If our eyes perceived the color spectrum of radio waves, we would witness a rather peculiar light show every time we look up at the sky: innumerable flashes of light that change color and disappear in fractions of a second.
These pulses of light have been known to astronomers for more than half a century, who have observed and studied them using radio telescopes. We know that 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.
As if it were a lighthouse embedded in the sky, each pulsar emits a light beam through its magnetic poles that our telescopes detect as regular pulses as the star rotates and said beam aligns with the Earth.
The color change of the flashes is due to the galactic material in the form of ionized gas between these stars and us, which acts as a prism that scatters radio waves. Thanks to the dispersion we can determine the amount of material in the trajectory of each pulse towards the Earth. For this reason, pulsars are really useful for studying the structure and composition of Earth. Milky Way.
FRB: flares from other galaxies
Much more recent is the discovery that some of the radio flashes we detect daily from Earth are not associated with pulsars in our galactic neighborhood. Astronomers call these mysterious events fast radio bursts (FRBs), and we’ve only known of their existence since the first reported detection in 2007.
Although they are morphologically similar, the scattering of FRBs can be enormous compared to pulses generated by pulsars, implying a large amount of gas in their paths. Another important difference is that the vast majority of FRBs do not repeat.
We now know that the fast radio bursts come from other galaxies, some billions of light-years from our own. The fact that we can detect them from Earth means that they are millions of times more luminous than the pulses of known pulsars, constituting some of the most powerful explosions in the entire Universe.
multiple theories
Little else can we say with certainty about the origin of FRBs. Until now, more than fifty theories have been proposed to explain this enigmatic phenomenon. The vast majority involve neutron stars, the family to which pulsars belong, as sources of FRBs. Other hypotheses invoke black holes as possible generators.
There are also experts who suggest that FRBs constitute evidence of still unknown physical objects and processes, of fundamental particles that until now are considered hypothetical and even of extraterrestrial intelligence.
The discovery that some FRBs repeat themselves (come from the same region of the universe) irregularly implies that at least some of the objects that generate them do not self-destruct. And this would be so despite the enormous amount of energy released during these explosions. It is not yet clear if these objects are different from the ones generated by the rest of the FRB.
The challenge of observing FRB
These flashes are a fairly common phenomenon. It is estimated that thousands of them are produced in the sky daily. So why do we know so little about them? How have they been able 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. Except for the few repetitive FRBs, it is impossible to predict when and where in the sky the next flash will appear.
Traditional radio telescopes, on the other hand, are made up of huge disks 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 (a star, a planet, a galaxy, etc.), but rather inefficient for detecting ephemeral and random signals like FRBs.
For this reason, until a few years ago, the FRB detection rate was so low that there were more theories about its origin than signals identified to study and compare them with the data from each of these models.
CHIME: the great FRB detector
The field of study of FRBs has changed radically since the launch of the CHIME (Canadian Hydrogen Intensity Mapping Experiment) radio telescope in 2018.
While it took a decade from its discovery to detect the first 50 FRBs using traditional radio telescopes distributed around the world, CHIME has located thousands in its first four years of operation. It has become the most powerful radio fast burst detector ever built.
Its unprecedented detection capacity is due to its distinctive design and a specialized supercomputer capable of performing almost a thousand trillion operations per second (the most powerful of its kind). These features give CHIME the same observing power as a thousand football-field-sized telescopes specialized in detecting extremely scattered signals.
New clues about its origin
This great avalanche of new detections has allowed a much more detailed study of the properties of FRBs.
For example, we now know that there are different classes and that those from repetitive sources tend to last longer and have more complex spectrotemporal structures. However, we do not yet have enough information to determine whether this is due to different generating objects, emission mechanisms, or propagation effects.
Two recent discoveries made thanks to CHIME have been key to understanding the origin and nature of this phenomenon.
The first, published in 2020, was the detection of an extremely luminous 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 those of an average pulsar. Basically, we are talking about the most powerful magnets that exist in the Universe.
Although technically this burst does not constitute an FRB, since it comes from our galaxy, this is the first time that an explosion comparable to that of an FRB has been directly associated with a particular object, in this case a magnetar.
The second, published this year, is the detection of an FRB with characteristics never before observed. The burst lasted more than three seconds, thousands of times longer than an average FRB. Furthermore, instead of a single pulse, it consisted of a train of pulses that were repeated strictly periodically every 0.2 seconds.
This behavior is remarkably similar to that of pulsar pulses, except that, for some as yet unknown reason, the ones now discovered are millions of times more intense.
These findings support the theory that neutron stars, whether they are pulsars or magnetars, can generate FRBs. However, not all of our observations are consistent with this theory, so current evidence does not rule out the possibility that there are other sources of these explosions in the Universe.
A new tool to study the confines of the Universe
Beyond 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.
FRB applications range from the detection of the tenuous gas that permeates intergalactic space to the study of the magnetic properties of the Universe on cosmological scales; and from the exploration of the earliest epochs of the evolution of the Universe to the study of the mysterious dark energy, responsible for the accelerated expansion of the cosmos in more recent times.
These signals from other galaxies have the potential to help us solve great puzzles in modern astrophysics and cosmology, many of which have profound implications for fundamental physics and our understanding of the Universe.
The discovery of FRBs has definitely opened a new window to explore the cosmos.
*This article was originally published onThe Conversation.