Astronomers recently discovered a neutron star that defies expectations. Called ASKAP J1935+2148, it rotates more slowly than any other known neutron star and takes nearly an hour to complete a complete rotation. This discovery 16,000 light years from Earth revolutionizes our understanding of neutron stars.
A neutron star like no other
Massive stars (about eight to ten times more massive than our Sun) that exhaust their nuclear fuel collapse under their own gravity, triggering a supernova explosion. This cataclysmic phenomenon leaves behind a extremely dense core composed mainly of neutrons. These neutron stars are so dense that a teaspoon of their material would weigh about a billion tons.
Neutron stars are known for their rapid rotation. Most of the approximately 3,000 such objects detected so far perform one rotation in about one secondbut some can perform up to several hundred complete rotations per second. This rapid rotation is due to the conservation of angular momentum during stellar collapse, a basic principle of physics which states that the angular momentum of an isolated system remains constant unless it is subjected to an external force. In the case of neutron stars, the gravitational collapse of the massive star at the end of its life compresses its mass and reduces its radius, which accelerates its rotation dramatically.
Recently, however, astronomers made a discovery that calls into question our knowledge of these enigmatic objects. This concerns a particular neutron star, named ASKAP J1935+2148, located 16,000 light years from Earth. Unlike other objects of this type, ASKAP J1935+2148 effectively almost an hour to complete a rotation. This surprising observation was made using the ASKAP radio telescope in Australia, and confirmed by the MeerKAT radio telescope in South Africa.
Challenges for current theories
The existence of a neutron star that rotates so slowly is unexpected and challenges current theories about their behavior. A hypothesis to explain this slowness is the advanced age of the star. As neutron stars age, they actually lose energy and slow down. ASKAP J1935+2148, however, continues to transmit despite its slow rotation, thus defying existing models.
Another possibility is that the observed object is not a neutron star, but a white dwarf with an extraordinarily powerful magnetic field. As a reminder, white dwarfs are the remains of smaller stars, like our Sun. However, they have never been observed with magnetic fields capable of producing such radio signals.
The discovery of ASKAP J1935+2148 could therefore encourage scientists to review their understanding of neutron stars and white dwarfs. This particular star shows that there is still much to learn about the evolution and characteristics of compact stars. Researchers will continue to study this star to better understand its nature and unique behavior.
Details of the study are published in the journal Nature Astronomy.
