UNLV astrophysicist Bing Zhang contributes to understanding the physical mechanisms of fast radio bursts in three papers published in Nature.
Fast radio bursts, or FRBs – powerful, millisecond-duration radio waves coming from deep space outside the Milky Way Galaxy – have been among the most mysterious astronomical phenomena ever observed. Since FRBs were first discovered in 2007, astronomers from around the world have used radio telescopes to trace the bursts and look for clues on where they come from and how they’re produced.
University of Nevada, Las Vegas (UNLV) astrophysicist Bing Zhang and international collaborators recently observed some of these mysterious sources, which led to a series of breakthrough discoveries reported in the journal Nature that may finally shed light into the physical mechanism of FRBs.
The first paper, for which Zhang is a corresponding author and leading theorist, was published in the October 28 issue of Nature.
“There are two main questions regarding the origin of FRBs,” said Zhang, whose team made the observation 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 in this paper.”
Two competing theories have been proposed to interpret the mechanism of FRBs. 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.
The team used 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 November 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.
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.
“We now know that the most magnetized objects in the universe, 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.
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 November 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.
To put it all into perspective, Zhang also worked with Nature to publish a single-author review of the various discoveries and their implications for the field of astronomy.
“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.”
Reference: “The physical mechanisms of fast radio bursts” by Bing Zhang, 4 November 2020, Nature.
Review here: https://arxiv.org/pdf/2011.03500.pdf .
“Two generic categories of radiation model invoking either magnetospheres of compact objects (neutron stars or black holes) or relativistic shocks launched from such objects have been much debated. The recent detection of a Galactic fast radio burst in association with a soft gamma-ray repeater suggests that magnetar engines can produce at least some, and probably all, fast radio bursts. Other engines that could produce fast radio bursts are not required, but are also not impossible.”
Zhang drwas an interesting historical parallel to gamma ray bursts [GRBs]. “The FRB field is following essentially the same path, but at an accelerated pace.”
“One natural question is whether magnetars can account for all the FRBs observed in the universe. The bursting rate of SGRs in the universe is about 10^2 − 10^3 times more often than the FRB rate.”
“One issue is that the number of the very active repeating FRB sources in the sky is too small to be consistent with the number of magnetars in the universe. This may require the existence of two populations of magnetars in a unified magnetar model for FRBs: a large population of regular magnetars (similar to Galactic magnetars) that contribute to the bulk of FRBs and a small population of special magnetars (those born from extreme explosions such as GRBs and superluminous supernovae) that power active repeaters. The FRB host galaxy data seem to be consistent with such a picture.”
“Despite rapid progress in the field, there are still several open questions regarding the origin of FRBs, which will drive the observational efforts and theoretical investigations in the field in the years to come:
(1) Are there genuinely non-repeating FRBs? If so, what could be the plausible source(s)?
(2) Are there engines other than magnetars that could power repeating FRBs? If so, what could be the plausible sources?
(3) How is FRB emission generated, from magnetospheres (pulsar-like mechanism) or relativistic shocks (GRB-like mechanisms)? What is the correct mechanism to produce coherent emission from FRBs?”
Instructions for reading this:
With no specific method to discern what knowledge has been acquired and comprehended by any individual mind, we must start at a win one, two, three momenta. Each of the following three parallels examples the same incredible process at three different sizes. We are no more at the center of the Universe than we are at the center of size. Smaller is to imagine one direction opposite of the direction that is Larger.
1 Here, at step one, we start with a verified par size example.
2 Here at step two, we step with a verified size smaller example.
3 Here at step three, we momenta an unverified size larger example.
1 When an object of X density outpaces the speed of sound, Sonic Boom.
2 When a charged particle of X density outpaces the speed of light, Cherenkov radiation.
3 When an object of X density outpaces the speed of radio waves, Fast Radio Burst.
1 A supersonic jet passes the speed of sound waves in a medium (air), Sonic Boom.
2 When an electron passes the speed of light waves in a medium (water), Cherenkov Radiation.
3 When a magnetar passes the speed of radio waves in a medium (?), Fast Radio Burst.
1 A sonic boom is audibly produced when a bullet travels faster than the speed of sound through (air).
2 An underwater nuclear reactor’s characteristic blue glow is visually produced when a charged particle travels faster than the speed of light through (water).
3 An intense burst of radio waves occurs when a cosmic object passes radio waves’ speed through (medium).
1 Refracted Sound Wave Aperture
Given that Mach 1 means the speed of sound, then Mach 2 is twice that speed. When a bullet, cracking bullwhip, or supersonic jet accelerates beyond Mach 1, an audible burst of sound, commonly known as a Sonic Boom, occurs.
2 Refracted Light Wave Aperture
Give that c means the speed that light travels in vacuum, a medium of empty space with no cosmological constant. Light travels through a vacuum faster than it travels through a body of water.
By example medium, sorted from faster to slower media speeds, light travels most rapidly in expansion > vacuum > air > water > glass. The rate that light shines through water is more rapid than it shines through glass. Electromagnetic radiation emits when a charged particle passes through an inhomogeneous media, such as the sample surface and an electron microscope’s vacuum.
In water, an electron that accelerates beyond the speed of light traveling in that water will procure Cherenkov Radiation (luminescence). This happens because the electron’s velocity breached light’s speed, aperture a less resistant path for the compacted light waves held in its wake to burst out as a radioactive blue glow. This same wave-related process causes sonic booms and fast radio bursts.
3 Refracted Radio Wave Aperture
Radio waves propagate like light waves. Astronomical bodies procure a fast radio burst when their speed surpasses radio’s phase velocity in a medium.
When we watch a simulation video of Fast Radio Bursts in actual time, we see bursts of light.
When we speed up a simulation video of Fast Radio Bursts enough, the simulation screen glows uniform color.
When we slow down a simulation video of Cherenkov Radiation enough, we see bursts of light.
When we watch a video with Cherenkov Radiation in actual time, we see the water glow a uniform color.