Are Mysterious Fast Radio Bursts Coming From the Collapse of Strange Star Crusts?

Fast Radio Bursts (FRBs) have fascinated astronomers ever since the first one was detected in 2007. This event was named the “Lorimer Burst” after it discoverer, Duncan Lorimer from West Virginia University. In radio astronomy, this phenomenon refers to transient radio pulses coming from distant cosmological sources, which typically last a few milliseconds on average.

Over two dozen events have been discovered since 2007 and scientists are still not sure what causes them – though theories range from exploding stars and black holes to pulsars and magnetars. However, according to a new study by a team of Chinese astronomers, FRBs may be linked to crusts forming around “strange stars”. According to a model they created, it is the collapse of these crusts that lead to high-energy bursts that can be seen light-years away.

The study, titled “Fast Radio Bursts from the collapse of Strange Star Crusts“, recently appeared in The Astrophysical Journal. The team was led by Yue Zhang of the  School of Astronomy and Space Science (SASC) at Nanjing University and included Jin-Jun Geng and Yong-Feng Huang – a postdoc and professor from the SASC and the Key Laboratory of Modern Astronomy and Astrophysics (also at Nanjing University), respectively.

As they state in their study, all previous attempts to explain FRBs have been unable to resolve where these strange phenomena come from. What’s more, no counterparts in other wavebands have been detected for non-repeating FRBs so far and research into their origins has been confounded by the study of repeating FRBs. This is due to the fact that the former are often attributed to catastrophic events, which are incapable of repeating.

In the case of the FRBs, these catastrophic events include “magnetar giant flares, the collapses of magnetized supramassive rotating neutron stars, binary neutron star mergers, binary white dwarf mergers, collisions between neutron stars and asteroids/comets, collisions between neutron stars and white dwarfs, and evaporation of primordial black holes.”

Alternately, in the case of the repeating FRBs, various models suggest that these could be caused by  “highly magnetized pulsars traveling through asteroid belts, neutron star-white dwarf binary mass transfer, and star quakes of pulsars.” For the sake of their study, the team proposed a new model whereby the build up and collapse of matter on certain types of neutron stars (aka. “strange stars”) could explain the behavior of FRBs. As they explain:

“It has been conjectured that strange quark matter (SQM), a kind of dense material composed of approximately equal numbers of up, down, and strange quarks, may have a lower energy per baryon than ordinary nuclear matter (such as 56 Fe) so that it may be the true ground state of hadronic matter. If this hypothesis is correct, then neutron stars (NSs) may actually be ‘strange stars'”.

This artist’s impression of the cosmic web, the filamentary structure that fills the entire Universe, showing radio sources associated with FRBs. Credit: M. Weiss/CfA

According to this model, strange stars build up a layer of hadronic (aka. “normal”) matter on their surface over time. As these SQM stars accrete matter from their environment, their crusts becomes heavier and heavier. Eventually, this leads the crust to collapse, leaving a hot and bare strange star that becomes a powerful source of electrons and positron pairs.

These pairs would then be released along with large amounts of magnetic energy over a very short timescale. The team further hypothesized that during a collapse, a fraction of magnetic energy would be transferred to the polar cap region of the SQM stars, where the magnetic field energy is released. This would cause the electrons and positrons to be accelerated to ultra-relativistic speeds, which would then expand along magnetic field lines to form a shell.

Beyond a certain distance from the star, coherent emission in radio bands will be produced, giving birth to an FRB event. They also theorize that this same phenomenon could give to rise to repeating FRBs. One possibility is that the crust of an SQM star could be reconstructed over time, thus allowing for repeated events. A second is that only small sections of crust collapse at any given time, thus resulting in repeated events.

As they conclude, further studies will be needed before this can be said either way:

Owing to this long reconstruction timescale, multiple FRB events from the same source seem not likely to happen in our scenario. Our model thus is more suitable for explaining the non- repeating FRBs… However, we should also note that during the collapse process, if only a small portion (in the polar cap region) of the crust falls onto the SQM core while the other portion of the crust remains stable, then the rebuilt timescale for the crust can be markedly reduced and repeating FRBs would still be possible.
The CHIME telescope, a massive radio telescope located in Penticton, British Columbia. Credit: CHIME/DRAO

Another thing that they claim will require further investigation is whether or not the collapse of a strange star’s crust could result in electromagnetic radiation other than radio waves. At present, any emissions in the X-ray and Gamma-ray bands would be too faint for current detectors to observe. For these reasons, further investigations of FRB sources with more sensitive instruments are needed.

