Shining a Light on the Universe using Fast Radio Bursts

Often we hear that the Universe works in mysterious ways, and yes, this is very true and a great starting point for science. There are things about the Universe that we observe and are yet to understand, which are then theorised, modelled, investigated and analysed by scientists to help make them mysteries no more. 

Sometimes, space-based objects or phenomena can lie in the fascinating waters between something we are yet to understand vs. something that has come through our established systems of science, being peer-reviewed and accepted into becoming the most up to date fact. 

Of course, new data might come through and change these facts, tweaking them to represent further observations and confirmed results, and so science continues to progress in these recognised steps over time. 

In the last 15 years a relatively new, not-yet-fully understood phenomenon has emerged, fuelling excitement and a race to learn more by global radio astronomers. What began as a discovery in the archival data of the CSIRO’s Parkes radio telescope that was several years old, has now bloomed into a research field with an army of astronomers around the world working around the clock to turn mystery into fact. 

Fast Radio Bursts. 

Bright giant pulses of radio frequencies that spike above the entire background noise of the Universe, emerging from mostly extreme distances to us (and thus corresponding to events that occurred a long time back). 

We still don’t fully understand what is causing these FRBs (as they’re colloquially shortened down to) but we know it must be something extremely energetic, yet remarkably small, roughly the size of a small rural town in diameter. 

Due to their excessive brightness in radio, and the cosmological distances in which they travel across space-time, astronomers have come to realise that even though we don’t yet fully understand what is causing them (there are several theories, one of which previously included ‘aliens’) we could utilise them as a tool to help shine a light (pun) on other mysterious phenomena we are also trying to understand and build knowledge against. 

Now, two Australian astrophysicists – Dr. Shivani Bhandari from Australia’s national science agency, CSIRO, and Dr. Chris Flynn from Swinburne University of Technology have completed an in-depth review of FRBs highlighting many of the recent scientific discoveries and outlining where future research into this emerging field can progress. The review paper provided an opportunity to outline the large and growing volume of knowledge-base of FRBs to date.

“I was invited by the editors of the Universe journal to review the cosmological applications of fast radio bursts and such an invitation was a great opportunity especially for an early career researcher like me,” said Shivani.

“It was really exciting to be able to take a step back and look at how far we’ve come in the last couple of years -- which have seen so much progress and new results coming in even while we were writing the review,” said Chris.

Unlike most serendipitous discoveries of astrophysical objects and phenomena, FRBs didn’t just pop out of a real-time observation with a telescope (though this finally did happen in 2015). In fact, it was more of an opportune discovery in 2007 when astronomers were looking through the archival data observed by CSIRO’s Parkes radio telescope (also known by its traditional Wiradjuri name, Murriyang) that they noticed a burst that occurred six years prior in July of 2001.

This single event, known as the Lorimer Burst (after Duncan Lorimer who found it) lasted very briefly, only 5 milliseconds but peaked above the noise floor as a 30-Jansky pulse (a Jansky is a unit of spectral flux density that is used in radio astronomy equivalent to 10^-26 watts per square metre per hertz).

The Lorimer Burst was the first of what would be many of these single, large radio pulses that would spawn research projects, telescope observations and an army of dedicated astrophysicists in pursuit of what could be causing them, and how we could apply them to learn about the Universe.

Along the way, several Australian telescopes have been finding FRBs – starting with the Parkes dish, but also including the oddly-shaped Upgraded Molonglo Observatory Synthesis Telescope (UTMOST) and more recently, the CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) – which has made several important discoveries in localisations of FRBs.

Internationally, the former Arecibo Telescope, Greenbank Telescope, DSA-10 telescope, Apertif Telescope and China’s 500-metre diameter telescope FAST, have all also contributed to FRB science. Special mention must also be made to the Canadian Hydrogen Intensity Mapping Experiment (CHIME) which has found and localised several FRBs, including repeating sources.

“CSIRO’s ASKAP telescope has certainly shown its presence in the field of FRBs,” said Shivani. 

“ASKAP’s wide field-of-view, real-time searches along with the interferometric localisation capability to precisely pinpoint the burst to its host galaxy has proven to be an excellent formula for conducting impactful studies of FRBs around the world.” 

“Our first UTMOST FRBs showed that they were not caused by local interference, but rather originated from well off the Earth, through the power of the radio array,” said Chris. 

“Now that we have returned UTMOST to a working “Mills Cross”, we will be able to find the host galaxies for many of the new FRBs, as part of a program to weigh the baryons in the Universe,” said Chris.

