The lighthouse mechanism and beam geometry
A pulsar is a neutron star that emits a focused beam of electromagnetic radiation from its magnetic poles. The beam is not intrinsic to the poles; instead, the intense magnetic field near the star's surface acts like a giant accelerator, funneling charged particles (electrons and positrons) outward along the field lines. These accelerated particles radiate across a wide spectrum, from radio waves to X-rays. As the neutron star rotates, the beam sweeps across space like a lighthouse. When the beam points toward Earth, we detect a pulse of radiation. When it points away, we see nothing. This creates the characteristic regular pulsing that gave pulsars their name.
The beam's geometry depends on the angle between the rotation axis and the magnetic axis. Most neutron stars have tilted magnetic axes, so the beam traces a wide cone as it rotates. Only pulsars whose beam can reach Earth are detected; many neutron stars rotate invisibly because their beams never point our way.
Pulsar slowdown and binary interactions
Pulsars gradually slow down over time as they radiate energy and lose angular momentum. A young pulsar might pulse thousands of times per second; old pulsars pulse once every few seconds. The slowdown rate reveals the pulsar's strength and age. Millisecond pulsars, some rotating 600 times per second, are old pulsars that have been spun up by material transferred from a binary companion. These recycled pulsars can serve as cosmic clocks, some keeping time more accurately than atomic clocks. Pulsar timing arrays (networks of dozens of monitored pulsars) are now sensitive enough to detect the distortion of spacetime from passing gravitational waves, providing a new window into the universe's most violent events.