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The Transient Radio Sky

 CSIRO’s 64-metre Parkes radio telescope showing an extragalactic radio burst

Artist’s composite of the CSIRO’s 64-metre Parkes radio telescope showing an extragalactic radio burst appearing briefly, far from the Milky Way’s disk. Credit: Swinburne Astronomy Productions. Background image: astrometry.fas.harvard.edu/skymaps/halpha

By Tara Murphy

With the ability to scan the entire sky each night, the Square Kilometre Array will enable astronomers to catch transient events like gamma-ray bursts and fast radio bursts, as well as phenomena that are so short-lived they have never been detected.

The changing sky has always fascinated humans: our ancient ancestors recorded the paths of the planets across the sky in great detail; Aboriginal astronomers used the movements of the stars to plan their travel to new food sources; and throughout history the appearance of “guest stars” – now known to be supernovae – was imbued with deep spiritual significance.

With modern telescopes we have been able to detect the remnants of supernova explosions such as SN 1604, which was observed not only by Kepler but also by Chinese and Korean astronomers. We are still able to detect the radiation from the exploded material as it crashes into the surrounding gas and dust.

The history of astronomy gives us fascinating examples of astronomical objects that vary on human timescales.

What Causes Variability?

Variability gives modern astrophysicists valuable insights into some of the most extreme processes in the universe. Transient objects that appear and disappear, and variable objects that show extreme changes in their brightness can be used as laboratories to investigate the physical processes that cause such variable behaviour.

Radio variability can be classified into a number of categories that represent the physics responsible for that variability:

  • Explosions are usually the cause of the most extreme variable events we observe. Explosions occur, for example, when a star forms a black hole at the end of its life.
  • The emission from a distant object appears variable because light on its way to Earth must travel through the gas and dust between stars and galaxies. This causes the brightness of the source to vary, much like our atmosphere causes stars to twinkle.
  • Accretion occurs when matter is absorbed by a dense object. This can happen in binary star systems in which one star is a normal star and the other is a compact object like a neutron star, white dwarf or black hole.
  • Stars and large planets such as Jupiter emit radiation when their magnetic fields trap and accelerate particles.

There may also be other processes that cause radio variability but these are currently unknown.

Designing a Survey for Variability

In radio astronomy, our ability to explore variability and transient behaviour has been limited by our telescopes. To look for variable phenomena, we want to observe as much of the sky as possible, as often as possible, so we need a telescope with a large collecting area that can see a large area of sky. Such a telescope would be able to conduct surveys rapidly and see very faint objects.

Typically we have had either highly sensitive telescopes that can see a small area of sky that must be revisited often, or telescopes that can see a larger area of sky but are not very sensitive. This means it has been difficult to survey the entire sky rapidly. For example, Sydney University’s Molonglo Sky Survey observed most of the southern sky at a radio frequency of 843 MHz, but it took 10 years to complete. To observe everything again and look for things that have changed would take another 10 years.

The Square Kilometre Array

The Square Kilometre Array will revolutionise the study of radio transients and variable phenomena. It is designed to be a survey machine, and will be able to survey the whole sky with amazing sensitivity every night. This will allow us to conduct a complete census of radio sources that change rapidly on human timescales. What do we expect to see?

Most radio sources don’t vary much, but some do, and we’d like to study these more to understand them. An example of these “known knowns” are gamma-ray bursts.

There are also mysterious objects that we have discovered, but we don’t yet understand what they are or know the source of their radio emissions. An example of these “known unknowns” are fast radio bursts, which were discovered with the Parkes Telescope.

Finally there is the possibility of making serendipitous discoveries of types of objects that we don’t even know are out there. Discovery of “unknown unknowns” could result in the next Nobel Prize, just as the discovery of pulsars did in 1974.

