Kilonova explosion caught on camera by Hubble
The explosive remnant of a stellar collision between two neutron stars – predicted to be the source of all the gold and platinum in our universe – may have been caught on camera by an international team of researchers.
The smoking gun was spotted by astronomers – who call it a kilonova because it’s about a thousand times brighter than an exploding white dwarf star – roughly 4 billion light years away in the constellation, Leo.
“We know for sure that neutron stars are going to merge every now and again, but what we weren’t sure of, is whether they’d actually produce all these heavy elements in the process,” Nial Tanvir, study co-author and astronomer at the U.K.’s University of Leicester, told SciFare.com.
Snapped by Hubble in the near-infrared – beyond the red colours we can see with our eyes and most telescopes – researchers said the fainter, supernova-like explosion may be from the decay of radioactive elements that were created, just a few days earlier, during the astronomical fender-bender.
“If this infrared transient we see is as it appears to be, it has all the right properties for being made up of these heavy, radioactive elements,” Tanvir said.
Gold and platinum are just two of many rapid-process elements that may have been cast out into the cosmos. Behind the radioactive glow and the kilonova, could be a trove of gold that some researchers estimate to be roughly equivalent to ten times the mass of our moon!
Having a smoking gun may help us better understand the roots all the r-process elements in our periodic table too – a large chunk of elements after iron are predicted to form this way.
“Modeling isn’t tremendously convincing that supernovae will produce large amounts of r-process elements and these neutron star binary mergers have always been seen as an alternative possibility,” Tanvir said. “The difficulty is deciding whether or not they really will produce kilonovae.”
Before they could capture a glimpse of a kilonova though, researchers needed to detect a short-duration gamma-ray burst.
Lasting, at most, a couple of seconds, the intense flashes are some of the earliest hints of a collision between super-dense stellar objects, like two neutron stars – researchers call them compact binary mergers. In theory, a neutron star colliding with a smaller black hole could produce a similar result too.
“It’s ripping out chunks of neutron star material and flinging them out,” Tanvir said. “That’s what gives rise to the kilonova.”
On 3 June 2013, a gamma-ray burst that’s predicted to have been about 100 billion times brighter than the kilonova was detected by the Swift telescope and sent out to researchers – an instrument on NASA’s Wind spacecraft also detected it and several ground-based telescopes tracked it in the following days.
The race to characterize the explosion officially launched.
In theory, the kilonova should rise in intensity a couple days after the gamma-ray burst – known technically as GRB 130603b – and glow at its peak for about a week, before plummeting dramatically.
Just ten days later on 13 June 2013, the first round of images were captured by Hubble and by 3 July 2013 they had a second round in both the optical and infrared spectrum.
“That allowed us to confirm that it really was transient light we were seeing and not just some structure in the galaxy,” Tanvir said.
When researchers measured the near-infrared light captured by Hubble in the second epoch’s images and compared it to the near-infrared light in the first, what may be the first evidence of that transient, kilonova, emerged – you can see their analysis in the first sequence of the video. The kilonova is in the circle.
What was clearly visible in mid-June had faded away by early July.
When they repeated their analysis with images from the visible spectrum of light, they couldn’t see the blast that’s estimated to be as much as 75,000 light years across. By comparison the Milky Way is 100,000 light years from end-to-end!
“That suggested that whatever it was that we were seeing in the infrared really was dark in the optical, Tanvir said. “That was important, essentially, because it effectively ruled out the possibility that this light could be the afterglow of the gamma-ray burst.”
Long-duration gamma ray bursts – the earliest signals associated with supernova blasts – are known to have a lingering afterglow and its absence essentially laid the foundation necessary for crowning the blast a kilonova.
“All of the evidence we’ve gathered in the last eight years, from Swift, has kind of pushed little bits and bobs of evidence in support of that idea, but without the crucial breakthrough,” Tanvir said. “I hope this is the crucial breakthrough.”
Detecting a kilonova has some very practical applications, for some of tomorrow’s big experiments – it’s not just a catastrophic explosion that generates an unimaginable amount of gold and other bling-like elements.
