Our radio telescopes pick up mysterious signals from distant galaxies: what is their origin?

If our eyes were able to perceive the color spectrum of radio waves, we would see a rather peculiar light show every time we look into the sky: countless flashes of light that change color and disappear in a fraction of a second.

These light pulses have been known to astronomers for more than half a century, who have observed and studied them with radio telescopes. We know they are generated by a special class of superdense stars in our galaxy called pulsars, which form when stars much larger than our Sun run out of fuel and collapse under their own weight.

Like a beacon embedded in the sky, each pulsar emits a beam of light through its magnetic poles, which our telescopes register as regular pulses as the star rotates and aligns with the Earth.

The color change in the flashes is due to galactic matter in the form of ionized gas between these stars and us, which acts as a prism to scatter radio waves. Thanks to the dispersion, we can determine the amount of material on the path of each pulse to the Earth. For this reason, pulsars are really useful for studying the structure and composition of the Milky Way.

FRB: flares from other galaxies

Much more recent is the discovery that some of the radio flares we detect daily from Earth are not associated with pulsars in our galactic neighborhood. Astronomers call these mysterious events fast radio bursts (FRB), and we only know of its existence since the first recorded sighting in 2007.

Although they are morphologically similar, the scattering of FRBs can be huge compared to the pulses generated by pulsars, implying a lot of gas in their path. Another important difference is that the vast majority of FRBs do not repeat.

We now know that fast radio bursts come from other galaxies a few billion light-years away from ours. The fact that we can detect them from Earth means that they are millions of times brighter than known pulsars, making them some of the most powerful explosions in the entire universe.

several theories

Little else can we say with certainty about the origin of FRB. So far, more than fifty theories have been proposed to explain this mysterious phenomenon. The vast majority include neutron stars, the family to which pulsars belong, as FRB sources. Other hypotheses suggest that black holes are possible generators.

There are also experts who suggest that FRBs are evidence of hitherto unknown physical objects and processes, fundamental particles that are still considered hypothetical, and even extraterrestrial intelligence.

The discovery that some FRBs repeat (come from the same region of the universe) irregularly means that at least some of the objects that generate them do not self-destruct. And this would be so, despite the huge amount of energy released in these explosions. It is not yet clear if these objects are different from the objects created by the rest of the FRB.

FRB Observation Problem

These outbreaks are fairly common. It is estimated that thousands of them are produced in the sky daily. So why do we know so little about them? How did they manage to elude our telescopes for so long?

It is the random nature of the vast majority of these radio bursts and their short duration that makes them extremely difficult to detect and study. With the exception of a few repeating FRBs, it is impossible to predict when and where the next flash will appear in the sky.

Traditional radio telescopes, on the other hand, are made up of huge discs designed to collect as much light as possible in a specific direction in the sky. This makes them extremely useful when we know exactly the object we want to observe (star, planet, galaxy, etc.), but rather inefficient for detecting ephemeral and random signals like FRBs.

For this reason, until a few years ago, the detection rate of FRB was so low that there were more theories about its origin than signals identified to study and compare them with data from each of these models.

CHIME: Great FRB Detector

The field of study of FRB has changed radically since the launch of the CHIME (Canadian Hydrogen Intensity Mapping Experiment) radio telescope in 2018.

Although it took a decade since its discovery to detect the first 50 FRBs with conventional radio telescopes distributed around the world, CHIME detected thousands in its first four years of operation. It became the most powerful fast radio pulse detector ever made.

Its unparalleled detection capability is due to its distinctive design and dedicated supercomputer capable of nearly a thousand trillion operations per second (the most powerful of its kind). These features give CHIME the same observational capability as a thousand football field-sized telescopes specialized in detecting extremely scattered signals.

New information about its origin

This huge avalanche of new discoveries has made it possible to study the properties of FRBs in much greater detail.

For example, we now know that there are different classes and that those from repeated sources tend to last longer and have more complex spectral-temporal structures. However, we do not yet have enough information to determine whether this is due to various generating objects, emission mechanisms, or propagation effects.

Two recent discoveries made thanks to CHIME are key to understanding the origin and nature of this phenomenon.

The first, published in 2020, was the detection of an extremely bright radio burst in the direction of a magnetar in our galaxy. Magnetars are also neutron stars, but their magnetic fields are thousands of times stronger than the average pulsar. Basically, we are talking about the most powerful magnets that exist in the universe.

While this burst is not technically an FRB since it originates from our galaxy, this is the first time that an explosion comparable to an FRB has been directly associated with a specific object, in this case a magnetar.

The second, published this year, is the discovery of an FRB with never-before-observed characteristics. The burst lasted over three seconds, thousands of times longer than the average FRB. In addition, instead of a single pulse, it consisted of a series of pulses that were repeated strictly periodically every 0.2 seconds.

This behavior is remarkably similar to that of pulsars, except that, for some as yet unknown reason, the pulses now detected are millions of times more intense.

These results support the theory that neutron stars, whether pulsars or magnetars, can generate FRBs. However, not all of our observations are consistent with this theory, so current data does not rule out the possibility of other sources of these explosions in the universe.

A new tool to explore the boundaries of the universe

In addition to understanding their origin, much of the interest of the scientific community in FRBs lies in the possibility of using them to study the structure of the cosmos in the same way that we do with the Milky Way thanks to pulsars.

The scope of the FRB ranges from detecting rarefied gas penetrating intergalactic space to studying the magnetic properties of the universe on a cosmological scale; and from exploring the earliest eras of the evolution of the universe to studying the mysterious dark energy responsible for the accelerated expansion of the cosmos in more recent times.

These signals from other galaxies can help us solve the great mysteries of modern astrophysics and cosmology, many of which have profound implications for fundamental physics and our understanding of the universe.

The discovery of the FRB has definitely opened a new window for space exploration.

Juan Mena-Parra, Associate Professor, Dunlap Institute, and David A. Dunlap, Department of Astronomy and Astrophysics, University of Toronto

This article was originally published on The Conversation. Read the original.

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