ON 28 March 2011, at 9.47 pm Eastern Standard Time, NASA’s Swift satellite spotted a searing burst of light in the corner of its eye. Within seconds it had alerted astronomers on the ground, and their telescopes were soon trained on a dazzling spectacle.
At the centre of a galaxy 3.8 billion light years away, a supermassive black hole was devouring a star that had strayed too close, tearing it apart to spark a gargantuan swirling firework. Then, suddenly, a narrow jet of radiation, pointed right at us, erupted from the black hole at close to the speed of light.
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Jaws dropped. We’d previously seen only a handful of these star-shredding displays, known as tidal disruption events, and only ever their afterglow. Now, watching the show from the start, astronomers could see exactly how the hot gas spiralled into the black hole.
Seeing such pyrotechnics is not as rare as it once was. A new generation of rapid-reaction observatories are belying the apparent calm of the night sky, revealing the universe at its most violent and dynamic. In the process, we’re discovering all kinds of weird flashes and flare-ups – cataclysmic outbursts that open a window on physics under the most extreme conditions possible.
“Our data is already full of things we can’t identify,” says Mathew Smith, an astronomer at the University of Southampton, UK.
Astronomy is traditionally a sedate affair. We train our telescopes on small patches of sky for long spells, trying to drink in as much faint light from distant objects as possible. That has taught us pretty much everything we know about the structure and evolution of the universe, but it means we tend to miss the real action – the bumps, bangs and bursts that come and go over days, hours or just seconds.
Even when we have serendipitously caught these “transient astronomical events”, it’s usually well after the party has started. That limits what we can learn. “In the first 24 hours or so, there are unique signatures of the physics that we can’t access if we don’t find them until later,” says Eric Bellm, an astrophysicist at the California Institute of Technology in Pasadena.
Swift was an early attempt to change that. Launched in 2004, it roams the heavens searching for flashes of high-energy photons called gamma-ray bursts, and automatically alerts other telescopes when it spots one. Since 2008, it has had a partner, the Fermi gamma-ray space telescope. Meanwhile, on the ground, specialised telescopes are scanning vast swathes of the sky, snapping picture after picture using wide-angle cameras.
“Rather than targeting just a handful of galaxies, we’re looking more systematically across the whole sky,” says Bellm, who works on the Zwicky Transient Facility, successor to the intermediate Palomar Transient Factory, north of San Diego, California. The PTF has spotted over a million transient phenomena since 2009.
“We’ve tended to miss the real action: the bumps, bangs and bursts“
That’s too many for astronomers to deal with, so facilities such as PTF and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in Hawaii use software that picks out interesting events in real time, then instantly notifies astronomers and automated observatories. The Las Cumbres Observatory Global Network (LCGOT), for instance, has seven telescopes dotted around the world and more to come, meaning at least one is always ready to follow up on a flare-up, no matter where in the sky it has appeared.
The result is a cosmological revolution in the making. Take our sightings of supernovae, the incredibly violent and bright explosions of massive dying stars. Occasional observations of their afterglow have told us that one particular class, type 1a supernovae, burn with predictable brightness, meaning we can use them as cosmic distance markers. That’s how we know the expansion of the universe is accelerating, apparently propelled by an enigmatic substance known as dark energy.
But what was once a trickle of supernovae is fast becoming a flood. “Ten years ago we were spotting 100 per year, now it’s more like 100 per week,” says at Harvard University. And among them are bizarre new species: dim ones that hardly make a bang, ultraviolet ones and those that explode as they smash into other stars.
Last year, for instance, astronomers used observations from Swift, PTF and the Las Cumbres network to catch a just as it started to explode, slamming into another star. The collision created an ultraviolet afterglow that allowed them to work out the size of the supernova’s companion – a missing piece of the puzzle in terms of figuring out what sets off some cosmic explosions.
Perhaps the most dramatic newcomers, however, are the super-luminous variety. The most brilliant of these was discovered last year, shining roughly 200 times as brightly as a typical supernova or .
Astronomers are scratching their heads over what powers such objects. Some put it down to the collapsing stars being extraordinarily massive and unstable, but a fresh oddity suggests something else.
“The brightest explosion was 570 billion times brighter than the sun“
In February, a team led by reported observations of a super-luminous supernova called DES14X3taz. It was originally picked up in 2014 by the , an all-sky telescope in Chile designed to probe the nature of dark energy by looking at the large-scale structure of the cosmos. When Smith followed up with the GTC telescope on La Palma, in the Canary Islands, he saw something bizarre. Normally supernovae start faint and get bright before slowly fading away, says Smith. “But this one didn’t.” Instead, it brightened, faded and brightened again. It seems to have exploded twice.
Smith thinks we were witnessing the birth of a magnetar – a highly magnetised, rapidly spinning version of a neutron star, the densest objects in the universe. He believes that the first flare-up was the initial shock wave hitting material the star had thrown off earlier, while the second flash was some of that material being knocked back onto the spinning star – which immediately threw it off again.
When Smith looked at other super-luminous supernovae observed by the Dark Energy Camera, he saw the same pattern, raising the prospect that the same physical mechanism could be behind all of them. If so, we might someday use super-luminous supernovae as cosmic distance markers, just as we use type 1a supernovae. Since they are so much brighter, the super-luminous ones would allow us to chart the universe’s expansion even further back in time.
Exotic supernovae, however, are not the only momentary marvels astronomers are catching up with. Intense photon flashes lasting more than 2 seconds, known as long gamma-ray bursts, are thought to come from the birth of black holes formed when very massive stars collapse. But we’ve also seen “ultra-long” bursts that last for hours, some of which may spring from the death of known as blue supergiants.
