A mysterious blast of radio waves has been traced to its source. Its origins suggest there are multiple causes of these signals, which have been puzzling astronomers for nearly 10 years. What’s more, it has given us the first direct measure of the density of the interstellar medium, confirming there is a whole lot of matter we can’t account for.
Fast radio bursts (FRB) are powerful but incredibly brief, so are hard to spot. The Parkes radio telescope in Australia made the first detection in 2001, but no one noticed the signal, lasting just 4.6 milliseconds, until 2007. Since then, astronomers have only found a dozen or so more, and just one – in May 2014 – was seen as it happened.
Even then, we couldn’t identify the source. “When you find them you have an idea of where they are on the sky, but there are lots of galaxies so you’re not really sure which one it is from,” says Evan Keane at the Jodrell Bank Observatory in the UK.
In principle, though, catching an FRB in the act lets astronomers point several telescopes at it in the hope of tracking it down. Now Keane and his team have got one in their sights.
On 18 April last year, the Parkes telescope picked up a signal clocking in at under a millisecond. Keane and colleagues had set up a system to alert other telescopes of an FRB detection, and using the Australia Telescope Compact Array, they found a radio afterglow that lasted six days. Switching to the Subaru telescope in Hawaii, they traced the signal to an elliptical galaxy around 7 billion light years from Earth.
Explanations for FRBs have ranged from pulsars to aliens. “There are probably more theories than there are fast radio bursts actually known,” Keane says. But elliptical galaxies are old, so if one is the source of an FRB, you can rule out anything linked with a young stellar population, such as an outburst from a pulsar or magnetar – types of neutron star that emit periodic signals. Instead, Keane thinks the blast of energy released by two neutron stars merging could have been the culprit in this case.
Mystery solved? Not quite. Most FRBs are so brief that radio telescopes don’t have high enough resolution to tell exactly when they start and end – the millisecond value is just the smallest time they can resolve, like the peak shutter speed on a camera.
But a 2011 FRB, recently unearthed in data from the Green Bank telescope in West Virginia, lasted at least a millisecond, suggesting a different cause – perhaps a flare from a magnetar. And since magnetars can flare up from time to time, that suggests that some FRB sources may be giving repeat performances – though no examples are known so far.
Looking with LIGO
Merging neutron stars should also release gravitational waves. Two weeks ago, the team running the Laser Interferometer Gravitational-Wave Observatory announced the first detection of these waves, albeit from merging black holes. Now that LIGO’s capability is proven, it could help us distinguish between different FRB sources.
“If these neutron star mergers also produce FRBs, we would expect them to coincide with LIGO detections,” says Maura McLaughlin of West Virginia University in Morgantown, who was part of the teams behind the first FRB discovery and the Green Bank find. Seeing the same event with multiple telescopes provides a fuller picture, she says. “We have very little information with radio FRB measurements – all we have is one brief pulse.”
While astronomers don’t yet know exactly what causes FRBs, they can still use them to explore the universe. FRBs arrive in a tight bunch of frequencies, but the peak of the signal is slightly shifted at each frequency. The extent of this shift, known as the dispersion measurement, varies depending on how much gas and dust lies along the radio waves’ path.
“You don’t really know what distance it has gone through, you just know how much stuff it has gone through,” says Keane. But because his team were also able to identify the FRB’s host galaxy, they know the distance and can thus calculate the density of the intergalactic medium. This measurement tells us the universe is 5 per cent ordinary matter (the rest is dark matter and dark energy). That figure is in line with a value previously inferred from Planck telescope data, but this is the first time it has been directly measured.
However, half of that ordinary matter isn’t emitting light – researchers know it as the “missing baryons”. It would have been very hard to sneak a look at this stuff without the help of passing FRBs. “Even if we can’t determine their origin, they are very useful cosmological probes,” says McLaughlin.
Journal reference: Nature, 10.1038/nature17140