Over the years, scientists have found that FRBs come from all locations across the sky which indicates they are not confined to our own Galaxy (otherwise they would appear mostly to come from the Galactic Plane). Additionally, most FRBs originate from large distances – with nearly all (except one candidate) originating at high redshifts of extragalactic origin.

The bursts themselves appear as a short-period millisecond flash with enough strength to outshine the background noise of radio sources across the region of sky being observed. Whilst most FRBs have been observed around the 1400 MHz frequency range, many others have fallen into the 400 – 800 MHz range as well. Some have even been detected as low as 110 - 188 MHz (by LOFAR in the Netherlands). 

Based on the extreme distances and large luminosities that are reflected in the data, the brightness temperature of the progenitor event that causes the burst should lie somewhere near 10³⁵ Kelvin and due to its short duration, be originating from a source that exhibits an emission radius no bigger than 30 – 3,000 km. To put it another way, whatever is causing these bursts is likely as small as a city, and in a single millisecond, putting out the equivalent energy output as the Sun does over a three-day period.

Most FRBs discovered so far have been one-off events, whilst some appear to repeat at irregular intervals. One FRB so far has been detected to repeat on a regular cycle of approximately every 16 or so days. This has led some astronomers to speculate that there might be two types of progenitor events that are causing FRBs – after all, if something small is blowing up in a violent explosion to trigger an FRB, it can’t keep repeating this process. Other astronomers think that all FRBs are repeaters, only that the giant pulses are what make it across the Universe for us to observe, whilst regular pulses remain smaller and lost across the background noise.

“Are there two populations of FRBs and what is the central relationship between them?” asks Shivani, as she mentions these are some of the burning and heavily investigated questions in radio astronomy at the moment. “The idea of two populations is certainly very exciting!”

“Teasing out clues to the origins of the FRBs themselves may not be easy, as the bursts can be so different,” adds Chris.

“Every little bit will help, such as the subtle differences between repeating and one-off events that only emerge when dozens to hundreds have been found.” 

Whilst the progenitor cause of what is actually creating FRBs still remains a topic of intense research and mystery, a number of theories and hypothesis have been put forward (Table 1) about what they could be, including cataclysmic and episodic triggers across different generations of stellar populations, or potentially non-stellar models. At one point, even Aliens were a consideration.

An interesting recent discovery that could shed some light on the progenitor events that cause FRBs, came from within our own Milky Way Galaxy, rather than from an external source. In 2020, an FRB was reported from the Magnetar SGR 1935+2154, measured with energy approximately 30 times less than the extragalactic sources – as would be expected from a more local source. Along with this FRB, an X-ray burst was also detected, helping scientists narrow down the cause to a burst that occurs outside the magnetar.

An interesting recent discovery that could shed some light on the progenitor events that cause FRBs, came from within our own Milky Way Galaxy, rather than from an external source. In 2020, an FRB was reported from the Magnetar SGR 1935+2154, measured with energy approximately 30 times less than the extragalactic sources – as would be expected from a more local source. Along with this FRB, an X-ray burst was also detected, helping scientists narrow down the cause to a burst that occurs outside the magnetar.

Now, this new review paper outlines how FRBs can be used, across a number of different aspects to learn more about the Universe at cosmological scales – including answering some of the most fundamental questions in astrophysics relating to dark matter and dark energy.

The premise here is to exploit the opportunity to use the FRBs as probes, telling us about the conditions of their host galaxies, as well as what they encounter as they traverse across the enormous distances they seem to originate from.

Solving The Missing Baryonic Matter Problem

In mid-2020, an international team led by Australian scientists used ASKAP’s detections of FRBs to measure the amount of matter in the intergalactic medium, helping provide some context to the missing baryon problem. Once quantified, this helped provide a crucial understanding of galaxy evolution models, as well as define what makes up the roughly five percent of matter composition in the Universe (this is not related to dark matter, which to date is still unknown by any means, and makes up approximately 27 percent of the matter composition of the Universe).

“A sample of just five localised FRBs with ASKAP were able to provide a direct detection of the missing baryons in the Universe,” said Shivani. “Just imagine what will happen with thousands of them — we will not only be able to locate the missing baryons but also map the whole cosmic web!”

Learning More About Circumstellar Galactic Mediums

FRBs also provide the opportunity to study the properties of the materials and gases that surround galaxies, known as the circumstellar galactic mediums (CGM). The CGM plays an important role in the capabilities of galaxies to form new stars – with feedback from supernovae and active galactic nuclei (AGN) pushing a lot of this material into a halo outside the galaxy, effectively starving it of its star formation ability.

