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For several years, astronomers have been recording signals from space, the origin of which remained very mysterious. They repeat much less frequently than the pulses of typical pulsars, and at the same time they exhibit a regularity that is difficult to explain by random processes. Now one of such objects has been precisely identified. This is the ASKAP J1745−5051 system, in which a white dwarf captures material from a small stellar companion and emits radio pulses and X-rays approximately every 1.4 hours.

They found a cosmic code

The newly described object belongs to a class of phenomena called long-period transient radio sources. In English, the name used is long-period radio transients, abbreviated as LPT. These are sources that emit repeating radio pulses, but they do so on time scales of minutes or hours, not milliseconds or seconds.

This is what made these signals problematic for astronomers. Classic pulsars, i.e. rapidly rotating neutron stars, can flash radio waves with extraordinary regularity. The problem is that with such long turnover periods, some models make it clear: the neutron beacon should already be extinguished. A neutron star spinning too slowly should not easily produce such signals. So astronomers had cosmic impulses, but they lacked a reliable mechanism.

ASKAP J1745−5051 may be a breakthrough because for the first time it was possible to so clearly link one of such signals with a specific type of system. This is not a lone pulsar, but a binary system consisting of a white dwarf and a small red dwarf that orbit very close to each other, and their interaction produces radio and X-ray signals.

A zombie star eats its neighbor

A white dwarf is sometimes called a zombie star because it is the dead core of an ancient star that still manages to cause chaos. It's the remnant of a Sun-like star that used up its fuel, shed its outer layers, and left behind a very dense core about the size of Earth but with a mass similar to that of the Sun.

In ASKAP J1745−5051, this white dwarf is not alone. It is accompanied by a larger, but much lighter, red dwarf star. Its mass is approximately 1/10 of the mass of the Sun. The two stars orbit each other extremely closely, completing a complete orbit in just over an hour.

Such closeness has its interesting consequences. A white dwarf attracts material from its companion. Gas torn from the red dwarf begins to flow toward the dense stellar remnant. Along the way, it heats up and emits X-rays. At the same time, the magnetic fields of both objects interact with charged matter, producing radio pulses.

The cosmos gives an impulse exactly every 1.4 hours

The most important feature of ASKAP J1745−5051 is its exceptional regularity. Radio and X-ray signals repeat in the system's orbital rhythm, approximately every 1.3-1.4 hours. This means that the emission is related to the motion of both stars around their common center of mass, and not to a random flare.

At the same time, radio and X-rays do not reach their maximum at the same time. If both signals came from exactly the same place and the same process, their rhythm should be more synchronized. Since they are shifted relative to each other, it means that they are created in different regions of the system.

The X-ray radiation is most likely related to accretion, i.e. the falling of material onto or near the white dwarf. The radio, in turn, is said to come from the area where the magnetic fields of both stars meet the ejected, charged material. There, particles can be accelerated and emit highly polarized beam radio pulses.

ASKAP saw what is easy to miss

The source was discovered thanks to the ASKAP radio telescope in Australia. This instrument is particularly useful for catching rare signals scattered across the sky, because it can observe large areas with good sensitivity and resolution. Such possibilities are needed for phenomena that are rare, periodic and can easily be missed in classical observations aimed at other purposes.

Later, more instruments were incorporated into the work. The following were used, among others: Australia Telescope Compact Array, South Africa's MeerKAT, optical telescopes in Chile, and space-based X-ray and ultraviolet observatories including Swift and Einstein Probe. This is important because radio alone would not be enough for a full diagnosis.

Astronomers had to show that there was an optical counterpart in the same place, that the spectrum matched an accreting system, that the hydrogen and helium lines typical of hot, excited material appeared, and that the X-rays behaved in harmony with the orbital motion.

Only the comparison of all observations allowed us to clearly identify the source of the signal. The data indicate that this is not a random radio burst, but a well-defined binary system in which the emission results from specific physical processes related to the accretion of matter and magnetic interactions.

A magnetic storm in a miniature system

The most important processes in this system take place in the space between both stars. A white dwarf has a strong magnetic field, and a red dwarf is not passive in this respect either. When objects orbit so closely together and material flows from one star to another at the same time, it creates an extremely dynamic environment full of charged particles, violent magnetic forces and intense radiation.

In such a system, plasma, i.e. a gas composed of charged particles, moves in magnetic fields, is heated, accelerated and channeled. This can lead to radio emissions with high coherence and strong polarization. Polarization tells how the vibrations of an electromagnetic wave are ordered. If the signal is highly polarized, this usually indicates ordered magnetic fields and a specific emission mechanism.