Pulsar

Composite Optical/X-ray image of the Crab Nebula, showing energy coming from the surrounding nebula, which is caused by the magnetic fields and particles from the central pulsar.

A pulsar is a highly magnetized, rotating neutron star or white dwarf which spin rapidly and produce huge electromagnetic radiation along a narrow beam. Neutron stars are very dense and have short, regular spins. This produces a very precise interval between pulses that range from roughly milliseconds to seconds for an individual pulsar. Currently, there is no accepted theory on how pulsars emit their radiation.

The pulses match the star's turns. The spinning causes a lighthouse effect, as the radiation is only seen at short intervals.

Discovery

The first pulsar was discovered on November 28, 1967, by Jocelyn Bell Burnell. She observed pulses separated by 1.33 seconds that originated from the same location in the sky. The source kept to sidereal time. In looking for the explanations for the pulses, the short period of the pulses eliminated most astrophysical sources of radiation, such as stars, and since the pulse followed sidereal time, it could not have been any man-made radio frequency interference. When observations with another telescope confirmed the emission, it eliminated any sort of instrumental effects.

Types of pulsars

There are currently three different kinds of pulsars that are known:

• Rotation-powered pulsars, where the radiation is caused by the loss of rotational energy; radiation is caused by the neutron star slowing down in the speed in which it turns
• Accretion-powered pulsars (which are most, but not all X-ray pulsars); where the gravitational potential energy of matter that falls onto the pulsar causes X-rays that can be received from Earth.
• Magnetars, where an extremely strong magnetic field loses energy, which causes the radiation.

Although all three classes of objects are neutron stars, their observable behavior and the underlying physics are quite different. There are, however, connections. For example, X-ray pulsars are probably old rotationally-powered pulsars that have already lost most of their power, and have only became visible again after their binary companions had expanded and began transferring matter on to the neutron star. The process of accretion can in turn transfer enough angular momentum to the neutron star to "recycle" it as a rotation-powered millisecond pulsar. As this matter lands on the neutron star, it is thought to "bury" the magnetic field of the neutron star (although the details are unclear), leaving millisecond pulsars with magnetic fields 1000-10,000 times weaker than average pulsars. This low magnetic field is less effective at slowing the pulsar's rotation, so millisecond pulsars live for billions of years, making them the oldest known pulsars. Millisecond pulsars are seen in globular clusters, which stopped forming neutron stars billions of years ago.

Significant pulsars

• PSR B1919+21, the first radio pulsar, with a pulse period of 1.337 seconds and a pulse width of 0.04 seconds. It was discovered in 1967.
• PSR 1913+16, which was the first binary pulsar discovered. This pulsars orbit is decaying at the exact rate predicted due to the emission of gravitational radiation by general relativity.
• PSR B1937+21, the first millisecond pulsar found.
• PSR J0437-4715, the brightest millisecond pulsar found.
• Cen X-3, the first X-ray pulsar found.
• PSR B1257+12, the first pulsar with planets found.
• PSR J0738-4042, the first pulsar observed to have been affected by asteroids.
• PSR J0737−3039, the first double pulsar binary system.
• PSR J1748-2446ad, the shortest period pulsar, with a period of ~0.0014 seconds or ~1.4 milliseconds (716 times a second).
• AR Scorpii, the longest period pulsar at 118.2 seconds, as well as the first and only known example of a white dwarf pulsar.
• PSR J0437-4715, a pulsar with the most stable period.
• PSR J0337+1715, the first millisecond pulsar with 2 stellar mass companions.
• PSR J1841-0500, a pulsar which stopped pulsing for 580 days. One of only two pulsars known to have stopped pulsing for more than a few minutes.
• PSR B1931+24, a pulsar which appears to have a cycle. It pulses for about a week and stops pulsing for about a month. One of only two pulsars known to have stopped pulsing for more than a few minutes.
• PSR J1903+0327, a ~2.15 ms pulsar discovered to be in a highly eccentric binary star system with a Sun-like star.
• PSR J2007+2722, a 40.8-hertz 'recycled' isolated pulsar was the first pulsar found by volunteers on data taken in February 2007 and analyzed by distributed computing project Einstein@Home.
• PSR J1311–3430, a millisecond pulsar which was the first discovered via gamma-ray pulsations and part of a binary system with the shortest orbital period.