The clashing magnetic fields of a white dwarf star and its neighboring red dwarf star are the source of signals from space that have remained a puzzle for over 20 years, radio astronomers in Australia have found.
The signals, or long-period radio transients, are a class of celestial radio emissions discovered in 2005. Most radio-producing objects release bursts that last for mere seconds or less, but long-period radio transients, about a dozen of which are known, produce radio waves in bursts lasting from minutes to over an hour.
Speculation had focused on highly magnetic pulsars called magnetars as the origin of these radio bursts, but now new research led by Kovi Rose of the University of Sydney, using the Australian SKA Pathfinder (ASKAP) radio telescope, has shown that symbiotic binaries are to blame for at least some long-period radio transients.
Symbiotic binaries feature a compact object — usually a white dwarf, which is the core remains of a sun-like star — stealing matter from a close companion star. This scenario often leads to a nova explosion when too much material accretes onto the surface of the white dwarf.
"Long-period radio transients have puzzled astronomers for years," said Rose, who is a postgrad student, in a statement. "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 system in question has been catalogued as ASKAP J1745-5051, and features a white dwarf that is about the diameter of Earth but a mass similar to that of our sun, accreting matter from a red dwarf star with a mass just a tenth of our sun's mass.
What makes ASKAP J1745-5051 stand out is that not only does it produce these long-period radio bursts, but it also produces blasts of X-rays.
"These emissions are all tied to the orbital motion of the system," said Rose. "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."
The X-rays are produced as matter spirals in from the red dwarf onto the white dwarf. As it gets closer to the white dwarf, gravity causes it to bunch up, friction increasing the temperature to hundreds of thousands, or even millions, of degrees, which is hot enough to emit X-rays. Exactly where it bunches depends on the relative positions of the white dwarf and red dwarf.
The origin of the radio waves is more complex. Both the white dwarf and the red dwarf have their own intrinsic magnetic fields. Their orbit around each other, which takes just 1.4 hours to complete, is not circular but strongly elliptical, meaning that at times the two objects are closer together than at other times. When they are close their magnetic fields clash, stripping charged particles from each other's surface. These charged particles then spiral around the magnetic-field lines and release a form of radio waves known as synchrotron radiation. The radio bursts last for the duration that the magnetic fields are in contact, every 1.4 hours.
While this explains ASKAP J1745-5051, it does not necessarily explain all long-period radio transients. For instance, only one other has been shown to produce X-rays. It is therefore possible that some other long-period radio transients have a different origin. However, Rose hopes that this new research will help distinguish between the different types.
"This system gives us a way to decode these signals," he said. "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."
The findings were published on June 1 in the journal Nature Astronomy.
