The fast-spinning cores of dead stars, pulsars, appear to emit radio signals not only from their poles but also from their outer regions. This richness in emissions contradicts our established knowledge of decades.
These pulsars are what we call neutron stars, the ultra-dense remnants of massive stars that have ended their lives. During their collapse, they develop extremely powerful magnetic fields. Spinning on their own at speeds that can reach several hundred rotations per second, they emit beams of radiation that sweep through space, like a lighthouse.
Artist's impression of a neutron star surrounded by a strong magnetic field (blue) and emitting a narrow beam of radio waves (magenta).
Credit: NASA Goddard/Walt Feimer
A group of scientists studied radio observations of about 200 very fast-spinning pulsars, comparing them with data collected in gamma rays. They discovered that for a third of these objects, the radio waves came from two or more zones around the star. In contrast, only 3% of slower-spinning neutron stars show comparable behavior, highlighting a specificity linked to rotation speed.
The match between the radio pulses and the gamma-ray emissions detected by NASA's Fermi space telescope shows that these two types of radiation share a common source, far from the poles. This observation indicates that pulsars produce radio waves both near their poles, which was already known, and within a 'current sheet' of charged particles, a swirling structure located at a greater distance from the star.
This discovery facilitates the detection of millisecond pulsars, as their radio waves are emitted in a wider range of directions, and no longer only in a narrow cone from the poles. Consequently, a pulsar no longer needs to be perfectly aligned with Earth to be detected via its radio emissions, which is an advantage for projects using pulsar networks, particularly for measuring gravitational waves.
An important physical problem remains to be solved: by what process are these radio pulses created so far from the neutron star, in turbulent environments? Understanding this mechanism is fundamental to fully exploiting these objects as high-precision instruments in astrophysics, as the authors recalled in their study published in
Monthly Notices of the Royal Astronomical Society.
The formation of neutron stars
Neutron stars are born from the violent end of massive stars. When a star several times the mass of the Sun exhausts its nuclear fuel, it can no longer counteract its own gravity. Its core then collapses on itself, triggering a spectacular explosion called a supernova.
This collapse compresses matter to an unprecedented degree, creating an object so concentrated that a single teaspoon of its substance would weigh millions of tons on Earth. The pressure is such that electrons and protons merge to form neutrons, hence the name neutron star.
This phenomenon also generates magnetic fields of rare intensity, among the strongest known. The star's rotation accelerates during the collapse, following a principle similar to that of a skater pulling in their arms to spin faster, which can lead to speeds of several hundred rotations per second.
These extraordinary characteristics make neutron stars unique environments for studying physics under conditions impossible to reproduce in a laboratory. Their observation helps test theories about very dense matter and fundamental forces.