These include the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope – located in Penticton, British Columbia – and the Square Kilometer Array (SQA) currently under construction in South Africa and Australia. These facilities, which are optimized for radio astronomy, are expected to reveal a great deal more about FRBs and other mysterious cosmic phenomena.

Further Reading: arXiv

Astronomers Have Detected the Brightest Fast Radio Burst Ever Seen. Still No Idea What’s Causing Them

Fast Radio Bursts (FRBs) have been one of the more puzzling and fascinating areas of astronomical study ever since the first was detected in 2007 (known as the Lorimer Burst). Much like gravitational waves, the study of these short-lived radio pulses (which last only a few milliseconds) is still in its infancy, and only a 33 events have been detected. What’s more, scientists are still not sure what accounts for them.

While some believe that they are entirely natural in origin, others have speculated that they could be evidence of extra-terrestrial activity. Regardless of their cause, according to a recent study, three FRBs were detected this month in Australia by the Parkes Observatory radio telescope in remote Australia. Of these three, one happened to be the most powerful FRB recorded to date.

The signals were detected on March 1st, March 9th, and March 11th, and were designated as FRB 180301, FRB 180309 and FRB 180311. Of these, the one recorded on March 9th (FRB 180309) was the brightest ever recorded, having a signal-to-noise ratio that was four times higher than the previous brightest FRB. This event, known as FRB 170827, was detected on August 27th, 2017, by the UTMOST array in Australia.

The Parkes radio telescope, one of the telescopes comprising CSIRO’s Australia Telescope National Facility. Credit: CSIRO

All three of these events were detected by the Parkes radio telescope, which is located in New South Wales about 380 kilometers (236 mi) from Sydney. As one of three telescopes that makes up the Australia Telescope National Facility, this telescope has been studying pulsars, rapidly spinning neutron stars, and conducting large-scale surveys of the sky since 1961. In recent years, it has been dedicated to the detection of FRBs in our Universe.

Considering how rare and short-lived FRBs are, recording three in the space of one month is quite the achievement. What’s more, the fact that the detections happened in real-time, rather than being discovered in archival data, is also impressive. Shortly after the event, Stefan Oslowski (of the Swinburne University of Technology) tweeted about this rather fortunate discovery (see below).

At present, none of the three events are believed to be “repeaters” – aka. Repeating Fast Radio Bursts. So far, only one FRB has been found to be repeating. This was none other than FRB 121102, which was first detected by the Arecibo radio telescope in Puerto Rico on November 2nd, 2012. In 2015, several more bursts were detected from this some source which had properties that were consistent with the original signal.

As noted, and in spite of all the events that have been detected, scientists are still not sure what causes these strange bursts. But with three more events detected, and the possibility that they could repeat in the near-future, scientists now have more events to pore over and base their theories on. And with next-generation arrays being constructed, a great many more events (and repeaters) are likely to be detected in the coming years.

These include the the Square Kilometer Array currently being built across Australia, New Zealand and South Africa, and the Five hundred meter Aperture Spherical Telescope (FAST) being build in China. With these telescopes  joining observatories like the Very Large Telescope (VLT), the Atacama Large Millimeter/submillimeter Array (ALMA) and venerated observatories like Arecibo, FRBs may not be mysterious for much longer!

Further Reading: The Astronomer’s Telegram, Science Alert

Fast Radio Bursts On Repeat – Aliens, Or A Rotating Neutron Star?

Very recently, a team of scientists from the Commonwealth Scientific and Industrial Research Organization (CSIRO) achieved an historic first by being able to pinpoint the source of fast radio bursts (FRBs). With the help of observatories around the world, they determined that these radio signals originated in an elliptical galaxy 6 billion light years from Earth. But as it turns out, this feat has been followed by yet another historic first.

In all previous cases where FRBs were detected, they appeared to be one-off events, lasting for mere milliseconds. However, after running the data from a recent FRB through a supercomputer, a team of scientists at McGill University in Montreal have determined that in this instance, the signal was repeating in nature. This finding has some serious implications for the astronomical community, and is also considered by some to be proof of extra-terrestrial intelligence.

FRBs have puzzled astronomers since they were first detected in 2007. This event, known as the Lorimer Burst, lasted a mere five milliseconds and appeared to be coming from a location near the Large Magellanic Cloud, billions of light years away. Since that time, a total of 16 FRBs have been detected. And in all but this one case, the duration was extremely short and was not followed up by any additional bursts.

