In less than a week as June became July, two teams of radio astronomers – one in Australia, the other in the US – announced they had independently accomplished a decade-long astronomical quest: identifying the sources of powerful blasts of intergalactic radiation known as fast radio bursts (FRBs).
FRBs are enormous blazes of radio energy that in a few milliseconds can broadcast as much energy in radio waves as the monthly output of the sun in all forms combined.
What causes them is unknown, but it can only be something dramatic, such as a collision between neutron stars, or even a neutron stars falling into a black hole. “For a long time, there were more theories than [known] bursts,” says Keith Bannister, an astronomer with CSIRO’s Australia Telescope National Facility (ATNF) and leader of the Australian team.
FRBs are surprisingly common, with perhaps 2000 of them pinpricking the sky every day, Bannister says, but only a tiny fraction are detectable, “because traditional radio telescopes only see a small fraction of the sky”.
Also, the vast majority are one-off events, making it incredibly difficult to figure out what galaxy they are coming from, once they are spotted.
In an effort to solve this problem, Bannister’s team equipped 36 identical 12-metre radio telescopes that together form the Australian Square Kilometre Array Pathfinder (ASKAP) in Western Australia with a “phased array feed” that allowed each dish to see 36 distinct patches of the sky at once, each about 200 times larger than the full moon.
They also upgraded their software to rapidly triangulate on an FRB via 0.1 nanosecond differences in the time it takes the signal to reach the various telescopes in the array: a method, Bannister says, that allowed them to pinpoint its origin to a precision of 1/50,000th of a degree – the width of a human hair, 200 metres away.
The US team took a different approach. Rather than retrofitting an existing telescope array, it built a new one from the ground up.
Due to the extreme brightness of FRBs, “Keith Bannister and I both realised that we can utilise relatively insensitive but wide field-of view telescopes to try to localise them,” says its leader, Vikram Ravi, a radio astronomer at California Institute of Technology, Pasadena.
His team therefore purchased 10 broad-field-of-view 4.5-meter radio antennas – instruments not much larger than the best satellite TV dishes – and laid them out in the Owens Valley of eastern California, at a total cost of under $US500,000.
“It was a shoestring experiment,” Ravi says. “I literally moved them in place and focused them by hand.” The ultimate goal, he adds, is to expand the project to include 110 such dishes.
Finding the sources of FRBs is important for two reasons. One is simply that it helps us figure out what causes them. The FRB located by Bannister’s team, for example, came not from the centre of its galaxy, but from its outskirts – “or at least its suburbs”, Bannister says. “This means our FRB wasn’t produced by a gigantic black hole at the galaxy’s centre.”
Ravi adds that both the FRBs come from mature, Milky Way style galaxies. That’s interesting because the only other FRB whose source has ever been identified – a repeating burster whose repeated bursts made it easier to localise – came from a very different type of galaxy. That one had 1000 times less mass but was in a “starburst” stage, in which it was forming new stars at an extremely rapid pace.
Based on that, one theory had been that FRBs came from the deaths of such galaxies’ most giant youthful stars, which live fast and die in blazes of glory known as superluminous supernovae.
But such gigantic explosions are uncommon in more mature galaxies, suggesting that in the case of the two FRBs identified by Bannister’s team and Ravi’s, superluminous supernovae probably didn’t play a role.
Localising the sources of FRBs is also important, Ravi says, because FRBs can be used as probes of the distribution of matter in the universe.
One of the big issues in astrophysics, he adds, is that most of the matter in the universe is invisible to us.
Much of that is dark matter, an enigmatic substance to date is detected only by its gravity, but the vast bulk of normal matter is also invisible, Ravi says. All that’s known is that it’s very hot – on the order of a million degrees or more – and very diffuse, partly contained in tenuous halos around galaxies, but possibly also dispersed throughout the intergalactic medium.
FRBs, Ravi says, offer a way to figure out where this unseen matter lies, and how it is distributed.
That’s because as the radio burst travels through this diffuse medium, different frequencies travel at slightly different speeds. It’s not a big difference, but it’s enough that an FRB signal can become stretched as it travels, with higher frequencies travelling faster, and lower frequencies travelling slower.
“We observe the burst arriving first at the high frequencies, then later at the low frequencies,” Ravi says, an effect that can stretch a millisecond FRB to nearly a second.
Different parts of the signal can also reach us by different paths, in which they start out travelling in a slightly different direction than the main part of the signal, then are refracted back into our own line of sight.
“It’s sort of like why stars twinkle,” Ravi says.
The effect is small, but it’s a sign that the medium through which the FRB signal propagated might have been “clumpy”, rather than uniformly distributed.
To figure all of this out, Ravi says, it’s really useful to know how far an FRB signal has been travelling before it reaches us (and to know how many other galaxies it has passed close to. That’s another reason why it’s useful to locate the source galaxies of as many such signals as possible.
Shami Chatterjee, a radio astronomer at Cornell University, Ithaca, New York, and leader of the team that located the source of the repeating FRB, agrees.
Bannister’s find (and by extension, Ravi’s), he says, is “a magnificent technical achievement” that should, among other things, open the floodgates to more such findings, allowing FRBs to live up to their promise as probes of the intergalactic medium.
“Once we have a few dozen,” he says, “FRBs will be one of the only viable probes of the intergalactic medium.”