“In the future, we’ll be able to use Kilonovae as the optical counterpart of gravitational wave sources,” Tanvir said. “Without the electromagnetic information, you really only have half the story on gravitational wave sources.”
Two advanced gravitational wave detectors are being set up – one in the United States, the other in Italy – and they essentially measure the catastrophic, ripple-like waves also predicted to be some of the earliest hints of a cataclysmic stellar interaction that bursts in all directions.
Gamma-ray bursts are great, but they’re launched into space with laser-like precision, so if Swift isn’t in the direction to detect it, researchers learn nothing. Like the gravitational waves predicted to exist by Einstein, the kilonova blasts out in all directions, making it easier to spot than a gamma-ray burst.
“Although they’re much fainter and harder to spot, at least we should be able to see them isotropically,” Tanvir said.
The discovery was first published in the journal, Nature, but it was research that’s so hot, it’s not even off the press yet that pointed them in the proper direction.
In a manuscript submitted to The Astrophysical Journal, a duo of astrophysicists from the Lawrence Berkeley National Laboratory at the University of California, Berkeley, predicted the flash that followed the celestial pileup might be detected in the near-infrared, instead of the visible range of light.
“There had been suggestions that it would eject this radioactive material and maybe we’d be able to see it, but people didn’t really know what it would look like,” Daniel Kasen, The Astrophysical Journal study co-author and assistant professor of astronomy and physics at Berkeley, told SciFare.com.
In addition to predicting the blast would be red, they also developed models that Tanvir and his team used to help understand the images that Hubble snapped.
That’s because the research was made publicly available at an online database called ArXiv.org – the X stands for the Greek letter Chi – and essentially allowed the community to confirm the result before it even checked out of the peer-review process.
“It's an interesting feeling to think it may be something that's not just in my head or on my computer,” Kasen, who also prefers blingnova as a name for the explosion, said. “It's something that's actually happening in the real world.”
In order to unmask the blast, they needed to figure out how a radioactive cloud of heavy elements might behave after a compact binary merger between two cosmic objects that are about one-and-a-half times the size of our sun.
“There’s really nothing else like it that we can observe,” Kasen said. “You expect it to have some glow, but what colour and on what timescale, is hard to say.”
Then, researchers had to figure out how the light created from the blast might interact with a radioactive dust cloud.
“We were kind of surprised to find that it was a lot different than anyone had suspected it would be,” Kasen said. “It was a lot different than any other supernova or typical astrophysics gas, because it’s got all of these heavy elements.”
The researchers said nearly all the light outside the near-infrared may be masked by an extreme version of the process that creates red sunsets. In the same way that dust in the atmosphere only allows red colours through, bling-like dust may also be blocking and reflecting the light away from Hubble.
Hubble’s now got a perfect record for finding the origins of gamma-ray bursts. In a 1999 paper that was also published in Nature, it was used to link long-duration gamma-ray bursts to supernovae. Joshua Bloom was part of that discovery, wasn’t involved in any of the new research and called it all impressive.
“It will give people a pretty good sense that at least some short gamma ray bursts come from the merger of compact objects,” Bloom, who’s also a professor of astronomy at Berkeley, told SciFare.com.
“The big question is when – if ever – will we get another good short burst, to be able to get a good spectrum?”
It’s sort of like the explosion’s barcode. When they scan it, they’ll see a list of elements in the dust and can compare them to those predicted by theory. The chance to capture a supernova spectrum took five years, but Bloom hopes nature is more prompt this time.
“Nobody really agrees what they’ll look like, but they’ll certainly look different than supernovae,” he said.
That may also explain why scientists – including Bloom and two astronomers who just studied GRB 130603b – couldn’t find the blast that followed GRB 080503. After Swift detected it on 3 May 2008, some of the world’s best observatories were poised to capture it, in all its glory and came up short.
