In the year 1054, skywatchers in China and Japan witnessed light from an exploding star reach Earth, creating a dazzling bright spot in the sky. More than a millennium later, scientists have now revealed amazing new details about the powerful and unexplained radio signals that eerily emanate from the remains of this ancient supernova.
For years, scientists have been baffled by extremely loud radio signals, known as giant radio pulses (GRPs), that can be traced to a special type of dead star known as a pulsar. Pulsars are compact, rapidly rotating remnants of supernovae that get their name from the clockwork pulses of radiation they emit from their poles, which have made them useful natural timepieces for astronomers who use their regular bursts to measure other celestial phenomena.
For reasons that remain unexplained, some pulsars occasionally spew out GRPs that are hundreds to thousands of times brighter than regular pulsar radio signals. Now, scientists have discovered that GRPs are many times more energetic than previously thought.
One of the most infamous sources of GRPs is the Crab Pulsar, the remnant of the supernova spotted by skywatchers in 1054, which sits inside the Crab Nebula some 6,500 light years from Earth. The Crab Pulsar is the only system that has shown hints that GRPs might be accompanied by enhanced emissions of other types of light, such as the visible band of the spectrum, providing a potential clue about the origin of these extra-loud radio signals. To investigate this tantalizing evidence further, scientists led by the RIKEN Cluster for Pioneering Research in Japan spent years searching for X-ray emissions that might coincide with GRPs, from both ground and space observatories.
The results revealed, for the first time, bursts of X-rays that seem connected to the radio pulses, which “implies that the total emitted energy from GRPs is tens to hundreds of times higher than previously known,” according to a study published on Thursday in Science. The discovery not only sheds new light on the mechanisms behind GRPs, but also yields information about another class of mysterious signals called fast radio bursts (FRBs).
“When we started this project, the high-energy emission associated with giant radio pulses was discovered/confirmed only in visible light but not in X-rays or gamma rays,” said lead author Teruaki Enoto, who heads the Extreme Natural Phenomena RIKEN Hakubi Research Team, in an email. “Even if there was a brightening in X-rays, we thought it would be very faint and challenging to detect.”
On the contrary, when the team’s observations were complete, the enhanced X-ray emission crossed the gold-standard five-sigma level of confidence, which means the odds are about 1 in 3.5 million that the discovery is a statistical fluke.
“As a simple prediction, we would expect a few percent enhancement, the same as that of visible light, but nature sometimes does things contrary to our expectations, so we weren't sure how it really works,” Enoto said. “As the data was accumulated and the enhancement emerged with the smaller statistical error, I was both pleased and relieved to know this answer.”
Enoto and his colleagues were able to make such precise measurements by using multiple specialized observatories. The team collected data from the Neutron star Interior Composition Explorer (NICER) X-ray observatory mounted on the International Space Station, as well as radio telescopes at the Kashima Space Technology Center and the Usuda Deep Space Center in Japan.
The unambiguous detection of GRP-coincident X-ray emission is completely unprecedented and raises a host of new questions about the mechanism that generates these loud pulses.
Shota Kisaka, a theoretical astrophysicist at Hiroshima University who co-authored the study, noted in an email that the enhanced X-rays and GRPs may have a common acceleration mechanism, but that a weird difference in the timing of both types of emission could also point to distinct origins. Future observations across multiple parts of the spectrum, from radio waves to gamma rays, could help to clarify the link between GRPs and their X-ray counterparts.
“How the enhancement rate changes with respect to the GRP flux is important for clarifying the mechanism,” Kisaka said. “The simultaneous observation with radio and gamma-ray will also help distinguish the models.”
“We are also now considering the possibility of using larger and sensitive radio telescopes to increase the number of GRPs for different flux levels,” he added.
The team’s conclusions about the link between GRPs and fast radio bursts were more clearcut. Some scientists have suggested that GRPs may be related to repeating FRBs, which emit many bright radio pulses from the same source, often in a periodic pattern. Repeaters are distinct from transient FRBs that burst into existence one time, for just a fraction of a second, and then are never observed by scientists again.
However, the new observations from the Crab Pulsar cast doubt on this association between GRPs and repeating FRBs.
“Our results disfavor the direct connection between GRPs from young and energetic pulsars and the repeating FRBs,” Kisaka said. “We showed that the total emitted energy from GRPs is ~100 times higher than previously known. If FRBs are accompanied by X-ray emission increases similar to Crab GRPs, the activity time-scale could be very short, which is inconsistent with observed repeating FRBs.”
“Therefore, we were able to deny one of the leading models, which is an important progress in enhancing the superiority of the other models,” he continued.
To that point, the X-ray discovery has opened up a new window into these inscrutable GRPs and related phenomena, and Enoto and his colleagues are eager to follow-up on the breakthrough with more observations.
“The study of giant radio pulses has been the domain of radio astronomy until now,” Enoto said, but this discovery shows that it is also an interesting topic in X-ray astronomy.”