A radio signal from space received every three hours was found not to originate from a neutron star.

The Earth receives an "echo" of various events occurring in space, and it is particularly fascinating to "listen" to radio signals that repeat with remarkable precision. When astronomers first started observing this phenomenon in the 1960s, it nearly caused a sensation: there were suspicions that messages from extraterrestrial civilizations had been detected. This led to the discovery of neutron stars, or pulsars — the remnants of "dead" massive stars.

When a massive star like Betelgeuse (which is about 15 times heavier than the Sun) "burns out," it sheds its outer layers (exploding as a supernova), while its core collapses to a diameter of just a few tens of kilometers, begins to spin at an immense speed, and emits powerful radiation from both poles. The axis of rotation wobbles significantly, causing the poles of this cosmic "top" to appear and disappear from our view, which creates the "pulse."

Neutron stars often complete hundreds of rotations per second, resulting in a corresponding frequency of "pulses." However, there are much more "sluggish" repeating radio signals coming from space. One such case was recently reported in The Astrophysical Journal Letters by an international team of astronomers led by researchers from Curtin University (Australia). This source is GLEAM-X J0704-37.

Each of its radio signals lasts about 30 to 60 seconds and repeats on average every 2.9 hours. Scientists have pinpointed its location: according to them, the source is situated 4,900 light-years away from us, in a "sparsely populated" part of the galaxy. This allowed for its observation. From the light spectrum, astronomers have determined that it is a red dwarf: a star smaller and "lighter" than the Sun, which can "burn" for billions, if not trillions, of years.

Red dwarfs are known for their flares, which make solar flares seem quite harmless, but these flares are irregular. Astronomers emphasized that such a star cannot emit these radio signals on its own. Therefore, they suspect that it has a companion.

As scientists explain, this companion cannot be a "living" star, meaning one that is burning nuclear fuel (this is referred to in astronomy as the main sequence): it would be detectable in visible light. Since it is not visible optically, it can be concluded that it must be the core of a "dead" star without its outer layers.

Of course, the first thought was that this might be a pulsar, or possibly a special type of pulsar with a super-strong magnetic field — a magnetar. However, astronomers have serious doubts about this. They noted that given the observed slow rotation, the neutron star would have to be very old, meaning it would no longer be capable of generating such "radio broadcasts."

The scientists concluded that the companion is a white dwarf. This is also the core of a "burned-out" star, but one with a smaller mass, roughly that of the Sun. Such cores compress to about the size of a planet and can possess a quite substantial magnetic field.

According to the researchers, the "pulsating" radio emission from this binary system could result from the interaction between the stars: material from the red dwarf falls onto the magnetic field of the white dwarf and gets accelerated. The three-hour periodicity should be explained either by the rotation of the white dwarf around its axis or by the motion of the pair around a common center of mass.