Host galaxies of Four Fast Radio Bursts revealed

The Spring of 2018 in the southeast corner of Australia started just like any other year - the warmth returned just as the winter broke and the days started to get longer. In Sydney, astrophysicist Dr. Shivani Bhandari logged in and monitored an array of 36-antennas, located some 3,000 km away, below the horizon to the west, in the radio-quiet desert of central Western Australia. 

Normally when scanning the sky for transient events, each of the 36 antennas of the array face different directions - surveying a large portion of the sky for anomalies that emerge from the distant past, having travelled across the distant universe. But lately, all 36-dishes have focused their gaze on selected targets - acting as single eye, with resolution unlike anything ever observed before. 

Like a bolt from the blue, a sudden flash of radio waves bursts across the telescope’s field of view from the edge of a distant galaxy. Its origin, located well beyond the outskirts of our own Milky Way - signifies a tremendously energetic event that sends a millisecond pulse of energy across intergalactic space, to arrive on Earth millions, sometimes billions of years later. 

Yet, no optical counterpart signal is detected. No high-energy signature is presented. Nothing can be seen to explain what caused it. 

By the time the next winter arrives in 2019, Shivani and her team would find a further two transient events - pinpointing these mysterious Fast Radio Bursts (FRBs) to their host galaxy.

To date, astrophysicists still don’t know what causes FRBs. A number of theories have been floated, and include a range that takes the human mind from the plausible to the impossible. These include:

The Parkes Radio Telescope, managed by Australia’s leading science agency - CSIRO,  would be the first instrument in the world to detect FRBs in 2001. However, that detection would remain in the archival data until Duncan Lorimer and his student David Narkevic would spend time in 2007, combing through the data and realising it was there. This find, known as the Lorimer Burst, would change the course of research for hundreds of radio astronomers - like Dr. Bhandari - over the next decade.

The latest findings, presented in Bhandari et al., highlight the global properties of the first sample of host galaxies of [apparently] non-repeating FRBs, in addition to ruling out the possibility that these bursts are caused by supermassive black holes. 

The events were localised by the Australian Square Kilometer Array Pathfinder (ASKAP) instrument, with the research aiming to provide insight about FRB host galaxies and their progenitor systems.  Astrophysicists involved with the paper looked at addressing questions such as reviewing if FRBs come from galaxies which are actively making stars - or the more quiet ones; and what region of galaxies do FRBs originate from and what does this tell us about the progenitor event?

“Our study has found that the current population of ASKAP localised FRBs live in massive galaxies which are forming fewer stars as compared to the galaxy of the repeating FRB 121102,” said Dr. Bhandari.

“We also found that FRBs typically reside in the outskirts of their hosts, ruling out progenitor models such as those related to the active supermassive black holes at the center of a galaxy,”

“The global properties of the hosts [galaxies] also suggest that a broader population of FRBs can arise from both young and old sources.” continued Dr. Bhandari.

Co-author CSIRO’s Professor Elaine Sadler said these fast radio bursts could not have come from a super-luminous stellar explosion, or from cosmic strings.

“Models such as mergers of compact objects like white dwarfs or neutron stars, or flares from magnetars created by such mergers, are still looking good,” Professor Sadler said. 

The research has also been praised by Dame Jocelyn Bell Burnell, who as a postgraduate student in 1967 was the first to detect rapidly spinning neutron stars now known as ‘pulsars’, praised the research.

“Positioning the sources of fast radio bursts is a huge technical achievement, and moves the field on enormously,” Dame Jocelyn said.

“We may not yet be clear exactly what is going on, but now, at last, options are being ruled out. This is a highly significant paper, thoroughly researched, and well written.”

To complete her research, Dr. Bhandari (who works for the CSIRO) and her team used the ASKAP telescope - an interferometer of 36 telescopes located in the red Earth desert of the designated radio quiet zone, in central Western Australia. 

CSIRO operates ASKAP as its most recently developed radio instrument, made up of 36 x 12m dish antennas spread across the flatlands of the Murchison Radio-astronomy Observatory (MRO). All antennas act as a single telescope, covering a large portion of the sky at any given time - which allows quick survey timeframes to be combined with a high level of sensitivity. This is ideal for looking out for FRBs - as scientists don’t know when they might flare up (there are repeating FRBs as well, which might have slightly more predictability).

ASKAP itself is a pathfinder project for the more ambitious Square Kilometre Array (SKA) telescope - one of science’s mega-projects, being constructed from later this year through to the end of the decade. Once completed, the SKA will be the largest scientific instrument in the world, spread across two continents (South Africa and Australia), acting as the world’s biggest ‘radio eye’ with the ability to peer to some of the earliest moments in the Universe’s history. 