These massive but metal-poor stars can be hundreds of times larger than our own sun because they retain a deep hydrogen atmosphere as they grow old – a huge cloud of gas that could fuel a long burst as it falls into the black hole created by the star’s collapse.
We have even gawped as a doomed star is sucked into a supermassive black hole. This was the jaw-dropping tidal disruption event spotted by Swift in March 2011, which generated another unusually long gamma-ray burst and an X-ray afterglow visible for more than a month.
That allowed a team led by at the University of Maryland to peer deep into the gravitational well of a black hole that is largely dormant, as opposed to one constantly swallowing material and emitting radiation. The researchers used the reflections of X-ray flares emitted during the event, known as reverberations, to map the turbulent flow of gas as it spiralled into the maw.
Spying on such cataclysms might tell us more about how space-time behaves around these gravitational monsters, potentially revealing something about the fabric of the universe. “The ultimate goal is to look at gravity in the most extreme environments in order to test general relativity,” says Kara.
Any departure from , which have yet to fail a test, would be a vital step towards a quantum theory of gravity, a long-sought explanation of how the force operates at the smallest scales.
Then there are short gamma-ray bursts, revealing collisions between extremely compact objects (see “Striking flares“) and, at the other end of the electromagnetic spectrum, fast radio bursts (see “Curious transmissions“). These come from mysterious sources that, if we can ever identify them, may show us what the universe is really made of.
This colourful cornucopia of explosions is just the warm-up act. The next generation of automated all-sky observatories should give us the whole show. Perhaps the most hotly anticipated is the Large Synoptic Survey Telescope, due to see first light in 2021. With a camera boasting a whopping 3200 megapixels, it will cover the whole sky in just a few nights, checking back regularly for anything out of the ordinary. “We’re going to jump from having hundreds of known objects to hundreds of thousands,” says Smith.
The Square Kilometre Array, with instruments in Africa and Australia, will set off a similar sea change in radio astronomy.
“Not only are we going to get a much better view of phenomena we already know about,” says Berger, “we’re going to see things we’ve never seen before.”
Striking flares
Short gamma-ray bursts illuminate the most violent collisions in the cosmos
Over the past few years, astronomers have cranked up the rate at which they are discovering short gamma-ray bursts, flashes of high-energy photons lasting no more than a couple of seconds. No one is quite sure what causes them, but most think they come from collisions between very dense neutron stars, which either merge together or collapse into a black hole and kick out a blast of gamma rays in the process.
For a long time that was hard to prove. Then theorists predicted that when these incredibly compact objects come together, they should produce a telltale infrared glow. And in June 2013, by training the Hubble Space Telescope on a flash spotted by Swift, a team led by Harvard’s Edo Berger caught a glimpse of one.
It was the strongest evidence yet that short gamma-ray bursts come from clashes between binary neutron stars. To see the real smoking gun, though, we would have to detect the gravitational waves that accompany such events, much as last year we picked up the ripples in space-time generated by two merging black holes.
Golden glow
The clash may also have been the first time we’ve seen the production of heavy metals such as gold and platinum. We know they can’t be forged through nuclear fusion alone, because most stars lack the neutron density and energy needed, but smash-ups between neutron stars could do the trick. Now we need to see more of them to really figure out whether they are the main source of heavy metals in the universe.
Recent observations suggest that could be tricky. When Eleonara Troja from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and colleagues followed up on another short gamma-ray burst from a neutron star collision, they showed that it produced a very narrow jet.
The upshot is that if the beam is not pointed in Earth’s direction, we’ll miss it. So although astronomers are seeing more short gamma-ray bursts than ever, it looks as if we’re still only spotting a tiny percentage of what’s out there.
Curious transmissions
Fast radio bursts could give us clues to missing matter, if only we can figure out where they come from
By far the most enigmatic of the fleeting phenomena we’re detecting are fast radio bursts – intense pulses of radio waves that last mere milliseconds yet carry as much energy as the sun gives out in a month. Right now we’ve only seen around 20 of them, but there could be 6000 to 8000 every day, says Institute for Radio Astronomy in Bonn, Germany.
The source of these chirps has mystified astronomers ever since the first was identified back in 2007. Black holes imploding into white holes? Messages from alien civilisations? Sparks from superconducting cosmic strings? Or just something mundane from inside our galaxy? “There are more models than events,” says Kramer.
Earlier this year, a team led by Kramer . Within 2 hours of FRB 150418 triggering the automatic alarm on the Parkes telescope in Australia, other radio telescopes were following up. Observations of the afterglow showed that it came from an elliptical galaxy 6.8 billion light years away. “If they come from that far away, they need a really extreme source of energy to power them and that makes it a lot more exciting and exotic,” says Harvard’s Edo Berger.
Passing through
Kramer’s team also showed how fast radio bursts might be used to test our ideas about dark matter – the mysterious substance that helps hold galaxies together. Once they’d identified the source galaxy, the researchers knew how far the signal had travelled to get here, and the way the radio waves were dispersed en route let them calculate the amount of matter – both ordinary and dark – in between. Their answers agreed with values provided by other techniques. “We believe that’s more than a coincidence,” Kramer says.
Berger is not convinced. He argues that the afterglow was actually a persistent emission from an active black hole in a distant galaxy and therefore not related to the fast radio burst. It just goes to show how little we know about these signals.
This article appeared in print under the headline “Flash bang wallop”
