Astronomers have cracked the case of a mysterious repeating radio signal that has been a mystery since it was uncovered last year.
The team tracked the signal back to a strange binary system containing a dead star or “white dwarf” and a red dwarf stellar companion. The radio pulse repeats every 2 hours and was first detected a decade ago. It came from the direction of the Big Dipper.
This new research indicates that the cause of this repeating radio signal is the magnetic fields of the white dwarf and its red dwarf stellar companion slamming together in this tight binary, designated ILTJ1101.
Previously, long-period radio bursts like this one had only been traced back to neutron stars, meaning this work puts an entirely new spin on their origins.
“There are several highly magnetized neutron stars, or magnetars, that are known to exhibit radio pulses with a period of a few seconds,” team member and Northwestern astrophysicist Charles Kilpatrick said in a statement. “Some astrophysicists also have argued that sources might emit pulses at regular time intervals because they are spinning, so we only see the radio emission when the source is rotated toward us.
“Now, we know at least some long-period radio transients come from binaries. We hope this motivates radio astronomers to localize new classes of sources that might arise from neutron star or magnetar binaries.”
The team’s research was published in the journal Nature Astronomy on Wednesday (March 12).
Digging up a dead star
Team leader Iris de Ruiter from the University of Sydney in Australia first discovered the signal in 2024 when she was searching through archival data collected by the Low Frequency Array (LOFAR). LOFAR is the largest radio telescope operating at the lowest frequencies that can be detected from Earth.
The pulse first appeared in LOFAR data in 2015, and after finding its first instance, de Ruiter found six more pulses from the same source.
These flashes of radio waves can last anywhere from several seconds to a few minutes. Despite the difference in duration, the pulses repeat regularly, once every two hours.
The pulses have some similarities with a cosmic phenomenon called “fast radio bursts” or FRBs,” but are much rarer.
“The radio pulses are very similar to FRBs, but they each have different lengths,” Kilpatrick said. “The pulses have much lower energies than FRBs and usually last for several seconds, as opposed to FRBs, which last milliseconds.
“There’s still a major question of whether there’s a continuum of objects between long-period radio transients and FRBs, or if they are distinct populations.”
The team wanted to know what the source of these regular radio pulses is, so they performed follow-up investigations with the Multiple Mirror Telescope (MMT) Observatory in Arizona and the McDonald Observatory in Texas.
This revealed the origin of the pulses was two stars located around 1,600 light-years from Earth that are pulsing in unison. The two stars whip around each other once every 125.5 minutes.
The researchers then further investigated the system for a full two-hour-long cycle using MMT discovering the true nature of this system.
Dead star is magnetically lashing its stellar companion
The team’s detailed observations allowed them to track the system’s movement in detail while gaining information from the red dwarf star by breaking its light down into different wavelengths or spectra.
“The spectroscopic lines in these data allowed us to determine that the red dwarf is moving back and forth very rapidly with exactly the same two-hour period as the radio pulses,” Kilpatrick said. “That is convincing evidence that the red dwarf is in a binary system.”
This back-and-forth rocking of this star seems to be the result of a barely visible companion in ILTJ1101 gravitationally tugging on it. The variation of the motion revealed to the team the mass of this very faint companion.
This allowed them to determine it is a white dwarf, a stellar remnant that is created when a star with around the mass of the sun reaches the end of its life and its collapses while its outer layers are shrugged off.
“In almost every scenario, its mass and the fact that it is too faint to see means it must be a white dwarf,” Kilpatrick explained. “This confirms the leading hypothesis for the white dwarf binary origin and is the first direct evidence we have for the progenitor systems of long-period radio transients.”
Astronomers are now planning to study the high-energy ultraviolet emissions of ILTJ1101. This could reveal the temperature of the white dwarf and additional details of red dwarf/white dwarf binaries like this one.
“It was especially cool to add new pieces to the puzzle,” team leader de Ruiter said. “We worked with experts from all kinds of astronomical disciplines.
“With different techniques and observations, we got a little closer to the solution step by step.”