The ASKAP project is currently developing, testing and learning answers to the many complex questions that are required to be resolved, as part of the development of the SKA - such as how even more data can be processed, how to utilise machine learning to automate target identification, and how to generate renewable energy to power the facility without introducing radio interference. 

Dr. Bhandari and her team have announced that these recent results obtained by using ASKAP,  feature a team of international collaboration and effort, including a number of Australian institutions, such as:

• CSIRO

• University of Sydney

• Macquarie University

• International Centre for Radio Astronomy Research (ICRAR)

• Swinburne University of Technology

In addition to utilising the ASKAP telescope for these findings, the research team also used the Australian Telescope Compact Array (ATCA - also operated by the CSIRO), as well as the Jansky Very Large Array - located in New Mexico, USA. 

A transient astronomical event is an object, event or phenomena which can last from milliseconds to several years (like a comet passing or a supernova explosion) - but it is not a (relatively) permanent fixture (like the stars and planets). 

Transients come and go from our views and within our lifetimes, and can be further broken down into those that can be predicted (like the upcoming solar eclipse of 2028) or unpredicted (like if Betelgeuse were to explode 10 years from now). 

Fast Radio Bursts are classed as transient events - their unpredictability, unknown progenitor origin and random spread across the sky means that they’re (mostly) unable to be forecast - and our telescopes have to be pointing in the direction of sources when an FRB flashes by. 

That’s exactly what the Commensal Real-time ASKAP Fast Transient (CRAFT) was designed for - to utilise ASKAP to search for fast transients (less than five seconds). These shorter timescales are usually associated with much more dramatic and energetic events across the Universe, usually generated under extreme circumstances and providing the opportunity to probe the cosmos with capabilities that could never be achieved here on Earth. 

Prior to late 2018, the CRAFT survey pointed the ASKAP dishes in different directions, each scanning a different portion of the sky with its beams - looking for any transient anomalies. But since then, the survey has been operating the telescope in Incoherent-Sum (ICS) mode, pointing all dishes at the same part of the sky then combining the signals through data processing. 

“The power from each of the 36 ASKAP dishes is summed together. Real-time searches for bright single pulses are then performed on this incoherent summed data (sum of powers). After a candidate is detected in the realtime search, a trigger is issued and the amplitude/phase information of the incident electric field (voltages) is downloaded from a 3-second long buffer for offline interferometric analysis, which leads to FRB localization.,” said Dr. Bhandari.

The CRAFT survey doesn’t only work with ASKAP though - once an FRB trigger is set off,  notifications of the celestial coordinates are sent to global members of the CRAFT project, who then activate optical wavelength telescopes which allows a team of global astronomers to quickly turn their focus towards the event in the hope of capturing more data about these elusive transients. 

So what are FRBs? From the information we have - we know that they originate from outside our galaxy, making them of extragalactic origin (though there is one Milky Way origin case currently being investigated and only observed at the end of April 2020). There are several pieces of evidence for this. The first is a paper published in astronomical Journal MNRAS in 2017, which states that the minimum distances that FRBs can be from the Earth, would be 10,000 km - meaning they certainly have an astronomical origin and are not considered interference at the observatory location.

Additionally, most of the visible material in the Milky Way resides in the Galactic plane - so a lot of what we observe from our own Galaxy occurs in this region. But FRBs come from all over the sky, so scientists (in the same paper above) have made the determination that they must be from extragalactic sources.

A more firm piece of evidence is to analyse what is known as the Dispersion Measure (DM). This method of measuring distances is often applied to pulsars (from within our galaxy) and is indicative of the number of electrons between us and the pulsar. It can be thought of as a 1 cm cylindrical tube stretching between us and the emitting object - and by counting the number of free electrons per unit in this tube, we could find the DM. 

All measured FRBs have shown a DM value that is in excess along the line of sight after accounting for the known DM values that the Milky Way contributes (using known models such as NE2001) - and as such scientists have confirmed they are from outside the Milky Way.

Unlike pulsars, only a handful of FRBs are known to repeat - that is, most FRBs present as a single giant, one-off pulse. There are many theories as to why this might be - it could be a single transient event that never re-occurs. Other discussions have stated that it could be because potentially other pulses are not so bright and we simply don’t observe them. 

New surveys using telescopes like MeerKAT,  CHIME, and UTMOST will continue to find new sources of FRBs and are designed to find repeaters, but the majority of known FRBs are single burst events. 

FRBs present themselves as a single peak across radio frequencies with a width only a few milliseconds long (and sometimes less than a millisecond). Given that we know that they have an extragalactic origin - the process of creating FRBs must be very energetic (especially, since we can detect them from across the Universe!). After traversing through the Universe, the signal reaches Earth with energy much weaker than the wifi signals operated in most homes. 

