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中性子星の衝突、重力波ではじめて観測：海外専門家コメント

欧米の国際研究チームは、宇宙の深遠にあり、極めて密度の高い中性子星どうしが衝突する現象を重力波の観測によってとらえたと発表しました。8月に米国のLIGOとイタリアのVIRGの2施設において同時に重力波を観測したことを受け、各国の望遠鏡で発信源方向からの光線を観測したところ、中性子星の合体によって重元素ができたと推定できるなど、多くの知見がもたらされたとのこと。7本の論文が10月17日付のNature（online版） とNature Astronomy（online版）に掲載。この件についての海外専門家コメント（原文）をお送りします。

論文リンク（抜粋）

Nature

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature24290.html

Nature Astronomy

https://www.nature.com/articles/s41550-017-0285-z

Dr Eric Howell

ARC DECRA Fellow in the School of Physics and Astrophysics, University of Western Australia

原文 

"The gravitational wave observations of black-hole collisions by the LIGO and Virgo detectors has been an incredible feat in technology and has accelerated our knowledge of the extreme and dynamic universe. However, on August 17th “colliding neutron stars invited themselves to the gravitational wave party and announced their presence with authority”.

We had not expected to detect such an event so close; as a result it was very loud. To be accompanied by a gamma-ray burst was incredible. The association between this type of short duration gamma-ray burst and neutron star collisions had been predicted for around 30 years. It is now confirmed. Thousands of gamma-ray bursts have been recorded, but this was the closest of this type ever observed. Much is unknown about these bursts and given previous understanding, this event simply should not have happened. 

The beamed fireball of a gamma ray burst can be observed at later times by x-ray, optical and radio telescopes. Astronomers from OzGrav teamed with other astronomers all over the world to work in unison to observe this amazing event – such coordination towards a common goal is a fantastic achievement and heartwarming. Over the next few years such events will become commonplace. Looking ahead, what we can learn about the universe in this new era of coordinated observations of gravitational wave sources is incredibly exciting."

Winthrop Professor David Blair

Node Director at the ARC Centre of Excellence for Gravitational Wave Discovery, and from the School of Physics, University of Western Australia

原文

"I started working on the first high sensitivity gravitational wave detectors in  the USA in 1973. I expected to spend a year or two detecting Einstein’s waves and then move on to something else.

We pinned our hopes on gravitational waves from neutron stars. This was our holy grail but it eluded us even when gravity waves from black holes had been detected. Forty four years later we have found the holy grail!

It is astonishing that a single very faint signal lasting a minute combined with an even briefer burst of gamma rays and a fading glow of light, can reveal so much: the scale of the universe, the speed of gravitational waves, the mechanism of gamma ray bursts, and the origin of gold."

Professor Susan Scott

Chief Investigator from the Research School of Physics and Engineering at The Australian National University

原文 

"Neutron stars are the densest stars in the universe. The astronomers have found many of them, alone but also in pairs orbiting each other. As they circle each other they radiate gravitational waves and their orbit shrinks. We knew that eventually many of them must smash together in violent collisions but we had never seen it happen. The astronomers simply did not know where to point their telescopes at the right time.
 
The LIGO and Virgo gravitational wave observatories can’t be pointed to a place on the sky, they just sit there and wait for something 'big' to happen in the Universe. And on 17 August this year something really big happened – a gravitational wave swept into our detectors which was the hallmark signal of two neutron stars colliding. It was our closest source and our strongest signal of our five detections announced so far.
 
We quickly sent details of a patch of sky to astronomy partners all over the world. What followed was an unprecedented avalanche of telescopes and satellites scrambling to scan this patch to pinpoint the source and image it with light, x-rays, radio and gamma rays. The age of multi-messenger astronomy had truly begun.”

Susan states no conflicts of interest, but says she is a member of the LIGO Scientific Collaboration.

Dr Paul Lasky

Lecturer and Future Fellow at the Monash Centre for Astrophysics, Monash University

原文 

"It is fair to say that this is one of the biggest astronomical discoveries of the century so far. In 2015 we made the first detection of gravitational waves which came from two massive black holes. This meant that only the gravitational waves were detected, but traditional telescopes saw nothing.

