All About Space
What did the team of telescopes really see?
Observing two neutron stars colliding in different wavelengths has revealed what occurred 1.7 billion light years away
“All neutron star mergers produce gravitational waves; this is something we’re extremely confident about”
Dr Van Eerten
States, and the University of Maryland at College Park (UMCP) tells All About Space. “Luckily the signal was also caught by NASA's Neil Gehrels Swift Observatory. Swift has a sharper view than Fermi and clearly pinpointed the GRB's position in the sky. This allowed astronomers to use other telescopes and observe that region of the sky in other wavelengths such as radio, optical and X-ray.
“The Magellan telescope detected the optical light 36 hours later, whereas NASA's Chandra X-ray Telescope saw its X-ray afterglow eight days after the gamma-ray burst was seen for the first time.” This chain of events was also helped by data taken by the Hubble Space Telescope; the Discovery Channel Telescope (DCT), a 4.3-metre (14.1-foot) telescope in Arizona, United States; the European Southern Observatory’s Very Large Telescope at the Paranal Observatory in Chile and the Gemini South telescope also situated in Chile at Cerro Pachón.
What led to this investigation, though? What made astronomers look at the signal and think that this warranted extra observation time using multiple telescopes? The answer to that is explained by a fellow researcher in the study, Dr Hendrik Van Eerten of the University of Bath, United Kingdom. “The X-ray instrument on board Swift detected a signal that remained unusually steady over a long period of time. This was so unexpected,” explains Van Eerten to All About Space. “A rapid fade-out, or at least any sort of fadeout, is more usual for 'short' GRBs – the type caused by merging neutron stars. To get a more in-depth observation in the X-rays, with better spatial resolution, the Chandra X-ray telescope was utilised.”
The gamma-ray burst
exhibited not only had a notably short fade-out, but it was extremely weak as well. In fact, it has one of the lowest energies ever detected by the Swift telescope.
Just over a day after the detection follow-up observations were conducted in bright optical and X-ray wavelengths. After such data had been collected and the options ruled out there was only one explanation to this conundrum, and the researchers announced that they had just witnessed the kilonova caused by the merger of two neutron stars 1.7 billion light years away from Earth.
Unlike a supernova – a stellar explosion that signals the end of a star’s life – a kilonova has a different process and emits different forms of energy. The most common form of supernova, a Type II, occurs when the star cannot maintain nuclear fusion and causes a collapse of outer material against its solid core, thus exhibiting the explosion that thrusts large amounts of material into the cosmos - this type of explosion can shine with the brightness of 10 billion Suns. What is left behind after a supernova can either become a black hole or a neutron star, which is a star that has
“Optical tells us about the kilonova; X-rays tell us about the jet and the newborn black hole”
Dr Eleonora Troja
a large mass tightly jammed into a sphere that is about the size of a city.
When two of these neutron stars collide they emit vastly greater amounts of energy, and the final bang at the end of its crescendo can result in a gamma-ray burst, which was spotted in GRB150101B. These events are the creators of some of the universe’s most exotic and heaviest elements. “It [the ejecta] is so hot, dense and neutron-rich that unstable massive nuclei are formed faster than they can decay. Eventually they do decay to more stable isotopes, just about everything with atomic numbers from about 30 to about 80,” explains Dr Geoffrey Ryan of the University of Maryland at College Park (UMCP), United States, to All About Space. “You've heard of gold, I'm sure. But there's also iridium, tungsten, silver, palladium, all the way down to rubidium and selenium. Even the lanthanides, like europium, are made.”
During a kilonova a fast-moving jet of energy is also emitted from the source and is normally firing from both ends of a newly formed black hole, perpendicular to its plane of rotation. “We can study these two, very different events [the kilonova and the jet] because we have the ability to see the sky through different lenses: optical tells us about the kilonova and the production of heavy elements; X-rays tell us about the jet and the newborn black hole,” says Troja.
On 17 August 2017 a worldwide fleet of telescopes all turned their heads to the constellation of Hydra to follow up on a gravitational-wave detection made by LIGO and Virgo. It was then that astronomers found the first-ever visible counterpart to a gravitational-wave detection. GW170817 was located 130 million light years from Earth. Astronomers were able to deduce that this event was also the result of two neutron stars merging.
The astronomers involved in the analysis of GRB150101B have claimed that it is a cosmic ‘relative’ of GW170817. Troja, Ryan and Van Eerten all agree that these two events are remarkably similar and could well be related. The main similarities are the unusually faint and short-lived GRB, but they also both emitted bright, blue optical light which lasted over a period of days – the X-ray emission lasting even longer – and their local environment also strikes a resemblance.
“Both GRB150101B [and GW170817] are dim for gamma-ray bursts, we believe because we are viewing them off-axis, away from the core of the jet. Both are also optically bright,” says Ryan. “In the case of GRB150101B, in optical wavelengths it appears brighter than you would expect for the afterglow alone, and the difference is about the brightness expected for a kilonova. Both events also occur in the outskirts of the same type of galaxy: old, luminous, and elliptical.”
What appears to be missing though is the detection of the far more valuable gravitational waves. These waves were predicted by Einstein’s general theory of relativity over a century ago which states that the most intense events in the universe, such as black holes or neutron stars merging, will create ripples in time and space. The LIGO mission was the first to prove this true with its first detection in September 2015. The Advanced LIGO experiment wasn’t fully functional until the month before the first gravitational-wave detection, however, meaning that the detectors missed the chance to view GRB150101B by eight months. But
even if LIGO was working and staring at that patch of sky at the right time, there is no certainty it would have detected anything due to the colossal distance between Earth and GRB150101B, which is over 1,000-times further away than GW170817! However, the researchers are still extremely confident that gravitational waves were still created, even if they weren’t able to detect them.
“All neutron star mergers produce gravitational waves; this is something the astronomical community is extremely confident about. The reason for this confidence is that, when it comes to predictions of gravitational waves, mergers of neutron stars and of black holes are extremely straightforward exercises in general relativity,” says Van Eerten. "What matters for us is whether these mergers occur sufficiently close enough to detect with gravitational-wave detectors.”
Whether these events are frequent or not is something that still needs to be determined. The evolution of telescopes and detectors has helped a lot with this research. Hopefully with the fine array of ground- and space-based telescopes available to modern astronomers the detections of such events could become more frequent. When asking Ryan what this discovery means for our understanding of science and the universe, he replies: “Short-term, identifying the shared properties will help us detect similar events in the future. Long-term, with more observations we can build up a cohesive picture of these events and better understand the physics that drives them.”
Both of these goals centre heavily on gathering as much data as possible. Now that astronomers know what they have to look out for it’s a matter of keeping a keen eye out for similar events and building a database of neutron star mergers throughout the universe. With a more comprehensive archive astronomers can improve on finding the visible counterpart to gravitational-wave signals. If another enormous event was on the verge of transpiring astronomers would be well-equipped to understand the true intricacies of the merger.
This includes understanding how the cosmos’ most exotic elements, which make up everyday life on Earth, are made.