An international team of astronomers discovers ‘Rosetta stone’ for mysterious cosmic signals

An international team led by the University of Sydney has uncovered the clearest evidence yet for the origin of an unusual class of cosmic signals. The team, with the participation of researchers from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE-CSIC),  has identified a rare stellar system that is providing scientists with a natural laboratory to study extreme physics. The findings are published in Nature Astronomy.

Using the Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope from the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the team discovered a small, dense star, called a white dwarf, shredding material from its larger, but less dense, companion star. As this material spirals in, it produces powerful bursts of radio waves in a cycle that repeats every 1.4 hours.

“Even more surprisingly, using X-ray observations with the Einstein Probe we’ve detected the same 1.4 hours modulation of the source in X ray”, says Nanda Rea, co-author, IEEC researcher at the ICE-CSIC and member of the European Space Agency (ESA) Einstein Probe Science Team, a collaboration between the Chinese Academy of Science (CAS), ESA, Max Planck and the Centre National d’Études Spatiales (CNES).

“For the first time we have pinpointed the origin of these signals, confirming the source to be a ‘cataclysmic variable’, or an accreting white dwarf binary system,” says lead author and PhD student Kovi Rose from the University of Sydney’s School of Physics and CSIRO.

“Long-period radio transients have puzzled astronomers for years. We’ve only found about a dozen, and their origins have been unclear. Now, we’ve been able to show that the source for one of these transients comes from a white dwarf actively pulling material from a companion star,” he adds. 

The newly identified system, named ASKAP J1745−5051, consists of a white dwarf—a dense stellar remnant roughly the size of Earth but with the mass close to that of the Sun—paired with a larger but lower-mass red dwarf star of about one-tenth the Sun’s mass. The two stars orbit each other extremely closely, completing a full orbit in just over an hour.

As material from the less massive star is drawn towards the white dwarf, it heats up and emits X-rays. At the same time, interactions between the stars’ magnetic fields generate regular bright radio bursts, never observed before in any of the hundreds of those ‘cataclysmic variable’ known to date in x-rays and optical observations.  

“What makes this system especially remarkable is that we can now connect the radio bursts to the physical processes occurring in the binary. The simultaneous radio and X-ray observations provide an unprecedented view of how magnetic fields, accretion and orbital motion interact, revealing behaviour we had never before observed in a cataclysmic variable”, says Rea.

The team found that the radio emission likely originates where the magnetic fields of the two stars meet and interact with the charged material being ripped from the companion star, producing tightly beamed bursts of radiation.

“Some similar objects had been linked to binary systems before, but this is the first one where we can clearly see both stars and the accretion process in action,” said Professor Murphy, Head of School at the University of Sydney School of Physics and Chief Investigator at the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

The system is also only the second known long-period radio transient to emit regular X-rays—and the first where the cause of the regularity has been confirmed.

This unique system was discovered using the ASKAP radio telescope and the Einstein Probe X-ray satellite. The researchers say that ASKAP J1745-5051 could act as a reference point for understanding other long-period radio transients.

“This system gives us a way to decode these signals. It could help us determine whether other long-period transients are more like pulsars or like white dwarf systems, acting like a stellar Rosetta stone,” says Rose, referring to the fragment of an ancient Egyptian stela that helped translate ancient hieroglyphics.

The discovery also provides a unique opportunity to study extreme plasma physics and magnetic interactions under conditions that cannot be replicated on Earth. The team plans further observations combining radio, optical and X-ray telescopes to better understand how these emissions are generated and whether similar mechanisms can explain the full population of long-period radio transients.

The team used CSIRO’s Australia Telescope Compact Array and ASKAP radio telescopes in Australia, the MeerKAT radio telescope in South Africa, the SOAR and Magellan optical telescopes in Chile, and the space-based Swift (UV/X-ray) and Einstein Probe (X-ray) telescopes.