Pulsar

I

Introduction

Pulsar, rapidly spinning neutron star, the light from which is emitted in highly directional cones, causing it to appear to be pulsed as the cone sweeps across the line of sight of the observer. The pulsation periods (the time in which the neutron star spins once on its axis) of all pulsars are remarkably stable and pulsars are often referred to as the “lighthouses” of the Galaxy. Their rapid spin (on average, a few seconds) is related to their formation in supernova explosions, following the catastrophic collapse of a massive star (typically 10 to 30 times the mass of the Sun) when it has exhausted its nuclear fuel (see Novae and Supernovae). Conservation of angular momentum in that collapse leads to the small (approximately 20 km across) neutron star core (which contains typically 1.4 solar masses) spinning tens of times per second immediately after the supernova. In fact, neutron stars spin down very slowly, and their emission is powered by the corresponding loss of rotational energy (although the actual physical process linking the two is still poorly understood). The spin-down rate is typically around 10^-15 sec per second, equivalent to taking about 30 million years for the period to increase by 1 second (a rate only measurable with the most precise atomic clocks on Earth). More than a thousand pulsars have been found in our Galaxy, with a few in neighbouring galaxies such as the Magellanic Clouds. Some have now been found spinning 500 to 600 times a second (the so-called “millisecond pulsars”). These are actually old pulsars that are in binary systems and have been spun up by the accretion of material from their companion star.

The individual radio pulses arrive at Earth at different times when observed at different radio frequencies. This is because the very low density, but ionized, material in interstellar space acts to delay and broaden the pulses. Using this effect it is possible to map out the structure of the Galaxy in those directions. The extremely high density of a neutron star also makes pulsars valuable tools with which to probe the extremes of gravitational physics.

II

The Discovery of Pulsars

Pulsars were discovered accidentally at radio wavelengths in 1967 by Jocelyn Bell (now Jocelyn Bell Burnell) and her doctorate supervisor Antony Hewish at the Mullard Radio Astronomy Observatory in Cambridge. A regular signal with a period of 1.337 seconds was quickly shown not to be of terrestrial origin, and was even (semi-seriously) thought to be a sign of extraterrestrial intelligence. However, this was rapidly disproved when their observations revealed several more quite distinct pulsars, and it was recognized that the signals must originate far outside our Solar System. Furthermore, such fast periods could not be associated with normal stars (which would break apart if spinning that fast), and therefore had to be associated with white dwarfs or neutron stars (the latter at that point still only a theoretical construct and never detected), either as a pulsation (the surface moving up and down) or a rotation of the star. Once the slow decrease in pulsar period was measured, this confirmed that the effect was due to spin-down of the pulsar's rotation, and the fastest pulsars (for example, the 33-ms pulsar in the centre of the Crab Nebula) indicated that they must be neutron stars (even white dwarfs cannot rotate that fast). Hewish subsequently received the 1974 Nobel Prize in Physics for his part in this discovery, but Bell's pivotal role is also now widely recognized.

III

Distribution and Properties of Pulsars

Of the more than 1,000 pulsars now known, all have periods in the range from 1.557 ms to just over 8 s, and are believed to represent barely 1 per cent of the total population of neutron stars in our Galaxy. Most of these are within a few kiloparsecs (that is, closer than 10,000 light-years), and a new one is formed every 50-300 years in our Galaxy following a supernova explosion. Only a handful of these are obviously associated with a supernova remnant (the most famous are the Crab Pulsar, formed in the supernova of ad 1054 which was recorded by Chinese astronomers, and the Vela Pulsar) because the remnant will fade from view on a timescale of less than 100,000 years, whereas the rotation of the neutron star can power the radio pulsations for millions of years. In fact the Crab Pulsar had been visible in optical photographs of the nebula (as the southern of the two 16th-magnitude stars near its centre) for many years prior to the discovery of radio pulsars, but it required special instrumentation to recognize that the light was flashing on and off 30 times per second.

As well as spinning up the neutron star in the supernova explosion, the original magnetic field of the massive star is compressed in the collapse to extraordinarily high values (typically a million million Gauss, far higher than any magnetic field that can be generated in Earth-based laboratories). Based on these models it is estimated that the ages of the observed pulsars range from a thousand years to a hundred million years. They also have high spatial velocities of several hundred kilometres per second (relative to their position in the Galaxy), and this is believed to be due to a “kick” received in the supernova explosion because the collapse is not symmetrical.

A handful of pulsars have been found in binary systems, thereby allowing accurate measurements to be made of their masses. All are around 1.4 solar masses, although current theories constrain them to be less than 3 solar masses. That their pulsed radiation is related to the rotation of the neutron star is now obvious, and it is necessary for the magnetic axis to be offset from the spin axis (see figure), but the actual mechanism for producing the radiation is complex and unclear. Electrons and protons are believed to be accelerated in the strong field region of the polar caps and leave the neutron star in a narrow cone, but are still held by the field until they reach the “light cylinder”. This is the radius at which a point fixed relative to the surface of the neutron star would be moving at the speed of light, and most of the emission comes from just inside this point. That the magnetic field is a key component is demonstrated by the high degree of polarization observed in the pulses, indicating that we are seeing synchrotron radiation, which is produced when charged particles spiral at high velocity in a magnetic field.

IV

X-Ray Pulsars

While the pulsars formed in supernova explosions are seen to emit their pulses across the entire electromagnetic spectrum, including X-rays, the term “X-ray pulsar” is normally used to denote the (often much slower) pulsations seen in X-ray binary systems (see X-Ray Astronomy). Here a neutron star is orbiting a (usually) massive star whose stellar wind provides material that can be accreted on to the neutron star. This matter does not fall uniformly on to the neutron star, but is constrained by the magnetic field to travel (at high velocity) on to the magnetic poles, producing an X-ray “hot spot” that leads to the observed pulsation (the physics is believed to be very similar to that in which the very much lower density solar wind of ionized gas is guided by the Earth's magnetosphere to the polar regions, thereby producing the aurorae).

However, there are a handful of “anomalous” X-ray pulsars, the nature of which is still a subject of investigation. They all have periods of around 7 s, but no evidence of a binary companion or motion has been found. They have also been linked to the “soft gamma-ray repeaters” and it is suggested that their behaviour is due to their having the most extreme magnetic fields of all (see Gamma-Ray Astronomy: Magnetars).