This time we managed to catch the collision of two neutron stars ― dead stellar remnants that weigh more than our Sun but are just 10 kilometers across. When these neutron stars merged they created a huge explosion which was seen using gamma-ray telescopes two seconds after the collision as seen by the gravitational-wave detectors.

Over the subsequent hours, days and weeks we saw this event across all different types of light, including x-rays, ultraviolet, optical and radio. The amount of physics that is being learned from this one collision is truly immense. This is a watershed moment in astrophysics, and brings us into the era of ‘multimessenger’ astronomy. "

Paul is part of the LIGO Scientific Collaboration, and has been actively working on the science that is to be announced on Tuesday morning. He is also an Associate Investigator of OzGrav.

 

Associate Professor Jeff Cooke

ARC Future Fellow at the Centre for Astrophysics & Supercomputing, Swinburne University of Technology

原文 

"On August 17, the LIGO and Virgo gravitational wave detectors recorded two neutron stars, for the first time, merging about 130 million light years away (one light year is about 10 trillion kilometres). However, this distance is 'very close' in astronomical terms and is essentially in our ‘back yard’. As such, it gave us a great view of the event. Although an event like this was predicted to be detected (eventually) by the very sensitive LIGO/Virgo detectors, no one expected it to occur so soon. Mainly because it's a very difficult detection and would have to occur very close, which would be very rare.

Gravitational wave detectors can detect black holes and neutron stars merging, but cannot locate them with great accuracy. As such, we don’t know exactly where in the vast universe they occur or, specifically, in which galaxy they occur. Until now, only black hole mergers have been detected and they do not produce light, so we have no way to locate them precisely or study them with our telescopes. Neutron star mergers do produce light and, as such, we can pinpoint where they are and study them in great detail. This was the first time astronomers can accurately locate a gravitational wave event and observe it in detail with telescopes.

About two seconds after the gravitational wave detection, a telescope in space (Fermi) detected a burst of gamma-rays. Astronomers suspected that this burst and the gravitational wave event were connected. For about 50 years, it had been theorised that highly energetic gamma-rays of light would occur when two neutron stars merge. The later confirmation of this connection solved a 50 year-old mystery.

Because the light was seen at the same time as the gravitational waves, we can confirm that gravitational waves travel at the speed of light through the universe.

Alerts of the gravitational wave event and the gamma-ray burst were sent out to teams of astronomers worldwide that had been preparing years for this. Astronomers worldwide dropped what they were doing to turn their telescopes to this event. We discovered a bright explosion called a kilonova (named so, because they are about 1000 (kilo) times brighter than a nova). Such explosions have been theorised to accompany neutron star mergers but have never been observed. With these observations, we witnessed a kilonova for the first time, confirming the theory, and we now understand how they work.

Astronomers stayed with the event, because it was expected to fade away forever, several days later. The event was observed with every type of telescope at every wavelength of light. It was identified and located in a galaxy named NGC 4993 that is located near our Milky Way galaxy and in the Southern Hemisphere of our night sky. We observed an explosion, a kilonova, unfold in front of our eyes, more beautifully than ever imagined. Kilonovae produce the heaviest elements around us today. The event enabled us to see where, and how, exotic elements such as gold, platinum, and uranium are formed.

Knowing the distance and the strength of the event, we now have another means to measure the expansion rate of the universe and its age.

Finally, the location of the event was a bit troublesome. It was in a direction in the sky near the Sun. This made things difficult for some telescopes to observe it, as they could only get snippets of time (about an hour or less) each night to catch it before it set with the Sun. As a result, a coordinated combination of telescopes worldwide and in space was needed to observe the event in a series of short segments as we raced the sunset around the world as the Earth turned." "

Professor Peter Veitch

University of Adelaide’s Head of Physics and Node-Leader of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)

原文 

"Before this first-ever detection of a binary neutron-star merger, electromagnetic telescopes could see the gamma rays emitted by a merger but didn't know what caused it.

When the LIGO-Virgo collaboration made the landmark detection of gravitational waves, we could identify the source of the gravitational waves but only knew approximately the location. Now we know both what happened and where it happened – multi-messenger gravitational astronomy has been born."

