Pulsar

Composite Optical/X-ray image of the Crab Nebula pulsar, showing surrounding nebular gases stirred by the pulsar's magnetic field and radiation.

Pulsars are rotating neutron stars that are observable as sources of electromagnetic radiation. The radiation is observed to consist of a regular series of pulses, believed to be in synchrony with the rotation of the star. As observed by radio telescopes, however, the pulses often tend to come and go (fade away), especially at long wavelengths, due to interstellar scintillation; this is similar to atmospheric scintillation in the visible wave bands (astrophysics) but is an effect of irregularities in the interstellar plasma rather than in the Earth's atmosphere. Because of this tendency to fade in and out, the regularity of the pulses was at first not at all obvious. Most observers ignored these pulses as probable noise from airplanes overhead (though none was seen), lightning strokes or local power glitches. Fortunately, Jocelyn Bell Burnell, a student of Antony Hewish, was astute enough to perceive that some families of these pulses (a family being associated with an observed patch of sky) were indeed regular sequences (with missing or unobserved members), and so the study of pulsars was born. Antony Hewish received a 1974 Nobel Prize for the work.

Astronomers classify pulsars according to the source of energy that powers the emission of radiation. There are three presently accepted classes:

• Rotation-powered pulsars, where the loss of rotational energy of the star powers radiation
• X-ray pulsars, where the gravitational potential energy of accreted matter is the energy source, and
• Magnetars, where the decay of an extremely strong magnetic field powers radiation.

Although all three classes of objects are neutron stars, their observable behaviour and the underlying physics are quite different. There are, however, connections. For example, X-ray pulsars are probably old rotation-powered pulsars that have already lost most of their energy, and have only become visible again after their binary companions expanded and begun 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.

Significant pulsars

• The first radio pulsar, CP 1919 (now known as PSR B1919+21), with a pulse period of 1.337 seconds and a pulse width of 0.04 second, was discovered in 1967 (Nature 217:709-713, 1968). A picture entitled "100 consecutive pulses from the pulsar CP 1919" appears on the front of Joy Division's album Unknown Pleasures
• The first binary pulsar, PSR B1913+16, confirming general relativity and proving the existence of gravitational waves
• The first millisecond pulsar, PSR B1937+21
• The first X-ray pulsar, Cen X-3
• The first millisecond X-ray pulsar, SAX J1808.4-3658
• The first pulsar with planets, PSR B1257+12
• The first double pulsar binary system, PSR J0737−3039
• The magnetar SGR 1806-20 produced the largest burst of energy in the Galaxy ever experimentally recorded on 27 December 2004