New research has provided clues that galactic magnetars –young pulsars that spin more slowly than ordinary pulsars and have the most powerful magnetic fields in the universe– lying in close proximity to a black hole, could perhaps be the source of “fast radio millisecond-long bursts,” or FRBs –high-energy blasts that originate beyond our galaxy but whose exact nature and origin is unknown.
“More than 60 of these surprise broadcasts have been recorded so far,” reports Dennis Overbye for the New York Times. “About the only thing astronomers agree on is that these signals probably are not extraterrestrials saying hello.”
Origin –“A Special Place”
“Perhaps the FRBs originated in some dense clump of matter or gas, such as the remains of an exploded star,” says astronomer Cherry Ng of the University of Toronto, about a 2019 study by the Canadian Hydrogen Intensity Mapping Experiment, or Chime, in British Columbia.. ” “Or maybe they arose near the black holes at the hearts of distant galaxies,” said Ng, who added that the burst had to have come from “some special place.”
“That could mean in some sort of dense clump like a supernova remnant or near the central black hole in a galaxy,” said Ng.
“Maybe we should thank our lucky stars, observes Overbye “that we do not live in such a ‘special place’ in our own galaxy.”
The Two Questions
“There are two main questions regarding the origin of FRBs,” said astrophysicist Bing Zhang at the University of Nevada, Las Vegas, whose team has made new observations the sources of Fast radio bursts, or FRBs –which led to a series of breakthrough discoveries that may finally shed light into the physical mechanism of FRBs–using the Five-hundred-meter Aperture Spherical Telescope (FAST) in Guizhou, China.
“The first is what are the engines of FRBs and the second is what is the mechanism to produce FRBs. We found the answer to the second question,” said Zhang, about the most enigmatic and powerful events in the cosmos. Until now, scientists don’t know what causes them it involves incredible energy–equivalent to the amount released by the Sun in 80 years.
Two competing theories have been proposed to interpret the mechanism of FRBs, reports UNLV: “One theory is that they’re similar to gamma-ray bursts (GRBs), the most powerful explosions in the universe. The other theory likens them more to radio pulsars, which are spinning neutron stars that emit bright, coherent radio pulses. The GRB-like models predict a non-varying polarization angle within each burst whereas the pulsar-like models predict variations of the polarization angle.”
GRB or Pulsar?
The team used China’s FAST to observe one repeating FRB source and discovered 11 bursts from it. Surprisingly, seven of the 11 bright bursts showed diverse polarization angle swings during each burst. The polarization angles not only varied in each burst, the variation patterns were also diverse among bursts.
“Our observations essentially rules out the GRB-like models and offers support to the pulsar-like models,” said K.-J. Lee from the Kavli Institute for Astronomy and Astrophysics, Peking University, and corresponding author of the paper.
Four other papers on FRBs were published in Nature on Nov. 4. These include multiple research articles published by the FAST team led by Zhang and collaborators from the National Astronomical Observatories of China and Peking University. Researchers affiliated with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 (STARE2) group also partnered on the publications.
“Much like the first paper advanced our understanding of the mechanism behind FRBs, these papers solved the challenge of their mysterious origin,” explained Zhang.
City-sized Neutron Stars — “Possess the Most Powerful Magnetic Fields in the Universe”
Magnetars are incredibly dense, city-sized neutron stars that possess the most powerful magnetic fields in the universe. Magnetars occasionally make short X-ray or soft gamma-ray bursts through dissipation of magnetic fields, so they have been long speculated as plausible sources to power FRBs during high-energy bursts.
The first conclusive evidence of this came on April 28, 2020, when an extremely bright radio burst was detected from a magnetar sitting right in our backyard – at a distance of about 30,000 light years from Earth in the Milky Way Galaxy. As expected, the FRB was associated with a bright X-ray burst.
Most Magnetized Objects in the Universe
“We now know that the , the so-called magnetars, can produce at least some or possibly all FRBs in the universe,” said Zhang.
The event was detected by CHIME and STARE2, two telescope arrays with many small radio telescopes that are suitable for detecting bright events from a large area of the sky.
Intriguing “Non-Detection” Discoveries
Zhang’s team has been using FAST to observe the magnetar source for some time. Unfortunately, when the FRB occurred, FAST was not looking at the source. Nonetheless, FAST made some intriguing “non-detection” discoveries and reported them in one of the Nov. 4 Nature articles. During the FAST observational campaign, there were another 29 X-ray bursts emitted from the magnetar. However, none of these bursts were accompanied by a radio burst.
“Our non-detections and the detections by the CHIME and STARE2 teams delineate a complete picture of FRB-magnetar associations,” Zhang said.
“Thanks to recent observational breakthroughs, the FRB theories can finally be reviewed critically,” said Zhang. “The mechanisms of producing FRBs are greatly narrowed down. Yet, many open questions remain. This will be an exciting field in the years to come.”
Nature.com Sources: No pulsed radio emission during a bursting phase of a Galactic magnetar,” . “The physical mechanisms of fast radio bursts,” .“Diverse polarization angle swings from a repeating fast radio burst source”
Image credit, top of page: is a NASA image of the Crab Nebula, powered by a quickly spinning, highly magnetized neutron star called a pulsar, which was formed when a massive star ran out of its nuclear fuel and collapsed. The combination of rapid rotation and a strong magnetic field in the Crab generates an intense electromagnetic field that creates jets of matter and anti-matter moving away from both the north and south poles of the pulsar, and an intense wind flowing out in the equatorial direction.
The latest image of the Crab is a composite with X-rays from Chandra (blue and white), NASA’s Hubble Space Telescope (purple) and NASA’s Spitzer Space Telescope (pink). The extent of the X-rays in this image is smaller than the others because extremely energetic electrons emitting X-rays radiate away their energy more quickly than the lower-energy electrons emitting optical and infrared light.