Associate Professor David Ottaway

Chief Investigator at the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), in The University of Adelaide

原文 

"We’ve now seen the first event using multi-messenger gravitational astronomy but with improved sensitivity we can observe many more of these cosmic events.

With more observations, we will be able to build a clear picture of the evolution of our stars and galaxies and the birth and development of the universe. Here at the University of Adelaide we working with LIGO and OzGrav colleagues to improve the sensitivity of the current detectors and developing the technology for the next generation of detectors."

Dr Douglas Bock

Director of CSIRO Astronomy and Space Science

原文 

"Here at CSIRO we are excited to be part of this discovery where we have turned some of the world’s best radio telescopes and joined them together with the gravity wave detectors and other telescopes around the world to try and understand the nature of gravity waves coming from across the Universe.
 
The Australia Telescope Compact Array, operated by CSIRO, just had its 25^th birthday but we are keeping it up to date so that it is ready for these kinds of discoveries – we can turn it with just a few hours to the most exciting things happening in the Universe.

We have a program where astronomers like Tara Murphy, at the University of Sydney, can come in with just a couple of hours’ notice and completely reschedule the telescope to follow up the latest exciting scientific discoveries while they are going on.
 
The current program with the Australia Telescope Compact Array has been going now for 40 hours since the gravity waves were first discovered. It’s an ongoing program in collaboration with telescopes all around the world.
 
Congratulations to Tara Murphy and the University of Sydney team for leading this Australian effort to follow up gravity waves, and understand what they are coming from, using radio telescopes here in Australia as part of an international effort.”

Associate Professor Tara Murphy

The University of Sydney and the Centre of Excellence for All-Sky Astrophysics (CAASTRO)

原文 

"[After seeing the report from LIGO] We immediately rang our team in Australia and told them to get onto the CSIRO telescope as soon as possible, then started planning our observations. We were lucky in a sense in that it was perfect timing, but you have to be at the top of your game to play in this space. It is intense, time-critical science.”
 

Dr Michael Spence

The University of Sydney's Vice-Chancellor and Principal

原文 

"This international discovery, with Sydney playing an integral role, demonstrates that the best science and modern innovation is intrinsically a collaborative effort.

What a terrific way to confirm that Einstein’s theory of relativity was correct, gain insights into massive bodies like black holes and, with this knowledge, start to re-think our understanding of the universe."
 

Professor Tamara Davis

ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO) and The University of Queensland

原文 

"This is a landmark discovery for astrophysics. It uncovers a whole new way of measuring the universe.  Even just this one event where we see the explosion that accompanied these gravitational waves, already confirms many predictions ― such as how the heavy elements were created, what happens when neutron stars collide, and how fast is the universe expanding.   

This is just the beginning, it’s as though we’ve discovered a new sense. We can feel the universe now, as well as seeing it. The biggest discoveries are still to come."  
 

Comments collected by our colleagues at the New Zealand SMC:

Professor David Wiltshire

Department of Physics & Astronomy, University of Canterbury

原文 

"The first discovery of gravitational waves from the merger of two neutron stars is an historic event. It is every bit as exciting as the first discovery of gravitational waves from merging black holes! Since this involves neutron stars that radiate light, for the first time we can also see what is going on in an extreme astronomical event that shakes up space-time.

This merger has been followed up by satellites and telescopes all over the planet. In one hit it solves a cosmic mystery – the origin of short duration gamma-ray bursts. We have seen flashes of gamma rays – like the ones that the Fermi satellite saw from this event – for decades without knowing what they are. Now we know. (At least in some cases!) But the event opens up other cosmic mysteries.

That is the nature of science. We will be learning new things for decades to come. It will be one hell of a ride.

One further cosmic gravitational wave first still awaits us: the merger of a black hole with a neutron star. Such events should also give out light and other electromagnetic waves. And they will be different to anything we have seen yet. When that happens, it will again open up completely new frontiers for 21st-century astronomy."

Dr J.J. Eldridge

astrophysicist, University of Auckland

原文 

"GW170817/GRB170817A/SSS17a was the event many astrophysicists have been waiting for since LIGO made the first detection of gravitational waves in 2015. This time we didn’t just detect the gravitational waves from two merging neutron stars, but have also seen the associated gamma-rays, optical light and radio emission from the possibly black holes and from the two exploding stars.