The Orphan Afterglows of Gamma-Ray Bursts

In 1963 the United States launched the first in a series of Vela satellites to monitor the Earth for breaches of the nuclear test ban treaty. The satellites were equipped with X-ray and gamma-ray detectors so they could detect and confirm nuclear bomb blasts. The satellites identified a number of events that did not fit the signature of nuclear bombs. Based on where the signals were coming from, scientists were able to rule out the Earth and the Sun as the sources of these events, and so the first detection of “cosmic gamma-ray bursts” was made.

It wasn’t until 30 years later that the BATSE space telescope was able to detect thousands of gamma-ray bursts, and found that they were distributed roughly evenly across the sky. This showed they were likely to be coming from sources outside the Milky Way.

The first X-ray and optical detections of the “afterglow” from gamma-ray bursts showed that they came from distant galaxies and are caused by the explosive death of a massive star at an early time in the universe. Stars throughout the universe explode as supernovae all the time, typically releasing ~1044 J of kinetic energy, but in gamma-ray bursts all this energy is channeled into two narrow jets of material moving at close to the speed of light along the star’s rotation axis. If one of these jets happened to point almost directly towards us we would see an emission that is millions of times brighter than normal.

Since we only see a gamma-ray burst if one of the jets is pointing towards us, we have probably only discovered a tiny fraction of the total population. However, the radio emission from the afterglow of a gamma-ray burst shines uniformly in all directions, not just in a discrete beam, so it might be possible to detect gamma-ray bursts directly with a radio telescope rather than by following-up a gamma-ray emission.

No one has yet detected one of these so-called “orphan afterglows” but simulations suggest that the Square Kilometre Array should be able to detect hundreds of them each year. We may even be able to detect gamma-ray bursts from “population III” stars – the earliest stars to form in the universe – and hence use gamma-ray bursts to see right back to the beginnings of cosmic time.

Hunting for Fast Radio Bursts

The Parkes Telescope is an Australian icon, and one of the things for which it is most famous is the large number of pulsars it has discovered. In 2007 astronomers looking through archival Parkes data found a highly unusual event in their data. This object, the first example of what are now called “fast radio bursts”, appeared as an incredibly bright burst of emissions lasting less than 5 milliseconds. The source of the burst is a complete mystery, and for many years it was uncertain whether it was of astronomical origin or caused by some terrestrial signal such as lightning or artificial radio-frequency interference.

Since then, nine more of these bursts have been found with the Parkes and Arecibo telescopes. Their origin is still unknown: they could be giant flares from energetic magnetised stars called “magnetars”, or they could be created when neutron stars collapse to form black holes, making them a relative of gamma-ray bursts. What we do know is that they are extremely bright and appear to come from very distant parts of the universe.

Extreme objects like gamma-ray bursts and fast radio bursts are interesting in their own right. However, they are also interesting as probes of the distant universe. Radio emissions from these objects have to travel through the gas between galaxies and stars to reach Earth. This gas is at a very low density, with fewer particles per cubic centimetre than the best vacuums we can create on Earth. However, because the distances travelled by the radiation from these events are so great, we can still detect the effects of this gas on the radiation we receive from these distant objects.

Being able to detect large numbers of fast radio bursts with the Square Kilometre Array means we could figure out what they are and then use them to map out some of the missing matter in the universe.

A Revolution in Radio Astronomy

The Square Kilometre Array will revolutionise radio astronomy. It will allow us to study transient and variable objects in a way that has never before been possible, and hence resolve many long-standing mysteries about extreme objects like gamma-ray bursts. The ability to carry out large-scale surveys opens up the exciting possibility of serendipitous discovery: we may detect things that are so short-lived that they have always been missed.

Tara Murphy is a Senior Lecturer at the University of Sydney. Artist’s composite of the CSIRO’s 64-metre Parkes radio telescope showing an extragalactic radio burst appearing briefly, far from the Milky Way’s disk. Credit: Swinburne Astronomy Productions. Background image: astrometry.fas.harvard.edu/skymaps/halpha