What scientists really want to know about the CGM is how much mass it contains, how densely it is distributed around galaxies in these halos, what ionisation state it is in and what elements it contains, as well as some of the dynamics – such as how fast it is inflowing or outflowing into the host galaxy.

Distant FRBs are excellent probes for this as the burst itself, when alignment is perfect with our view from here on Earth, will pass through these outer halo regions of intermediate galaxies. One such example occurred with an ASKAP observation of FRB 181112, in which a burst from a distant galaxy passed through the outskirts of a more localised galaxy, allowing the magnetic field of the foreground galaxy to be calculated.  

“The CGM contains a pretty big reservoir of material that will eventually fall into galaxies, and is the reason that galaxies are in many cases still lively with star formation and other evolutionary processes. Knowing about the CGM allows us to predict the fate of galaxies,” said Chris.

What Can The Host Galaxy Tell Us About FRBs?

Due to the lack of further information that comes from FRB detections that occur at great distances (i.e. there is no counterpart electromagnetic signals yet detected), the best opportunity for astronomers to work out what is causing them, is to pinpoint the location of where they occur in their host galaxies, and class this against the type of galaxy.

Are FRBs coming from actively star forming galaxies, or older, more quenched structures? Are they coming from the inner regions of galaxies – where most supermassive black holes reside, or the outer regions of galaxies, well away from the central engine? These types of questions help focus on what the cause of FRBs could be.

In another study involving ASKAP published in 2020, Shivani and her team were able to localise four non-repeating FRBs to the outskirts of their host galaxies, all of which appeared to be 9 – 10 times the mass of our own Milky Way. Additionally, these galaxies appear to not be showing star formation activities, with the findings suggesting that the progenitor sources might likely be from the general stellar population, rather than young star populations.

“Precise localisation of FRBs to their host galaxy is the first step towards investigating FRB sources,” said Shivani.  

“This enables the in-depth study of the host’s stellar population, type and shape; the position of the FRB with respect to the centre of the galaxy; and the properties of the immediate surrounding of the FRB progenitor.”

Using FRBs As A Cosmological Tool

Probing galaxies and the materials between them are not the only usage for FRBs, with grander, cosmological uses also put forward by scientists – but these are yet to be achieved as they require coupling with other messenger signals or the detection of thousands of FRBs to occur in conjunction.

This includes observing FRBs with Gamma-Ray Burst (GRB) or gravitational-wave counterpart signals, allowing some of the parameters of dark energy equations of state or baryon acoustic oscillation (BAO) to be further constrained. To date, this analysis has not been achieved, however, some FRB analysis/research have helped provide a better understanding of cosmological parameters like how galactic halos and dark matter can be accounted for.

The study of FRBs is now starting to emerge as a primary field of astrophysics, given the nature of all the different science that can be utilised from these short, loud, bursts of radio waves – science that can tell us about what is causing them, as well as help us understand the environments between us and them.

Whilst the field is emerging, it is still very much in its younger days – with new FRB event detections occurring at least once per month (at the time of writing, several ATELs were issued about new FRB localisations using a number of different telescopes).

Once a larger number of FRBs are detected, anywhere between the range of 102 – 104, then some clever cosmological applications can be applied, including detection of He II reionization, using FRBs as cosmic rulers, or constraining cosmological measurements like the Hubble Constant.

A number of existing telescopes, like UTMOST are currently underway in receiving upgrades that will help improve sensitivity and thus increase FRB detection rates in the not too distant future.

“We could not localise FRBs to their host galaxies at all prior to the upgrade [of UTMOST] - the region from which they potentially could come contained thousands of galaxies on the sky,” said Chris. 

“With the new system, we’ll be able to zero in on the exact galaxy, and connect FRB science to all the things we know about galaxies from multi-wavelength research over many decades.”

New telescopes, such as DSA-2000, the Hydrogen Intensity and Real-time Analysis experiment (HIRAX), and the Canadian Hydrogen Observatory and Radio Transient Detector (CHORD) are also set to increase detection rates to dozens of samples per day, for a range of different scientific purposes. This also leads to the eventual commencement of the discovery of FRBs by the Square Kilometre Array – which is expected to detect 104 – 106 FRBs during its operations.

Then, whatever Fast Radio Bursts turn out to be – we will truly have a remarkable tool at our disposal to test and understand the Universe with. A means by which we can answer some of the mysteries of the cosmos, turning them into science and fact. 

Literally, a powerful flashlight that helps us illuminate the dark.