Preface

Pulsars were one of the four great astronomical discoveries of the 1960s. In less than twenty years, research related to them led twice to the award of the Nobel Prize in Physics and attracted worldwide attention.

The discovery of pulsars confirmed the existence of neutron stars. A neutron star has a mass comparable to that of the Sun, but a radius of only about ten kilometres. That makes it an archetypal compact object of extremely high density. Neutron stars also possess extraordinary physical conditions: enormous pressure, enormous temperature, ultra-strong magnetic fields, and intense radiation. They form natural laboratories for extreme physics that cannot be reproduced on Earth.

The discovery of radio pulsar binaries, together with precise measurements of changes in their orbital periods, confirmed the existence of gravitational radiation. The search for gravitational waves, pursued by scientists for more than half a century, first achieved observational success through pulsar studies.

The discovery of millisecond pulsars revealed a second major class of pulsar. Radio pulsars and X-ray pulsars had once been regarded as unrelated phenomena. Once millisecond pulsars were shown to evolve from X-ray binaries, the connection between the two became direct and unavoidable.

The discovery of planetary systems around pulsars excited scientists again. The search for planetary systems beyond the Solar System has long been a major scientific goal, yet one of the earliest and most successful cases emerged unexpectedly from pulsar observations.

After pulsars were discovered, pulsars were gradually found inside supernova remnants, confirming that supernova explosions are the main formation channel for neutron stars. The discovery of the pulsar in the Crab Nebula was especially important because it resolved the long-standing mystery of the Crab Nebula's power source.

Pulsars have stable periods. Although those periods increase slowly with time, the changes are extremely small. Some millisecond pulsars slow down by only a fraction of a microsecond over several years. Their stability is good enough to rival atomic clocks, making them natural standard clocks in the sky and possible practical time standards in the form of a "pulsar clock."

Pulsars have also become powerful tools for studying the interstellar medium and are often described as probes of that medium. Observations have already traced the distribution of free-electron density and average magnetic fields across the Galaxy.

Their radiation properties are especially rich and fascinating. Pulsar emission is confined to a very narrow region, like the beams from a lighthouse at sea, turning pulsars into rotating beacons in cosmic space.

A rapidly rotating pulsar has a magnetosphere that corotates with it. That magnetosphere is filled with charge-separated high-energy plasma. Radiation processes occur in the region enclosed by the open magnetic field lines above the polar cap. A pulsar is only about ten kilometres in radius, far too small to have its spatial structure imaged directly with present-day telescopes.

Yet by analysing pulsar pulse shapes, it is possible to infer the geometry, size, structure, and height of the emission region, along with the radiation mechanism and magnetospheric structure. A few pulsars have also been detected in optical, X-ray, and $\gamma$-ray pulses. The pulsar magnetosphere is a strange mixture of extreme relativistic charged particles, strong plasma waves, and intense magnetic fields, under conditions that few other astronomical objects can match.

This course introduces, in a relatively systematic way, both the basic knowledge of pulsars and later developments in the field.