This new discovery shows again the importance of LIGO and VIRGO’s work over decades. The refinement of their highly sensitive work allows us to be able to detect these gravitational waves.

We don’t even know the full story yet. The observations are ongoing and only in the coming months and years will we really begin to fully understand how exciting this object is. The fact that we have so much information from so many different sources will allow us to piece together in a way we have never been able to before. It’s going to take a lot of time and a lot of effort!

This event alone has already answered questions on the nature and structure of neutron stars and confirmed that merging neutron stars look like we’ve always expected them to, as a short gamma-ray burst and a type of explosion called a 'kilonova'.

The observations of the kilonova, the explosive afterglow, also confirms something else. In the explosion we have seen evidence for large amounts of heavy elements. These events mostly create elements such as gold, silver and platinum. In this particular event, it’s likely that 100s or 1000s of Earth masses of gold and other elements were made. If the rate of neutron stars mergers is as high as we now think, these dying stars are now the source of most of these elements in the Universe.

We’re all made of stardust, but gold, silver and platinum are made of neutron stardust!

Researchers in NZ have not played a role in the detection of this event but we are highly interested in gravitational waves. For example, at the University of Auckland Assoc. Prof. Renate Meyer has worked on the problem of extracting the faint gravitational waves signals from the much larger terrestrial signals that jostle the instrument, a problem that is like trying to hear a whispered conversation in a noisy room. 

While in my own group a PhD student John Bray and a MSc student Petra Tang are both working to predict from models what the rate of these events should be, this event is showing that our models need to be improved to match the high rate of such mergers we now expect in the Universe."
 

Professor Matt Visser

School of Mathematics and Statistics, Victoria University of Wellington

原文 

"The LIGO-Virgo collaboration is to be congratulated on the first parallel detection of a neutron-star/neutron-star inspiral and merger in both gravitational waves and electromagnetic waves (gamma rays, optical, and radio).

From my personal point of view; the most important point is that this gives us an independent way of measuring cosmological distances, independent of supernova physics. 

We now have an *independent* way of checking/verifying the accelerated expansion of the universe.

While this is only one data point for now, surely more will follow. This type of observation will give us a new way to measure the Hubble parameter, deceleration parameter, etc.
 
A second critical point is that this observation rather conclusively demonstrates that at least some gamma-ray bursts, (GRBs), specifically some of the 'short' GRBs, are caused (as was expected) by neutron-star/neutron-star collisions. 
 
There are also many other implications ― from the 'cooking' of heavy elements (heavier than iron) in the subsequent kilo-nova, to direct precision tests on the speed of gravitational waves (expected to exactly equal the speed of light). 
 
It is expected that this observational technique will quickly become a standard part of mainstream astronomy, we hope to see many more of these events over the lifetime of the LIGO-Virgo project and its successors."
 

Associate Professor Renate Meyer

Department of Statistics, University of Auckland

原文 

"With the recent stunning detection of a binary neutron star merger, Advanced LIGO/Virgo have ushered in an exciting new era of astrophysics that highlights the importance of effective collaborations between gravitational wave observatories and other electromagnetic telescopes such as Fermi GBM, INTEGRAL, Chandra, Hubble, Swope, DECam etc.

Arguably, this detection and its follow-up observations of gamma rays, X-rays, ultraviolet, optical, infrared, and radio waves have an even higher significance for fundamental physics than the very first detection of gravitational waves in September 2015.

The new event confirmed many significant hypotheses that had only been conjectured before, for instance the origin of short gamma-ray bursts, the production of heavy elements such as gold in the universe by binary neutron star mergers, an estimate of the Hubble constant, independent of the cosmic distance ladder, and that gravitational waves travel at the speed of light.

From a statistician’s perspective, statistical noise characterisation has proven important for accurate estimation of waveform parameters such as the luminosity distance and sky location which allowed the telescopes to zone in on a localised region of the sky.

A truly exciting new era for astrophysics with the promise of many more discoveries to be made!"

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