“We didn’t find any theory that allowed us to appeal to this as the simplest interpretation,” Bloom said. “The conclusion of that paper was that it was a weird afterglow.”
If history is any indication of what might happen in the future, it probably won’t come during regular working hours either – fortunately technology has made it easier to respond to an overnight event.
Today, Bloom has programmed a computer to respond to Swift events and in turn, it points a telescope in Arizona at the burst – he also has graduate students on the frontline. That’s because most telescope data is distributed to the field all at once and he said that makes publishing results ultracompetitive.
“I remember buying a whole bunch of pagers for my group at Caltech in 1997,” Bloom said.
As such, it shouldn’t be surprising that in a different manuscript accepted to be published in The Astrophysical Journal Letters – but not yet published – a team of Harvard astronomers are also staking a claim as its discoverers.
“My feeling is that we were the first ones to say there is a source there and it is indicative of this kilonova process,” Edo Berger, study co-author and associate professor of astronomy and natural sciences at the Harvard-Smithsonian Center for Astrophysics and Harvard University, told SciFare.com.
“It’s nice to see the group led by Tanvir is essentially reaching the same conclusion.”
This is where it gets uncomfortable. After Swift detected the gamma-ray burst, it was Tanvir and his colleagues who submitted the application for time-sensitive, director’s discretionary time with Hubble. You can read a portion of the application here.
Once the first set of images were snapped and released, Tanvir and his group announced they had obtained them using the same instant messaging service that Swift used to first notify the community. You can read it here.
What’s noticeably absent is the word kilonova, but the researchers do mention finding a source and that Hubble’s slated to snap a second image of it – long-duration gamma-ray bursts are known to have an afterglow. If it faded away, they said it would rule out an afterglow and pave the way for a kilonova.
Less than eight hours later, the Harvard group had scooped the Hubble data and posted their own analysis of the first photo – you can read it here. By 17 June 2013 they had submitted it, along with data from their set of ground-based telescopes, to the journal and published their results on ArXiv.org.
“When this group initially reported the first observations, they said there’s nothing there,” Berger said. “We looked at the data and said that source cannot be anything other than the counterpart to this gamma-ray burst.”
“Because of that discrepancy, we essentially said we need to write this up and publish it,” Berger added.
Both teams had data from ground-based telescopes that had given them insight into the short-lived afterglow they measured in the visible light – a small blast was observed, but quickly faded and never got that bright.
Because the Hubble image didn’t fall anywhere near the afterglow measurements though, the Harvard group ruled it out.
“The optical emission, early on, faded so rapidly that there was no way it could be the afterglow,” Berger said. “If we extrapolate the initial decline rate to the time of the HST observations, our predicted brightness of the source is about 25 or 30 times fainter than what's observed.”
Rather than wait for an image in July – once the kilonova had faded away – the Harvard team wanted Hubble to snap more images sooner.
“The second observation was done so late in the game that there would be no detection,” Berger said. “We actually wrote to the director of the space telescope and asked for additional observations before this very late observation in early July.”
Because the first round of images were made publicly available, they were fair game – so, although it’s an odd situation, there was no wrongdoing in using the data that another team had requested. Complimentary analyses are even welcomed by the scientific community.
As soon as they’re ready, researchers who study ancient DNA post the genomes online so that everyone can start working on them. The first journal results that glean information from them are reserved for the group that first posted it though. No similar agreement exists in astronomy – so it’s all fair game.
“When these kinds of requests for observations go in, essentially, they make the data immediately public to the community, so it gets analyzed in the most robust way,” Berger said.
That may not always be the case. At the request of Tanvir and his team, the second round of Hubble images were released to them first and to the rest of the community a month later.
“For the second epoch, we made the case that it was important to allow time for a careful analysis,” Tanvir said. “So, that request was granted.”
Just hours after the second round of pictures were made available to the community on 3 August 2013, the Harvard team had uploaded an updated version of the manuscript to ArXiv.org that included both sets of Hubble images.
The thumbnail and cover image were courtesy: Dana Berry/SkyWorks Digital, Inc.