By calculating the width of the signal, astrophysicists are able to work out how big the object must be that is emitting them - and have narrowed it down to a range of a few tens to 3,000 km. Unimaginable that an object with a maximum diameter of 3,000 km can produce such an enormous blast of radio waves - all the way across the Universe, and not show up in any other form of electromagnetic radiation (example, there is no visible light counterpart signal). 

What could cause such a colossal output of energy - and not blow itself up in a spectacular display of light? And given that there are now several repeater FRBs, how could it survive such a powerful event (producing the blast over and over)? 

It would be impossible to directly observe a 3,000 km object from all the way across the Universe, so scientists try to learn about what causes FRBs by studying their environments - and in particular, their host galaxies. 

Are FRBs associated with star formation - and if so, should we then find them in active star-forming galaxies? Or are they associated with the end of the life-cycle of stars, and thus, we should find a higher population in more mature galaxies?

“In a star-forming region, new stars are actively born. If FRBs are only coming from active star-forming galaxies, that may hint to a young stellar progenitor. A young stellar progenitor may have a polluted and dense environment which will give rise to high dispersion measure (DM) and strong magnetic fields in the immediate vicinity of an FRB source in their host galaxy. This is predicted for the progenitor of the repeating FRB 121102,” said Dr. Bhandari. 

“However, we find that FRBs can also come from galaxies with a general stellar population (all ages), with clean environments and hence very less host galaxy contribution to the total DM of an FRB.”

An alternate theory put forward has been that FRBs can be the result of progenitor events associated with the compact objects located at the centre of galaxies - their supermassive black holes, esp. if the galaxy has an active galactic nucleus (AGN) - when the supermassive black hole is consuming matter and unleashing a high volume of radiation.

Discussions amongst radio astronomers have also led to the possibility that there could be a variety of FRB classes - a subset of similar events with different outputs. This theory could explain why there are repeater events vs. non-repeating events. 

According to the FRB Catalogue, approximately 110 FRBs have been detected, using a number of different telescopes around the world - including ASKAP, Parkes, Molonglo, CHIME, GBT, Pushchino, Apertif, SRT, and DSA-10. 

By studying these events scientists have been able to determine that a handful are repeaters whilst most are one-off bursters. In one case of a repeater, FRB 121102, the host galaxy was studied to showcase it resides in a dwarf galaxy - whilst the second repeater, FRB 180916 does not. However, a review of some of the one-off FRB events indicates that there is a wide range of different progenitor galaxy types in which these events originate - for example, some are found in spiral-type galaxies which can be smaller than or equal to our own Milky Way. 

Determining where FRBs originate from, with reference to their own host galaxy, could indicate if these bursts are the by-product of AGN events, or something else. If scientists were to notice that the FRBs are coming from the central nucleus region of their host galaxies - then this could mean that the supermassive black hole / AGN theory could hold merit. However, if they came from the outer suburban regions of their host galaxies, this could mean something else all together. 

The paper presented by Bhandari et al. has now localised four FRBs to their host galaxies to arcsecond accuracy, using ASKAP and through the detection of a single pulse. All of these host galaxies have approximately similar or less mass than our Milky Way Galaxy and are at distances of approx. 1.5 - 5 billion light-years from Earth. 

As listed in Table 2 below, the star formation rates of the host galaxies are fairly different from the dwarf galaxy that hosts the repeating FRB 121102, with these galaxies producing a modest 2 solar masses of stars per year (a maximum value on the current sample of analysed galaxies). 

The localisation of the FRBs on their host galaxies has indicated that these are offset from their galactic centre by a range roughly 6,000 - 22,000 light-years. For comparison, our Solar System is offset from the Milky Way core (known as Sgr A*) by a value of 26,000 light-years. 

Once the FRBs were localised to a region, the research team turned to international collaborators with different instruments - optical telescopes - to image the host galaxies. First, images were captured using the FOcal Reducer/low dispersion Spectograph2 (FORS2) and X-shooter - mounted upon the Very Large Telescope, located at the European Southern Observatory in Chile. 

In addition to FORS2/X-shooter, the Sinistro instrument mounted on the 1-metre telescope at the Las Cumbres Observatory (LCOGT). 

Optical spectroscopy of the host galaxies was determined using the Keck Cosmic Web Imager (KCWI) instrument - located high atop Manua Kea in Hawaii, and the Gemini-Multi Object Spectrograph (GMOS) mounted on the Gemini-South telescope, located in Chile. Lastly, the Magellan Echellette Spectrograph was also used, located upon the Magellan Baade Telescope - also in Chile. 

