Pulsar

A pulsar, which originally stood for pulsating radio source, is a rapidly rotating neutron star, whose electromagnetic radiation is observed in regularly spaced interval, or pulses. Pulsars are closely related to magnetars, the main difference being the strenght of the object's magnetic field.

History

Pulsars were discovered by Jocelyn Bell[?] and Antony Hewish[?] in 1967 while they were using a radio array to study the scintillation of quasars. They found a very regular signal, consisting of pulses of radiation at a rate of one in every few seconds. Terrestrial origin of the signal was ruled out because the time it took the object to reappear was a sidereal day instead of a solar day. The original name for the object was "LGM", Little Green Men, thinking of it as a beacon made by some extraterrestrial intelligence. After more speculation, an agreement was reached that the only natural object that could be responsible was a neutron star, a kind of object up to then only hypothesized.

In the 1980s a new discovery was made, the millisecond pulsars, that, as their name indicates, instead of having periods of a few seconds, have periods of a few milliseconds.

Also important was the discovery of a pulsar in a binary system. The high precision of the measurements allowed astronomers to calculate the loss of orbital energy of the system, which is thought to be emitted as gravitational waves.

Theory

There is general agreement that what we observe as a pulse is what happens when a beam of radiation points in our direction, once for every rotation of the neutron star. The origin of the beam is related to the misalignment of the rotation axis and the axis of the magnetic field of the star; the beam is emitted from the poles of the neutron star's magnetic field, which may be offset from the rotational poles by a wide angle. The source of energy of the magnetic field is the rotational energy of the neutron star, which slows down over time as the energy is emitted.

Millisecond pulsars are thought to have been spun up to high rotational speed by infalling matter pulled off of a companion star.

Of interest to the study of the state of the matter in a neutron stars are the glitches observed in the rotation velocity of the neutron star. This velocity is decreasing slowly but steadily, except by sudden variations. These were for a time believed to be "starquakes" due to the adjustment of the crust of the neutron star. Models where the glitch is due to a decoupling of the possibly superconducting interior of the star have also been advanced.

In 2003 observations of the Crab nebula pulsar's signal revealed "sub-pulses" within the main signal with durations of only nanoseconds. It is thought that these nanosecond pulses are emitted by regions on the pulsar's surface 60cm in diameter or smaller, making them the smallest structures outside the solar system to be measured.

Importance

As mentioned above, the discovery of pulsars allowed astronomers to study an object never observed before, the neutron star. This kind of object is the only place where the behaviour of matter at nuclear density can be observed (though not directly). Also, millisecond pulsars have allowed one test of general relativity in conditions of an intense gravitational field.