.

Pulsars form when massive stars explode in core-collapse supernovae, leaving a rapidly rotating neutron star with a strong magnetic field. Owing to beaming and selection effects, only a fraction of pulsars are observable from Earth, and discoveries continue to refine estimates of their true population.