Pulsars: why dead stars flash like cosmic lighthouses

A stellar remnant with extreme properties

A neutron star can form when the core of a massive star collapses following its evolution and does not directly form a black hole. In the process, a mass roughly on a stellar scale is concentrated in a body only a few dozen kilometres across. Matter there exists in states far beyond everyday experience. The name neutron star describes the dominant role of neutron-rich matter, but is no substitute for a fully established description of its entire interior. Questions remain open, especially at the highest densities. Such stars can have strong magnetic fields and rapid rotation. Both affect their surroundings and the radiation produced. A pulsar is therefore not a completely different type of star, but a neutron star whose radiation reaches us as a regular sequence of pulses when the alignment is suitable.

The lighthouse effect produces the flashing

A neutron star's magnetic axis does not have to coincide with its rotation axis. Beams of radiation can therefore sweep across the sky as it rotates. If one regularly crosses our line of sight, a telescope records individual pulses. The star itself does not rhythmically switch off completely. Some pulsars rotate several times per second, and particularly fast examples even many hundreds of times. Their rotational energy decreases over the long term, which can cause a measurable slowdown. In binary systems, the transfer of matter can also speed a neutron star up again. The vivid lighthouse analogy explains the geometry well, but not every detail of how the radiation is produced. These remain a subject of research. Nor do we see every neutron star as a pulsar, because alignment, radiation strength and observing frequency determine whether a periodic signal reaches us.

Precise arrival times become a measuring instrument

For particularly stable pulsars, the expected arrival times of the pulses can be modelled very precisely. Deviations may indicate movement in a binary system, effects of the interstellar plasma or changes in rotation. For such analyses, distance and proper motion need to be known as accurately as possible. Radio interferometry projects therefore measure the positions of suitable pulsars over long periods. Pulsar timing arrays analyse many widely distributed cosmic timekeepers together to search for characteristically correlated signals. This method opens up different frequency ranges from those accessible to kilometre-long laser interferometers on Earth. A single irregular pulse is not in itself evidence of a wave in spacetime. Only the shared structure of many carefully corrected time series allows such conclusions. Pulsars are thus both extreme astrophysical objects and sensitive tools for investigating their surroundings and the cosmos.

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