RAPID RADIO SPACE BURSTS
Extreme radio-wave bursts from remote galaxies involve the same energy as 80 years of solar radiation – and astronomers have no idea why. New telescopes are to search the sky to find the origin of the bursts.
First Parkes and now Western Australia’s ASKAP array have led the hunt for these mysterious rapid radio bursts from space. What makes them, and what can we learn?
If human eyes could see what a radio telescope sees, the sky would be different. The disc of the Milky Way would be brighter, and we would see smoke rings around exploding stars and gas flows from black holes. And at regular intervals, everything would be outshone by powerful fast radio bursts – for one millionth of a second.
For some 20 years, astronomers have been puzzled by these enigmatic bursts; no familiar astronomical phenomena can explain the massive energy charges. Now, scientists are using two new telescopes, ASKAP here in Australia and CHIME in Canada, in an effort to solve the mystery. In the course of a single year, they have discovered as many bursts as older telescopes found in 20 years. The major harvest confirms that the bursts are due neither to radio noise from Earth, nor Solar System phenomena, nor violent events in the Milky Way – indeed the inexplicable signals have travelled through space for billions of light years, coming from
remote galaxies. So, astronomers are beginning to narrow down where the signals might come from: black holes, neutron stars, intelligent life... or something completely different.
Bursts reveal the size of the source
The long distances to the bursts and their brief duration make it incredibly difficult to study the sources directly, even where the bursts come from. On the other hand, the duration of the bursts could reveal the sizes of the sources.
A fast radio burst of 10 microseconds could only come from a source where the surface light can escape within this brief period of time, and the speed of light of 300,000km/s therefore indicates that the source has a maximum diameter of 3,000km – much less than the Sun’s 1.4 million kilometres.
The briefest burst ever observed lasted just 0.03 microseconds, indicating a source with a mere 10km diameter. So the bursts must come from extremely compact objects – such as evaporating black holes, or collisions between neutron stars. These small stars have diameters of 10km to 20km, but as their neutrons are just as densely packed as atomic nuclei, the neutron stars achieve twice the Sun’s mass.
The theories are challenged by one of the discovered radio sources, which has flashed 20 times since its discovery in 2016. Clearly the repeated bursts cannot come from individual destructive events such as neutron star collisions. They must come from a type of mechanism that does not destroy the radio source, so it can flash over and over again.
Telescopes finecomb the sky
The mystery of these fast radio bursts can only be solved via more observations, and so radio telescopes throughout the world are being used in an intensive search to solve the mystery. The Australian Square Kilometre Array Pathfinder (ASKAP) telescope in 2017 gave astronomers improved "glasses". Before ASKAP, a total of 34
fast radio bursts had been spotted, but in the past year alone the telescope has captured 20 new ones. The CSIRO-run facility in the MidWest region of Western Australia consists of 36 linked parabolic antennas of 12 metres diameter. The efficiency of the network is due to a large field of vision that covers an area of the sky 1,000 times the size of the full moon.
Moreover, the accuracy is impressive. ASKAP can locate the source of a fast radio burst within a thousandth of a degree – corresponding to pointing out the width of a human hair at a distance of 10 metres. The accurate observations mean that optical telescopes stand a good chance of finding the galaxies that emit the bursts. Before ASKAP, only the source that has flashed repeatedly since 2016 had been located, in a dwarf galaxy some 3.7 billion light years away.
Most newly-discovered fast radio bursts came from remote galaxies at least seven billion light years from Earth. The most powerful burst was twice as intense as the previous record holder, emitting what corresponds to 80 years of solar radiation in a few microseconds. The telescope also captured the closest fast radio burst so far, from the ESO 601-GO36 galaxy, located only 120 million light years away.
However, ASKAP will very soon be outdistanced by the new Canadian Hydrogen Intensity Mapping Experiment, CHIME, which over a test period of three weeks found 13 fast radio bursts, bringing the total number to 67. The telescope even found another repeating radio source, with six bursts in three weeks.
Astronomers’ present theories suggest that fast radio bursts are triggered somewhere in the universe a few times every minute. When CHIME has been fully introduced later this year, astronomers hope to capture some 12 new fast radio bursts every day.
Early bursts from Parkes' lunches
Fast radio bursts are among astronomy’s major unsolved mysteries. Since they were discovered in data from the 64-metre Parkes radio telescope in New South Wales in 2007, they have been shrouded in mystery and scepticism. Back then, most astronomers believed that the fast radio burst was just a measuring error – particularly after 16 signals that resembled the fast radio burst proved to come from scientists opening the microwave oven in the lunch room prematurely. But in 2014, the Arecibo telescope in Puerto Rico, which has a parabolic antenna of 305 metres, spotted a new fast radio burst that sped up the search for the extremely high-energy bursts.
The imaginative explanations point in many directions. Some astronomers talk about neutron stars that pulsate, collide, or cause short circuits due to super-powerful magnetic fields. Others believe in evaporating black holes. Scientists have even proposed that they emanate from sophisticated civilisations... without going into detail as to how the ultra-brief energy discharges that make the Sun seem weak might be produced.
Journey reveals hidden matter
The road to solving the mystery requires more fast radio bursts and careful analyses that can reveal whether the bursts are always caused by a specific mechanism, or by several different types of phenomena.
No matter what the sources prove to be, the bursts could form an important piece of the astronomical puzzle. On their way through space, the radio signals' wavelengths are delayed by extremely thin, almost invisible hydrogen clouds between the galaxies. Astronomers can only find 60% of the universe’s visible matter, whereas the hidden matter is hiding in the clouds, causing time lag. Fast radio bursts from remote galaxies throughout the universe can be used to determine the extent and volume of the clouds and reveal if they include all that visible matter which astronomers cannot find.
Many mysteries must be solved on the way, but the thousands of bursts that should be captured by the new telescopes could soon determine if black holes, special star types, or alien civilisations are the answer.