A mysterious repeating radio signal in the Milky Way once baffled astronomers. A new study sheds light on its nature. It turned out that it is emitted from a very rare object in the Universe. According to the article by astrophysicist Jonathan Katz from Washington University in St. Louis, the signal called GLEAM-X J162759.5-523504.3 is a candidate for the role of the first true white dwarf pulsar. Learn more at arXiv
“Since the early days of pulsar astronomy, there have been suggestions that a magnetic white dwarf may exhibit activity similar to pulsar”, Katz writes in his article.
The signal has a period of 1091 seconds, and its impulses show low-frequency (72-215 MHz) radiation, with a brightness temperature of 1016 K, supposing coherent radiation. It doesn’t have a dual companion to interact with. Therefore, it meets the criteria of a classical pulsar, although its period is hundreds of times longer than any of them.
Formation of black holes, white dwarfs and neutron stars
When a star dies, there are several possible consequences. If the mass of the luminary is about 30 times the mass of the Sun, the core collapses into a black hole. The death of a star that has a mass from 8 to 30 times of the Sun leads to a neutron star with a diameter of about 20 km and about 1.4 times the mass of the Sun. The star’s core that has a mass at least 8 times that of the Sun collapses into a white dwarf with a mass 1.5 times that of the Sun, and then the star’s core collapses into a sphere the size of the Earth and the Moon.
What are pulsars?
Pulsars are a subtype of neutron stars. These are neutron stars that rotate at breakneck speed and are angled in such a way that the rays of bright radio waves that come from the magnetic poles drift by the Earth at every rotation — on a scale from seconds to milliseconds.
Scientists asked themselves whether it is possible to observe similar behavior in white dwarfs? In 2016, they approached the star called AR Scorpio. Trapped in a binary system with a red dwarf star 380 light years away from Earth, AR Scorpio has a periodicity of several minutes.
Feature of the detected white dwarf pulsar
However, as Katz notes, its binary orbit is closer than the pulsars of neutron stars in binary systems have, and the periodic signal lacks coherence. This means that the physical processes that produce the signals may differ from traditional radio pulsars.
GLEAM-X J162759.5-523504.3 is located approximately 4.000 light years away from Earth. From January to March 2018, data collected by the Murchison Widefield Array in the Australian desert showed that it pulsates brightly for about 30 to 60 seconds every 18.18 minutes — this is one of the brightest objects in the low-frequency radio sky.
It did not match the profile of any known astronomical object. But the research team that discovered it thought it might be a hypothetical object known as a magnetar with too long a period. It is a neutron star with an extremely powerful magnetic field. But this explanation was completely wrong.
As an option, pulsar was considered. But there are two main issues. The first is a long rotation period. Second, the impulses were too bright for the pulsar of a neutron star. Both of these problems, according to Katz, can be solved if the object is actually a white dwarf.
If this is so, it will be the first discovered white dwarf that shares the physics and emission process of traditional radio pulsars. This means that GLEAM-X J162759.5-523504.3 may be a long range goal for optical observations.
Earlier, astronomers made an amazing discovery in a cosmic particle accelerator.