Is Fast Radio Burst a new storyteller of the cosmos?

Just when we thought we had chronicled all the beasts of the cosmos, including black holes, a new one—the "Fast Radio Burst"—is howling at us. The blips of the radiation emitted by the new beast are short-lived—less than five-thousandth of a second in duration. Hence the moniker "fast." The "radio" portion of the label is due to the fact that the emission has long wavelength distinctive of radio waves. It is called a "burst" because the blips disappear as quickly as they appear, without any warning whatsoever.

The Fast Radio Burst, or FRB for short, was discovered in 2007 by astronomers while sifting through six-year-old archival data gathered with a radio telescope at Parkes Observatory in Australia. The puzzling discovery was initially thought to be a telescopic artefact, a trick of light masquerading as a cosmic oddity. Since then, numerous FRBs shooting across the sky have been detected at observatories elsewhere. Astronomers at the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, located in British Columbia, had spotted hundreds of bursts since it came online in July 2018. According to a member of the team, they "lit up our telescope like a Christmas tree."

While astrophysicists were brainstorming to understand these fleeting eruptions as a rare one-off episode, they were thrown off balance by the discovery of bursts that repeat at regular intervals. The first repeater, as they are called, was detected in 2012 at the Arecibo Observatory in Puerto Rico. In February of this year, CHIME detected a repeater that maintained a 16-day schedule, roaring for 4 days followed by 12 days of silence. The repeaters led astronomers to conclude that they "had to be something where the engine that produced [them] survived for the next flash."

Releasing as much energy in one-thousandth of a second as the Sun releases in nearly a hundred years, the FRBs have become a cause célèbre in scientific circles over the past few years. While extremely energetic at their source, the strength of the bursts decreases dramatically due to interaction with interstellar electrons and collision with other particles along the way, to the extent that by the time the bursts reach the Earth, they become low-energy radio waves.

There have been suggestions that the bursts could be generated by advanced alien civilisations trying to communicate with us. The alien hypothesis does not hold water because, among other things, in order to generate such ridiculously high-energy signals, their transmitters have to be unbelievably large, perhaps as large as our planet! Surely, by now, our space-based observatories, particularly the ever-reliable Hubble Telescope, and other deep space probes would have discovered planets housing Earth-sized transmitters.

Instead, most astronomers favour natural explanations of the incidents. They believe that the bursts could be coming from colliding stars or fast-spinning neutron stars, which are the remnants of giant stars that died in a powerful luminous explosion known as supernova. Neutron stars are ultra-dense with roughly twice the Sun's mass crammed into a sphere barely 20 kilometres in diameter.

Assuming FRBs last one-thousandth of a second and because the speed of radio waves is 300,000 kilometres per second, any object producing them could not be larger than 300 kilometres across—the distance radio waves can cover in one-thousandth of a second. This suggests that the source is considerably smaller than an ordinary star. It is, therefore, quite likely that the bursts are indeed coming from neutron stars. 

Out of several dozen theories postulated to explain these rare occurrences, the one that is tenable posits that the source of Fast Radio Bursts could be magnetars, which are rapidly rotating neutron stars endowed with the strongest magnetic field observed in the Universe, as much as one thousand trillion times stronger than Earth's magnetic field. Moreover, magnetars are known to erupt without warning, some for hours and others for months, before dimming and disappearing. Magnetars do not remain insanely magnetic for long, though. They have a lifespan of approximately 10,000 years, after which they settle down and become normal neutron stars.

Astronomers at CHIME did detect a one-off Fast Radio Burst that appeared to flare out of a magnetar inside our home galaxy, the Milky Way. However, scientists believe that the source of all blasts may not be magnetars. In fact, a star-forming dwarf galaxy that lies three billion light years away from Earth has been identified by researchers as the source of a repeater. (A light year is the distance travelled by light in one year and is equal to 9.46 trillion kilometres.)

But this is not the end of the story. Although the ability to determine the location of the Fast Radio Bursts is a big leap towards solving the mystery, how they are produced still remains unknown. That is because establishing an acceptable theory of an astronomical phenomenon based on a few observations, as in the case of FRBs, is not easy. More observational data and detection of additional heretofore unseen patterns of the bursts are needed to nail down their source, as well as uncover the physics behind the creation of these enigmatic, terrifying cosmic monsters. In the meantime, all we can do is wait and watch with awe the fearsome display of their power.

To date, we have two storytellers of the Universe—Red Shift and Cosmic Microwave Background Radiation (CMBR). A red shift is the drift of the wavelength of a radiation toward longer values if sources of radiation are receding from the observer. The CMBR, on the other hand, is a relic of the hot, short wavelength radiation that emanated 378,000 years after the Big Bang and got red-shifted to cold, long wavelength microwaves characteristic of a temperature of negative 270 degrees Celsius.

By observing red shift in the spectra of radiation from far away galaxies, the first storyteller revealed to us that the Universe is expanding. The second storyteller, the CMBR, allows us to take a peek deep into space, and thus back into time. Now it seems Fast Radio Bursts have the potential to be the third storyteller to narrate hitherto untold tales of the cosmos.

Quamrul Haider is a professor of physics at Fordham University, New York.