Astrophysicists are puzzled by a peculiar radio source that abruptly altered its rotation speed.

In 2006, astrophysicists discovered a new type of cosmic objects that emit radio bursts at much lower frequencies than expected. These sources were named "long-period radio transients" (transient radio bursts). They are characterized by extraordinarily long rotation periods, ranging from several minutes to an hour.

The radiation emitted by radio transients is very similar to that of radio pulsars — a class of neutron stars. Consequently, some scientists consider radio transients to be neutron stars.

The discovery of these new sources has puzzled researchers, as it was previously believed that radio emission pulses cease when the rotation of the radio source slows down — taking more than a minute for one complete rotation. In such a case, the object crosses the “death line” — a critical threshold beyond which, due to a low rotation speed or weak magnetic field, a neutron star no longer produces pulses. However, researchers have never observed such events.

The main question concerning scientists is: how do such objects continue to emit in the radio range despite their slow rotation? There is currently no answer.

Neutron stars generate streams of radio emissions that reach Earth in the form of periodically repeating bursts — known as pulses. This occurs due to the rotation of the magnetic poles of neutron stars. To an observer, the radiation from the source appears to blink — it disappears and reappears, with this "pulsation" occurring at a consistent periodicity.

In 2024, an international team of astrophysicists led by Manisha Caleb (Manisha Caleb) from the University of Sydney (Australia) discovered an unusual long-period source of radio emission, ASKAP J1935+214, which rotated very slowly — completing one rotation every 53.8 minutes.

According to the researchers, ASKAP J1935+214 is located approximately 13,000 light-years from Earth and likely has a diameter of 10 to 20 kilometers. During the radio transient, scientists recorded three distinct states: a strong polarized pulse lasting 10-50 seconds, a weaker (26 times) pulse lasting 370 milliseconds, and then a gap without pulses.

Now Caleb and her colleagues have reported that ASKAP J1839-0756 has begun to rotate at a new record-low speed — one rotation every 6.45 hours, which is atypical for radio transients. Nevertheless, it continues to emit in the radio range. No known cosmic radio source has exhibited such behavior before. Furthermore, the authors of the study revealed another peculiarity of the object.

Typically, the radiation from a neutron star is emitted in the form of two relatively narrow beams from one of its magnetic poles. However, ASKAP J1839-0756 is different. This source emits additional pulses from the opposite magnetic pole. This additional signal is weaker and is referred to as an "interpulse," as it appears precisely between the main pulses. ASKAP J1839-0756 is the first object of this kind with an interpulse.

Initially, Caleb's team speculated that the new source was a magnetized white dwarf — an unsuccessful neutron star that lacked the mass to become a neutron star. Such states are typical for white dwarfs. However, researchers have never directly recorded radio emissions from such objects.

Then the researchers proposed that ASKAP J1839-0756 is a magnetar, a rare class of neutron stars with magnetic fields trillions of times stronger than the most powerful MRI machines on Earth. Previously, scientists had discovered slow magnetars that rotate around their axes every 6.67 hours, but none of them emitted radio waves at frequencies as low as those of ASKAP J1839-0756.

“This object completely changes our understanding of the mechanisms of radio wave emission in neutron stars. If it is a magnetar, then it is definitely unique,” Caleb explained.

The authors of the study noted that in the near future, due to the atypical characteristics of ASKAP J1839-0756, scientists will need to reassess the accumulated knowledge about the formation of neutron stars over the past 60 years and rethink modern ideas about the evolution of these bodies.

The results of Caleb's team's research have been published in the journal Nature Astronomy.