ASKAP localises two new Fast Radio Bursts to their host galaxies

The process of science is rather enlightening. With new discoveries come new questions, leading to further observations, which then provide new insights that raise more questions, and so on the process ebbs and flows as time rolls forward.

One of the more recent mysteries in astrophysics is that of the enigmatic population of phenomena known as fast radio bursts (FRBs) – powerfully bright flashes of radio light, that last only milliseconds and reach us from across the Universe.

To date, we still don’t know what could be causing them – but we do have a number of theories of what it could be and are slowly ruling out what it can’t be. A key to understanding what the source of these bright bursts of energy could be resides in understanding the types of host galaxies that they come from – by understanding their environments, we could then start to draw conclusions about what kind of object could be causing them.

For example, do they come from galaxies that are active and rich in star formation, which could point to the progenitor source being from newborn stellar objects? Or are they from the older, redder galaxies populated with older stars, indicating that they are being generated as part of the ageing star population?

Australian astronomers have now led an international team in using both the ASKAP radio telescope (owned and operated by Australia’s national science agency, CSIRO) and the Karl G. Jansky VLA instrument to add a further three FRB localisations to the small list of growing cases, that gives us just that little bit more information about what types of galaxies these events are coming from.

“FRBs do not prefer to come from a specific galaxy type. The host galaxies are of various shapes and only moderately star-forming,” said lead author, Dr Shivani Bhandari. “We also discovered no statistically significant differences in the properties of galaxies hosting repeating and non-repeating FRBs.”

As well as the discoveries of the three new localised sources in the papers, Shivani and her team added these to the existing population of FRBs which have been localised to their host galaxies and conducted an analysis on the global properties of this population revealing some interesting results.

The story of FRBs began as a serendipitous discovery in the archival data of the CSIRO Parkes radio telescope. In 2007, astronomers were combing through data that was a few years old (2001 to be exact) when they noticed a brief, 5-millisecond spike 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).

Once the first one was noticed for the first time, scientists knew what they could look for - and soon many observations started following. Whilst the first published records of FRBs occurred 14 years ago, this is still considered a relatively young field in astrophysics. 

One of the first things that data on FRBs started reflecting was that they were originating from all locations across the sky - which was a really important learning because it meant that they were not confined only to our galaxy (otherwise, they’d appear mostly across the Galactic plane).

And by using a measure of the dispersion of the electron density column along the line of sight, as well as timing the short duration of the pulse, astronomers were able to establish that the object that is causing them must be no bigger than 3,000-kilometres across whilst originating at great distances from us.

“According to a recent estimate, few thousands of FRBs occur per day across the sky, therefore the chances of discovering FRBs in other galaxies are higher than finding one in our Milky Way. Also, such events will be strongly affected by the turbulent gas in the Milky Way making their discovery even harder. The discovery of an FRB-like burst in the direction of a magnetar in our galaxy was both unusual and fascinating,” said Dr Bhandari.

The size of the emission region is constrained by the speed of light in a given time. Thus, for a pulse width of 0.1ms-10ms, the size of the emission region must be less than the light travel time, defining the minimum size to be 30 - 3,000-kilometres,” she added.

What could be smaller than the Earth but give off such an incredibly powerful burst of energy, that it could literally cross intergalactic space to reach us? 

Over time, and through a larger population of FRBs catalogued, we soon realised that they can fall into two categories - those which repeat (which makes it interesting if the progenitors are cataclysmic, after all, how many times can something blow up?), and those which do not repeat. Currently, there is discussion that maybe all FRBs are repeaters but we only get to see the single biggest pulse they emit, whilst others are lost to the background noise. 

What scientists think they might be related to are compact massive object merger events (like when neutron stars collide), the accretion-induced collapse of white dwarfs and core-collapse supernovae. 

And by using these populations, they’ve ruled out long gamma-ray bursts and super-luminous supernovae as the progenitor production models. 

