Mysterious repeating radio signal traced to 'vampire' star that's slowly eating its companion

A pair of spiralling stars could be a blueprint for decoding mysterious bursts of radio energy coming from space, according to new research.

Long-period transients have puzzled radio astronomers since they were first detected in 2022. These objects emit strong pulses of radiation every few minutes or hours, each burst lasting only a few seconds. They are much slower than the likes of fast radio bursts, which are intense bursts of energy that flicker for mere milliseconds.

There are many theories about what these transients are and what powers them. It's possible they are slowly rotating remnants of dead stars (pulsars) or the revolving cores of collapsed stars (white dwarfs). Previous work on other long-period transients has suggested that they could be white dwarfs in a slow dance with another object.

Now, a recent study published Jun. 1 in the journal Nature Astronomy argues that a newly discovered long-period transient named ASKAP J1745−5051 is a type of star system called a magnetic cataclysmic variable. This type of variable is a binary system, in which a white dwarf is stripping material off its companion star. It is the first time a long-period transient has been confirmed in an accreting system, where one object is stealing material from another.

The discovery is "a Rosetta Stone to help us decipher the missing bits of information in other long-period transients, both in the dozen or so that we've discovered, and the new ones that we're going to keep discovering," Kovi Rose, lead author on the study and a doctoral candidate at the University of Sydney, told Live Science. (The Rosetta Stone is an ancient Egyptian slab of stone that helped scholars to decipher hieroglyphics.)

Something unexplained

Rose analyzed more than 3 million sources of radio waves using the Australian SKA Pathfinder telescope (ASKAP), and whittled that number down to 100 sources whose light was circularly polarised, meaning it corkscrews as it travels toward Earth.

Out of those candidates, there were two objects Rose couldn't immediately identify. The first turned out to be an ultra-cool brown dwarf, which is only as hot as a pizza oven. The second was ASKAP J1745−5051. It was "something we couldn't explain, and when we eventually managed to explain it, it was more interesting than we could have hoped for," Rose said.

Using data from several radio telescopes, as well as optical and X-ray observations, Rose and collaborators showed that the object produces powerful bursts of radiation in both radio waves and X-rays every 1.3 hours. The object is more than a thousand light-years away.

"You’d see these beautiful pairs of pulses – pulse-pulse, pulse-pulse, pulse-pulse, and then it switched off," Rose explained.

However, there were a few strange features in the system that have helped researchers uncover what is behind the periodic signals.

For one thing, the radio waves and X-rays peaked at different times, showing that they originate in different parts of the system, Rose said. The team argued that the radio emissions are produced when the stars' magnetic fields meet and interact with the material that is being stripped off the smaller star. This material emits X-rays as it is heated up by the white dwarf.

Further analysis showed that the emissions pointed to the presence of helium and hydrogen, and they drifted in frequency. "The wavelengths would shift to higher frequencies and then back to lower frequencies, which is a tell-tale sign of a binary system," Rose said. That was a "smoking gun," indicating that it was two stars circling each other.

"This paper gives us a strong connection for at least some long period transients with white dwarf binary systems," Marcin Glowacki, an astronomer with the Royal Observatory, Edinburgh at the University of Edinburgh who was not involved in the research, told Live Science in an email. "We also see new behavior in this [transient], which has only been seen in one other [transient] to date, with emission drifting up and down in frequency. This behavior can give us important clues on the immediate plasma or accretion environment" of this long-period transient.

However, he was surprised that ASKAP J1745−5051 was identified as a cataclysmic variable. "As the paper states, 50 other cataclysmic variables have been seen to produce radio emissions, but none with periodic behavior as seen here and they are typically much fainter."

Patrick Woudt, an astronomer at the University of Cape Town in South Africa who specializes in cataclysmic variables and who was not part of the study, told Live Science in an email that ASKAP J1745-5051 provided a bridge between long-period transients and cataclysmic variables.

"ASKAP J1745-5051 is a few orders of magnitude brighter than the compact variables observed at radio frequencies, so what makes ASKAP J1745-5051 so unique amongst the magnetic CVs?" he asked. "That is a very interesting question to address with future observations, not only at radio frequencies, but also at UV, optical, X-ray wavelengths."

Going forward, Rose said that he plans to investigate ASKAP J1745−5051's X-ray behavior and further characterize the system to find out what other secrets it's keeping.