An international team of astronomers examining data from the Five Hundred Meter Aperture Spherical Radio Telescope (FAST) in southwest China has documented 1,652 radio waves of energy from a cosmic source about 3 billion d ‘light years.
Captured over a 47-day period between August and October 2019, the immense collection of rapid radio bursts (FRB) recently reported in Nature is more than the number documented in all other studies combined. Already, the team behind the sightings have used this new treasure to attempt to dissect the causes of these brief and mysterious celestial explosions.
First detected in 2007, FRBs are confusing flickers of radio waves from across the cosmos that last for thousandths of a second. They appear several times a day and each burst releases staggering amounts of energy. Some appear to be point explosions, while a smaller number of sources are known to repeat.
The source FAST zoomed in for this study – FRB 121102 – is a repeater. The first repetitive source discovered and most studied, FRB 121102 typically sends a burst of bursts for 90 days, followed by a silent period of 67 days. “Repeaters are interesting because we want to understand behavior, how these guys make FRBs,” says study co-author Bing Zhang (University of Nevada, Las Vegas). “The more repeated bursts we get, the more we can learn.”
When they plotted the energy of each of the 1,652 bursts versus the burst rate, the researchers spotted a surprising characteristic. They couldn’t fit the power distribution to a single, best-fit curve. Instead, they had to use two curves. The high-energy bursts matched the distributions in the previous results, unlike the low-energy bursts.
âThis suggests that there are probably two processes at play because you have two different types of energy distributions,â says Pragya Chawla (University of Amsterdam), who was not involved in the study. âThe source is probably emitting low energy bursts in one direction and high energy bursts in another. “
Shards of a magnetar?
Over 50 ideas have been put forward to explain what these burst sources are, ranging from those formulated in established theory, such as stray pulsars or neutron star mergers, to wacky concepts, like beams used to power alien light sails. Until recently, choosing the most likely scenarios was almost impossible. But that all changed last year when an FRB was found inside the Milky Way, like reported in Nature. The source of this brief burst was the magnetar SGR 1935 + 2154, a highly magnetized neutron star.
âBased on this observation, I think at least some of the FRBs are produced by magnetars,â Chawla adds. In fact, many astronomers, including Zhang, now believe that magnetars are the primary source of candidate FRB. Does the new FRB 121102 data support the magnetar image?
In a Press release, co-principal investigator Pei Wang (Chinese Academy of Sciences National Astronomical Observatories) said the study “seriously limits the possibility that FRB 121102 came from an isolated compact object” such as a magnetar, because too many energy was released during the observation period and there was no periodic pattern for the bursts; which would otherwise suggest that the source is spinning or orbiting at a defined rate.
For his part, Zhang is less pessimistic. Before this work, astronomers had put forward two main mechanisms that could explain how a magnetar could produce the bursts of FRB 121102. One of them suggested that the FRBs are generated from the region where the magnetic field d ‘a magnetar dominates. The other postulates that shocks traveling near the speed of light propagate beyond the magnetic influence of the magnetar, then collide with electrons to emit the characteristic bursts.
The new study makes the latter possibility highly unlikely. “The shock models predict relatively low radio emission efficiency – a factor of, say, 10,000 more wasted energy,” says Zhang. âThe magnetosphere models have a much higher efficiency. “
Based on the frequency and energy of the 1,652 bursts observed, the team calculated that if shocks created FRBs, they would consume about 38% of the magnetar’s total energy in less than two months. “It’s crazy because this source has been around for at least 10 years,” adds Zhang. In contrast, Zhang says that if the observed FRBs were generated from the star’s magnetosphere, the total energy emitted “is quite correct.”
To catch a nearby gust
To get to the bottom of this mystery and finally shed light on the nature of FRBs, Zhang and his colleagues hope that bursts will begin to appear closer to home in the near future. âFRB 121102 is a little too far away,â he says. âIf we can detect, say, a thousand bursts from a nearby source, then we can learn more. “
Unfortunately, FAST only observes an area one-tenth the angular size of the Full Moon, making it a poor FRB hunter. For this, other radio telescopes, such as the Canadian Hydrogen Intensity Mapping Experiment (CHIME) on which Charwal is working, are important. Observing an area about 1,000 times the size of the Moon, CHIME spots hundreds of FRBs per year. But, of these, only a handful are from neighboring galaxies, and so far only one is from the Milky Way. (FAST has yet to spot a Milky Way Magnetar FRB due to unlucky timing.)
When CHIME finally focuses on an appropriate nearby FRB source, the enormous sensitivity of FAST can then be used to detect thousands of low-energy bursts that would otherwise go unnoticed and, hopefully, determine how those bursts originate once and for all. all.
Though hesitant to reveal details of ongoing investigations, Zhang teases that by using this approach, they’ve already spotted something intriguing. “All I can say is, please stay tuned, there should be more excitement.”