But to date, the mystery of what could be causing them still remains an excellent unsolved topic of astrophysics. 

In the meantime, however, astronomers have found a clever way to utilise FRB signals that traverse intergalactic space to probe any structures and mediums that lie between us and those galaxies. This has included solving the missing baryonic matter problem, learning more about circumstellar galactic mediums, and using FRBs as cosmological tools. 

Localisation of FRBs is no easy feat in itself and only made possible by using an array of radio telescopes (known as an interferometer) to really pinpoint the location of where they originate in their host galaxies.

Of the small handful that have been localised, we know that they come from galaxies at redshifts which are about z = 0.03 - 0.66, which equates to a look-back distance of approximately 413 million to 9 billion light-years from us. This property is important, as it gives us information about FRBs, galaxies and the stuff that lies between us over the last nine billion years of the Universe’s history. 

Scientists have also been able to quantify the broad range of colours the galaxies exhibit (an indicator of the types of stars it contains), the total stellar mass of these galaxies (an indicator of which sized galaxies produce FRBs) and even the star-formation rate of galaxies. 

In most cases, FRBs were found to be offset from the centres of their host galaxies (which indicates that they are likely not related to the central supermassive black hole), and that they do not come from regions where lots of star-formation is occurring. Interestingly, high-resolution studies of a sample of FRBs with the Hubble Space Telescope shows a portion of FRBs seem to come from spiral galaxies, with the localisation of the events found to originate in the spiral arm regions. But in other cases, the galaxy resembles a blob and scientists require further info to study the galaxy types before making a confident conclusion on the host galaxy properties. 

“So far, FRBs have been observed in dwarf, spiral, and lenticular galaxies, indicating that FRB host galaxies are diverse and that bursts do not prefer a specific galaxy type,’ said Dr Bhandari. 

“However, the data also point to a lack of massive and old early-type elliptical galaxies, suggesting that the sources of FRBs are not solely dominated by old stars found in such galaxies.”

Now, in this recent paper, led by Shivani and her team - three new cases are being added to the population of localised FRBs, increasing the number to 16 cases in which we know where the FRB originates from, within its own host galaxy.  

The first (FRB20180301A) was discovered as part of the Breakthrough Listen project using the Parkes radio telescope and then followed up with CHINA’s FAST to confirm it was a repeater source. Shivani and her team performed a follow-up observation using the VLA instrument (an array made up of 27 active radio dishes) and found it to be about 10.8 kiloparsecs (kpc) from its host galaxy centre. 

The team also discovered two new FRBs (FRB20191228A and FRB20200906A) using ASKAP (another array, located in Western Australia and featuring 36 radio antennas) to localise these events to be about 5.7 kpc and 5.9 kpc offset from their respective host galaxy centres.  

“These FRBs are coming from the outskirts of their host galaxies, which are about 3-4 billion light-years away, and are shining a light on the ionised plasma in the intervening medium, which cannot be studied in any other way.”

“The 36 x 12-m dishes of ASKAP join hands together and function as an ‘interferometer’.  When an FRB signal is detected, the rawest form of the data containing the burst is recorded from each telescope and replayed to create an image of the FRB, which leads to its localisation,” added Dr Bhandari.

A recently large population of FRBs (535) detected with the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) project shone a little extra light on both sub-populations - indicating that the repeater sources had wider pulse widths and narrower bandwidths relative to those that did not repeat. 

This new data might be the first sign that there might even be two kinds of progenitor events that trigger FRBs. 

“A much larger sample of FRBs and their host galaxies, long-term monitoring of repeating FRBs, higher time resolution and wider bandwidth studies of FRBs, zooming in on the local environment of nearby FRBs, and multi-wavelength and multi-messenger connections are some of the most promising avenues for unravelling the mystery of FRB sources.”

And so the process repeats. A new discovery, which raises more questions, which leads to further observations, which creates further insights …. And on and on we go with the science.

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site on which ASKAP is located.