The NSF's Arecibo Observatory, which is located in Puerto Rico, is the world largest radio telescope. Credit: NAIC
The NSF’s Arecibo Observatory, which is located in Puerto Rico, is the world largest radio telescope. Credit: NAIC

Because of their short duration and one-off nature, many scientists have reasoned that FRBs must be the result of cataclysmic events – such as a star going supernova or a neutron star collapsing into a black hole. However, after sifting through data obtained by the Arecibo radio telescope in Puerto Rico, a team of students from McGill University – led by PhD student Paul Scholz – determined that an FRB detected in 2012 did not conform to this pattern.

In an article published in Nature, Scholz and his associates describe how this particular signal – FRB 121102 – was followed by several bursts with properties that were consistent with the original signal. Running the data which was gathered in May and June through a supercomputer at the McGill High Performance Computing Center, they determined that FRB 121102 had emitted a total of 10 new bursts after its initial detection.

This would seem to indicate that FRBs have more than just one cause, which presents some rather interesting possibilities. As Paul Scholz told Universe Today via email:

“All previous Fast Radio Bursts have only been one-time events, so a lot of explanations for them have involved a cataclysmic event that destroys the source of the bursts, such as a neutron star collapsing into a black hole. Our discovery of repeating bursts from FRB 121102 shows that the source cannot have been destroyed and it must have been due to a phenomenon that can repeat, such as bright pulses from a rotating neutron star.”

The Parkes Telescope in New South Wales, Australia. Credit: Roger Ressmeyer/Corbis
The Parkes Telescope in New South Wales, Australia. Credit: Roger Ressmeyer/Corbis

Another possibility which is making the rounds is that this signal is not natural in origin. Since their discovery, FRBs and other “transient signals” – i.e. seemingly random and temporary signals – from the Universe have been the subject of speculation. As would be expected, there have been some who have suggested that they might be the long sought-after proof that extra-terrestrial civilizations exist.

For example, in 1967, after receiving a strange reading from a radio array in a Cambridge field, astrophysicist Jocelyn Bell Burnell and her team considered the possibility that what they were seeing was an alien message. This would later be shown to be incorrect – it was, in fact, the first discovery of a pulsar. However, the possibility these signals are alien in origin has remained fixed in the public (and scientific) imagination.

This has certainly been the case since the discovery of FRBs. In an article published by New Scientists in April of 2015 – titled “Cosmic Radio Plays An Alien Tune” – writer and astrophysicist Sarah Scoles explores the possibility of whether or not the strange regularity of some FRBs that appeared to be coming from within the Milky Way could be seen as evidence of alien intelligence.

However, the likelihood that these signals are being sent by extra-terrestrials is quite low. For one, FRBs are not an effective way to send a message. As Dr. Maura McLaughlin of West Virginia University – who was part of the first FRB discovery –  has explained, it takes a lot of energy to make a signal that spreads across lots of frequencies (which is a distinguishing feature of FRBs).

Scientists have been exploring the possibility that radio bursts
For decades, scientists have been exploring the possibility that radio bursts are signals from alien civilizations. Credit: AdamBurn/DeviantArt

And if these bursts came from outside of our galaxy, which certainly seems to be the case, they would have to be incredibly energetic to get this far. As Dr. McLaughlin explained to Universe Today via email:

“The total amount of power required to produce just one FRB pulse is as much as the Sun produces in a month! Although we might expect extraterrestrial civilizations to send short-duration signals, sending a signal over the very wide radio bandwidths over which FRBs are detected would require an improbably immense amount of energy. We expect that extraterrestrial civilizations would transmit over a very narrow range of radio frequencies, much like a radio station on Earth. 

But regardless of whether these signals are natural or extra-terrestrial in origin, they do present some rather exciting possibilities for astronomical research and our knowledge of the Universe. Moving forward, Scholz and his team hope to identify the galaxy where the radio bursts originated, and plans to use test out some recently-developed techniques in the process.

“Next we would like to localize the source of the bursts to identify the galaxy that they are coming from,” he said. “This will let us know about the environment around the source. To do this, we need to use radio interferometry to get a precise enough sky location. But, to do this we need to detect a burst while we are looking at the source with such a radio telescope array. Since the source is not always bursting we will have to wait until we get a detection of a burst while we are looking with radio interferometry. So, if we’re patient, eventually we should be able to pinpoint the galaxy that the bursts are coming from.”

In the end, we may find that rapid burst radio waves are a more common occurrence than we thought. In all likelihood, they are being regularly emitted by rare and powerful stellar objects, ones which we’ve only begun to notice. As for the other possibility? Well, we’re not saying it’s aliens, but we’re quite sure others will be!

Credit: History.com/memegenerator.com
Credit: History.com/memegenerator.com
Further Reading: McGill University