FAST RADIO BURST DETAILS

“The localization of ASKAP FRBs is accurate to about less than an arcsecond, which not only leads to reliable identification of their galaxies but also their location within them. This is necessary to study the immediate FRB environment in their hosts,” said Dr. Bhandari.

“In the future, we need a much larger sample of host galaxies of both repeating and non-repeating FRBs to understand their progenitors.”

Given the most recent findings indicate that these FRBs (and others detected thus far) do not originate in the central region of host galaxies, the possibility that the progenitor event could be associated with supermassive black holes or AGNs in host galaxies is reduced.  

Supermassive black holes / AGNs have only been observed in the central region of galaxies, so the new evidence indicates that these processes could not be linked in from the populations of FRBs thus observed. 

This doesn’t rule out the possibility altogether, because future FRBs might be detected from these regions - but from all FRB cases so far observed - the likelihood is very little that the two phenomena are associated.

So what does Dr. Bhandari believe to be the progenitor cause of FRBs?

“Millisecond duration of FRBs confines their emission region to be within the tens to the thousands of kilometres range. Given the fact that some of the FRBs repeat, a population of them can come from progenitor models involving neutron stars or their cousins - magnetars,” she said. 

After a massive star goes supernova, the compact remnant object - usually no bigger than a small city (roughly 20km in diameter) is a neutron star (or pulsar if it is rotating with energetic beams emanating from its poles). These objects are incredibly dense, made up of 1.4 times the mass of the Sun, confined to a very small volume. A single teaspoon of a neutron star would weigh as much as a cube of Earth with sides of 800m. 

Neutron stars (and pulsars) have huge magnetic fields - many billions of times that of the Earth - to compare, the value of Earth’s magnetic field is between 0.25 - 1 Gauss, and the value of an MRI scanner is about 10,000 gauss. Neutron stars have values in the billions and trillions of times this. 

And yet, they are not the most magnetic object in the Universe. That title belongs to a handful of items, called magnetars. These neutron stars (or pulsars) have a magnetic field that is another 1,000 fold or so stronger. It’s said that if a magnetar was to be placed between the Earth and the Moon, every single credit card on the planet would be wiped clean. 

The scenarios that Dr. Bhandari thinks might be triggering the rise of FRBs are the result of two neutron stars merging together - an event that we’ve now observed in both electromagnetic and gravitational wavelengths. Could this event be also causing an enormous blast of short, millisecond-length radio waves? 

Or could it be the birth of a magnetar, during the violent core-collapse supernova of a massive star, that is orientated in just the right direction for us to observe it here on Earth, billions of years after the event? 

The only way to know more is to be able to find more FRBs and study their host galaxies, narrowing down the list of progenitor events and objects. 

Ongoing research continues into the mysterious nature of FRBs. Telescopes like ASKAP and its South African counterpart, MeerKAT, are now fine-tuning their capabilities and data processing techniques to recognise these events as they occur and notify global collaborative partners to swing their telescopes in the direction of the burst in real-time, hoping to unravel any sign of what could be the counter electromagnetic signal. Catching the progenitor in the act, so to speak. 

Even older telescopes, such as Australia’s UTMOST are also being upgraded to be able to detect FRBs with higher accuracy in localising, sifting through the volumes of data in faster turnaround times. 

Astrophysicists are currently training machine learning algorithms to search for FRB signals, hidden in the data generated out of telescopes in real-time, and once it has acquired confidence and capability - this application can be tuned to the petabytes of archival data collected by global telescopes over the last quarter-century. The likely outcome is that FRBs have been observed in the past, but never followed up - potentially considered random noise or a signal of unknown origin or relevance.

For Dr. Bhandari and her team, the work continues using the ASKAP - looking deep into patches of the sky above the radio-quiet, red desert skies of central Western Australia - hoping to catch a sudden flash of an FRB in real-time, so that they can localise them offline and then identify their host galaxies. 

The future results of these searches will reveal more about the conditions and environments of FRB host galaxies - hopefully answering some of our most fundamental questions about these mysterious phenomena. 

What could be the cause of such magnificent blasts of energy that can transcend billions of years of space and time? Why is the emission diameter so small? Why do some galaxies have these events, whilst others do not? (or maybe they all do and we haven’t observed them as yet) And are there two populations of FRB classes - those that are single bursters, and those that repeat?

Questions like these lead to fundamental shifts in our understanding of nature and the universe - rewriting textbooks as our common understanding of science takes a jump, fueled by the unknown of unknowns. 

We know something powerful is causing fast radio bursts in galaxies outside the Milky Way.

As with all human nature - we now need to know why.