Universe Today
An international team using the Australian Square Kilometer Array Pathfinder (ASKAP) Telescope has identified a star system that enables the study of extreme physics: a white dwarf pulling material away from its larger companion. As the material spirals in and accretes onto the white dwarf, it produces powerful bursts of radio waves and X-rays in a cycle that repeats every 1.4 hours. In addition to being a natural laboratory, this system helped them identify the source of a class of mysterious cosmic signals.
They're known as long-period radio transients (LPTs), coherent bursts of polarized radio emission that repeat over regular intervals. Astronomers have been searching for the source of these signals for over 20 years, and now they've found one that explains all of the unusual behavior observed in them. The newly identified system (ASKAP J1745−5051) is a binary consisting of a white dwarf and a red dwarf star of about 0.10 Solar masses that orbit each other with a period of just over an hour.
The team was led by PhD student Kovi Rose from the University of Sydney and the Commonwealth Scientific and Industrial Research Organization (CSIRO). He was joined by researchers from the SKA Observatory (SKAO), the Australia Telescope National Facility (ATNF), the Sydney Institute for Astronomy (SiFA), the ARC Center of Excellence for Gravitational Wave Discovery (OzGrav), the International Center for Radio Astronomy Research (ICRAR), the Dunlap Institute for Astronomy and Astrophysics, the Chinese Academy of Sciences (CAS), and multiple institutes and universities worldwide.
Artists’ impression of the white dwarf binary ASKAP J1745-5051. The smaller, denser white dwarf is accreting material from the larger but less dense red dwarf. Credit: Carl Knox (OzGrav/Swinburne) and Dr Joshua Preston Pritchard (CSIRO).
Unlike Fast Radio Bursts (FRBs), which typically last for milliseconds to a few seconds, long-period radio signals can last for minutes to hours. When astronomers first detected an LPRT in 2005, these signals were thought to be due to slow-spinning neutron stars with powerful magnetic fields (aka. magnetars). However, current astronomical models suggest that such signals would not originate in magnetar systems. An alternative explanation was that they originate in binary systems, in which a white dwarf rapidly orbits a companion star. This new discovery reinforces this latter hypothesis.
Several such signals have been detected to date, mainly in remote parts of the Milky Way. ASKAP J1745-5051 is also only the second known LRST to emit X-rays regularly, and the first one where the cause of the regularity has been confirmed. Said Rose:
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 star. 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.
The ASKAP telescope combines a degree of coverage, resolution, and sensitivity that is unparalleled in radio astronomy, allowing astronomers to detect unusual signals that would otherwise be missed. When examining ASKAP J1745−5051, the team found that heated material drawn from the red dwarf causes it to emit X-rays, while interaction between the two stars' magnetic fields and the charged material produces tightly beamed bursts of radio waves. This causes the radio signals to repeat at regular intervals.
*An artist’s impression of fast radio bursts in the sky above the SKA precursor ASKAP. This “fly’s eye” configuration allows the telescope to see much more of the sky at one time. Credit: OzGrav/Swinburne University of Technology*
“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 co-author Professor Murphy, the Head of School at the University of Sydney School of Physics and Chief Investigator at OzGrav. The discovery also provides a unique opportunity to study extreme physics by allowing scientists to test their understanding of how matter behaves in strong magnetic fields and under intense gravitational forces.
Rose also says that ASKAP J1745-5051 could act as a reference point for understanding other long-period radio transients, making it a "Rosetta Stone" for interpreting LPRTs:
These emissions are all tied to the orbital motion of the system. But interestingly, the radio and X-ray signals don’t peak at the same time, which tells us they’re being produced in different regions of the system. 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,” said Mr Rose, referring to the archaeological object discovered in Egypt that helped translate ancient hieroglyphics.
In the near future, the team plans to combine radio, optical, and X-ray observations of ASKAP J1745-5051 to understand LRPTs better. “Each new discovery is helping us piece together the bigger picture,” said Rose. “We’re only just beginning to understand this new class of cosmic events.”
Further Reading: University of Sydney, Nature Astronomy
