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Inside the Lair of a Mysterious Cosmic Radio Burster

A flash from the Fast Radio Burst source FRB 121102 travelling towards the 100-metre Green Bank telescope in the USA.  The burst shows a complicated structure, with multiple peaks that may be created during the burst’s emission or imparted during its 3-billion-light-year journey to us. This burst was detected using a new recording system developed by the Breakthrough Listen project. Credit: Danielle Futselaar/Shutterstock

A flash from the Fast Radio Burst source FRB 121102 travelling towards the 100-metre Green Bank telescope in the USA.  The burst shows a complicated structure, with multiple peaks that may be created during the burst’s emission or imparted during its 3-billion-light-year journey to us. This burst was detected using a new recording system developed by the Breakthrough Listen project. Credit: Danielle Futselaar/Shutterstock

By Charlotte Sobey

Two of the world’s largest radio telescopes have unveiled the astonishingly extreme and unusual environment of a mysterious source of repeating radio bursts emanating from 3 billion light-years away.

Once every 10 seconds, mysterious astrophysical objects blast swift, sudden radio flashes from across deep space. We call these signals “fast radio bursts’ (FRBs). New observations from two of the world’s largest radio telescopes have led us to a remarkable discovery about a unique FRB factory. We found that the source is embedded in a hot, dense environment with an exceptionally strong magnetic field. A compelling explanation is that it is situated in the neighbourhood of a massive black hole (https://goo.gl/NC1BKw).

The detection of this unique FRB, formally known as FRB 121102 or informally as the “Spitler burst” after its discoverer (https://goo.gl/KcD7MS), was a breakthrough for FRB science. It is the only FRB signal that keeps repeating. All other FRBs found so far have been single, millisecond, flash-in-the-pan events, even after hundreds of hours of follow-up observations. FRB 121102 provided us with an exclusive chance to carefully monitor and scrutinise the signals.

Last year we were able to pinpoint the source of FRB 121102 to a dwarf galaxy, much smaller than our own Milky Way, more than 3 billion light-years away. At this distance, an immense amount of energy must power each burst – roughly as much energy in a single millisecond as the Sun releases in an entire day. In fact, if one went off on the other side of our galaxy, it would disrupt radios on Earth!

The burst was initially discovered using the 305-metre-diameter Arecibo Observatory in Puerto Rico. Its localisation was possible by tuning more radio telescopes towards the source, including the Very Large Array in the USA and the European VLBI Network, linking telescopes across the globe. These provided a precise zoom-in on the burst’s position – to an angular size equivalent to a road train parked on the Moon. By accessing visible light data from the Gemini and Hubble Space telescopes we learned that the FRB source resides in an area of massive star formation and death – a stellar nursery and graveyard within the dwarf galaxy.

Since its discovery, FRB 121102 has been monitored using several radio telescopes, including the Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia. By employing specialist equipment and observing modes, we could record the bursts in the rawest possible form. The microsecond structures within the FRBs require high resolution to capture them, and this generates a vast volume of data – the equivalent of 18,000 DVDs per hour!

The region creating the FRBs must be tiny by astrophysical standards – less than just 10 km across. Although none of the bursts look alike – they vary in brightness, shape and structure – the shortest time span of the signals is about 30 microseconds. The size of the emission region must therefore be smaller than the distance that light travels over that time.

Amazingly, we learned more about the object creating these astonishingly powerful signals by detecting the bursts at the highest radio frequencies to date. This was crucial for studying FRB 121102, which is 100% polarised: the light from FRB 121102 vibrates in only one direction. In contrast, the Sun’s light is unpolarised – the electromagnetic waves vibrate in all directions perpendicular to their course. Wearing polarised glasses reduces the amount of light that reaches your eyes because they only transmit waves that vibrate in one direction.

The behaviour of FRB 121102’s polarised light presented us with an insight into the object’s environment within its host dwarf galaxy. We discovered that the object is embedded in an exceptionally magnetised and dense plasma of ionised gas. As an electromagnetic wave passes through a plasma with a magnetic field, the vibration angle of the wave twists. This is known as Faraday rotation (after Michael Faraday, who discovered it). Detecting the bursts at higher radio frequencies than ever before enabled us to measure a tremendous amount of Faraday rotation imparted by the extreme environment – among the largest ever measured in a radio source and 500 times that for other FRBs.

Considerable Faraday twisting and “dispersion” allowed us to estimate that the magnetic field strength in FRB 121102’s environment is a few milliGauss – about 1000 times larger than the magnetic field strength in our solar system’s neighbourhood. An effect that all FRBs share is that their signals are dispersed: intervening plasma (material in the host galaxy, intergalactic medium, our galaxy and solar system) causes the lower radio frequencies (longer wavelengths) of light to become delayed and arrive at the telescope later. The delay due to this dispersion was the first clue that FRBs originated from outside our galaxy because the delays are too large to be explained by material in the Milky Way alone.

We expect that the source of FRB 121102 is located within an extremely magnetised environment because the amount of Faraday twisting varies by about 10% over the timescale of a few months. Another surprising finding was that the Faraday twisting imparted on the FRB is very clean – there appears to be little interference to the signal throughout its 3-billion-light-year journey to us.

The leading hypotheses about FRB 121102’s extreme environment are that it is near a massive black hole or cocooned in a nebula of unprecedented power. The dynamic region close to the massive black hole in the centre of our galaxy shows nearly as much Faraday twisting, so the source of the FRB may be embedded in a similar environment.

The other conjecture that cannot be ruled out yet is a nebula that would have to be a million times more energetic than the Crab Nebula (a bright supernova remnant a few thousand light years from Earth that can be seen with binoculars). Ongoing monitoring of the repeating FRB over the next year will help us distinguish between the possible scenarios.

The most perplexing FRB puzzle is what physical object and mechanism create the immensely powerful signals? Currently, fewer FRBs have been discovered than there are theories about their possible progenitors. Suggestions include astrophysical origins such as unusually bright pulses from pulsars, merging binary neutron stars, or annihilating mini-black holes.

More exotic, but unlikely, scenarios include alien life communication or structures. FRBs originate from widely separated areas in the sky, and it is doubtful that many separate civilisations living across the universe would communicate via the same means. FRBs also require an immense amount of energy that would more likely be generated by an extreme astrophysical object.

The repeating FRB 121102 has also shed some light on the possible origin of the signals. The process cannot involve a cataclysmic event, where the object is destroyed in the process. A neutron star origin is one leading theory because the signals are somewhat similar – short, polarised bursts generated in an area a few kilometres in size.

However, there are still complications. For example, unlike pulsars, no periodicity has been found in FRB 121102. Eighteen bursts were detected in the first 30 minutes of one observation, but none were seen in 5 hours afterwards. The signal would also have to be about a million times brighter than the brightest pulsars in our galaxy or amplified through space through a cosmic “magnifying glass”.

However, FRB 121102 is quite atypical for an FRB and may belong to a unique class to the others, much like the different types of gamma-ray bursts released from supernovae or merging binary neutron stars.

As international collaborations continue the great FRB hunt using radio telescopes around Australia and worldwide, the future is bright for solving these and other fascinating mysteries about our universe.


Charlotte Sobey is a Research Associate at the Curtin University node of the International Centre for Radio Astronomy Research and the CSIRO.