Pulsar Timing
Introduction
A pulsar is a highly magnetized, rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when the beam of emission is pointing towards the Earth, much the way a lighthouse can be seen only when the light is pointed in the direction of the observer. This is called the lighthouse effect. The precise periods of pulsars make them useful tools. Observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of gravitational waves.
Discovery
The first pulsar was observed on November 28, 1967, by astronomers Jocelyn Bell and Antony Hewish. The discovery of pulsars was one of the most surprising in the history of astronomy. The existence of neutron stars was first proposed by Walter Baade and Fritz Zwicky in 1934, when they argued that a small, dense star consisting primarily of neutrons would result from a supernova explosion. However, it was not until the discovery of pulsars that these ideas were broadly accepted.
Characteristics
Pulsars are known for their extreme conditions. The density of a neutron star is comparable to that of an atomic nucleus. The strong gravitational field means that the neutron star is an almost perfect sphere, with a mountain higher than a millimeter causing the star to break up. The magnetic field strength of a pulsar is a trillion times stronger than that of the Earth, and the electric field in the magnetosphere is also much stronger than any artificial one on Earth.
Pulsar Timing
Pulsar timing is the regular monitoring of the pulsed emission from pulsars. It is a powerful tool in the study of a variety of phenomena, including tests of general relativity, the study of the interstellar medium, and the detection of gravitational waves.
The timing of pulsars is possible because of their remarkable stability. The rotation period of a pulsar is so regular that it can be measured to a precision of a few nanoseconds. This makes pulsars some of the most accurate clocks in the universe.
Pulsar timing involves the collection of time of arrival (TOA) data from a pulsar, which is then analyzed using a pulsar timing model. This model includes a variety of parameters, including the pulsar's location, its spin frequency and spin-down rate, and any binary parameters if the pulsar is in a binary system.
Pulsar Timing Arrays
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 several reasons for constructing a PTA. The most common is to detect ultra-low frequency (10−9–10−8 Hz) gravitational waves, which are predicted by current models of galactic supermassive black hole binaries.
There are currently three major pulsar timing array projects in operation: the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA). These projects are combining their data in an effort to make a direct detection of gravitational waves.
See Also
Neutron star Gravitational wave General relativity