Illustration: ESA 2002/Medialab

DTU researchers involved in historic discovery in outer space

Wednesday 18 Oct 17


Søren Brandt
Senior Scientist
DTU Space
+45 45 25 97 10


Jérôme Chenevez
Associate Professor
DTU Space
+45 45 25 97 03

INTEGRAL and neutron stars

In 1995, the work on the INTEGRAL space craft started, headed by the European Space Agency, ESA.

DTU Space was charged with the development and construction of X-ray detectors for INTEGRAL, called JEM-X (Joint European Monitors for X-rays). DTU Space is still involved in the scientific part of the successful mission.

The mission was launched into space in 2002 to examine black holes and neutron stars. Since then, it has orbited Earth approx. 10,000 to 180,000 km into space.

The instruments on board detect, among other things, the short gamma-ray bursts that last less than two seconds.

The mission has now really proven its worth with the successful discoveries of the correlation between short gamma-ray bursts, neutron stars and gravitational waves.

The colliding neutron stars are only around 10 km in diameter. But their mass is enormous. The mass of a star like this is 50 per cent greater than the mass of the sun. On Earth, a piece of a neutron star the size of a die would weigh 100 million tonnes.

Short gamma-ray bursts are the gamma radiation in the form of photons that is formed when the neutron stars collide and then spread as light waves through space. Their duration makes them difficult to study. At the same time, gravitational waves are formed when a part of the mass of the neutron stars is converted into energy.

The INTEGRAL space craft has measured signals originating from a collision of two neutron stars. For the first time ever, gravitational waves and gamma rays have been recorded from the same event. With the discovery of this phenomenon, which Einstein predicted, and for which the Nobel prize was recently awarded, the DTU researchers have secured their place in history.

It will surely be quite a party at the conference in Venice that researchers from DTU Space are currently attending together with a number of other research institutions from around the globe. Shortly after the opening event on 16 October, the world sensation of neutron stars and gravitational waves was announced.

That it has once again been possible to measure gravitational waves is big news in itself. But the most significant discovery is that they—for the first time—now have been observed in connection with a collision of two neutron stars. Until now, gravitational waves were only recorded in connection with black holes.

At the same time, it has been demonstrated that a massive amount of energy in the form of gamma-ray bursts is also discharged. In other words, when these stars collide, it generates both gamma-ray bursts and gravitational waves.

A large part of the new discoveries is based on data from the INTEGRAL mission under the auspices of the European Space Agency, ESA, in which DTU is involved.

“So far, we have not known with certainty how the short gamma-ray bursts are formed. But we know that now. It is probably when two neutron stars melt together and gravitational waves are formed,” explains Søren Brandt, astrophysicist and Senior Researcher at DTU Space.

“We know that because the LIGO detector on Earth detected gravitational waves, which can only be from a neutron star collision, and because we then recorded gamma-ray bursts with INTEGRAL shortly after.”

It is a great discovery which ties up many loose ends in astrophysics and supports Einstein’s general theory of relativity.

The discovery has just been announced at the scientific conference in Venice and in a number of scientific articles in the Astrophysical Journal. Researchers at DTU Søren Brandt and Jérôme Chevenez co-authored these.

The work is part of a major complex of discoveries involving hundreds of researchers that are based on observations from both space and Earth headed by the European Space Agency, ESA, and NASA in the USA.

DTU Space is a contributor to the four articles on the new discoveries that are now being published.

Gamma-ray bursts discovered by chance

Gamma-ray bursts have long been a bit of a mystery.

They were first detected by chance in the 1960s by American spying equipment in space that was keeping an eye on the former Soviet Union’s secret nuclear testing. At that time, no one had any idea what this energy discharge was.

Since then, both long and short gamma-ray bursts have been defined.

Short gamma-ray bursts are gamma rays in the form of photons. They occur when two neutron stars collide. Then, the photons spread as light waves through space.

The studies of these star collisions contribute to the research into black holes as part of the work on studying, among other things, gravitational waves.

With the new discoveries, researchers have come closer to answering questions such as how heavy elements such as uranium and gold are formed, what the inexplicable dark energy—which is believed to constitute most of the universe—is and why the universe is expanding at ever greater speeds.

In the late 1990s, researchers learned that long gamma-ray bursts arose in connection with dying stars, supernovas. But short gamma-ray bursts, which lasted less than two seconds, could still not be explained. One theory was that they were formed when neutron stars collided. And now, 50 years after their discovery, this theory has been confirmed.

A signal from the past

The collision that has now been recorded took place approx. 125 million years ago many billion kilometres from Earth. In other words, the signal from the event travelled 125 million light years before it reached INTEGRAL and was recorded.

From its orbit around Earth, INTEGRAL has registered gamma rays several times before, without being able to establish whether they came from neutron stars.

"This is a new era for astrophysics. We have better equipment to study the universe, and now we have coinciding measurements from both Earth and space that confirm the phenomenon predicted by Einstein."
Søren Brandt, astrophysicist and senior researcher at DTU Space

But this summer, gravitational waves were once again recorded on Earth on two occasions, namely by the two LIGO detectors in the USA and a similar system in Italy.

On 14 August, gravitational waves were recorded for the fourth time since they were observed for the first time in 2015. As soon as Søren Brandt and his colleagues received this information, they asked the ESA’s experts to adjust the control program for INTEGRAL so that it pointed towards the area in the universe from which gravitational waves had been recorded.

“We had INTEGRAL adjusted, but we did not see anything. This is presumably because the gravitational waves were from black holes and not from neutron stars,” Søren Brandt explains.
Then, three days later, on 17 August, came the breakthrough. For the second time that month, gravitational waves were recorded by the detectors in the USA and Italy.

“Two seconds after gravitational waves were detected on the ground, photons from short gamma-ray bursts came in from the side and were recorded by the detector on INTEGRAL,” says Søren Brandt.
“It was quite unique. Both gamma-ray bursts and gravitational waves were recorded from the same event, and then we were sure that they had to come from the fusion of neutron stars and not from black holes.”

Black holes do not emit gamma-ray bursts. This means that the event on 17 August establishes a clear link between gravitational waves, short gamma-ray bursts and neutron stars. So, today’s announcement reveals the news that gravitational waves have been measured for the fifth time and that—for the first time—they come from a collision of neutron stars.

A new era for astrophysics

Gravitational waves, which are part of Einstein’s general theory of relativity, are formed in three types of events according to the theory:

When two black holes rotate around each other, when neutron stars collide, or when a black hole and a neutron star collide. In these processes, some of the gigantic masses are converted to energy which is then released in a burst of gravitational waves.

The work on the LIGO machine—and the first direct detection of gravitational waves in 2015—was recently rewarded with the Nobel Prize in physics.

“This is a new era for astrophysics. We have better equipment to study the universe, and now we have coinciding measurements from both Earth and space that confirm the phenomenon predicted by Einstein,” says Søren Brandt.

“If you stretch your imagination, this may be compared with previous major breakthroughs in astronomy. As when Galileo in the 17th century revolutionized the world of astronomy and the perception of the universe by using advanced technology in the form of the first proper telescope. With this, craters on the surface of the moon and the moons orbiting Jupiter could be observed for the first time.”

DTU Space has been part of the INTEGRAL project from the outset. Both in the work on constructing the detectors on board and in the scientific work after the launch.

The main occasion at this year’s astrophysics conference was actually that tomorrow marks the 15-year anniversary of the launch into space of the INTEGRAL mission.

This should still be celebrated. But with the new revolutionary discovery, there is much more reason to party than the researchers had dared hope for when they received their invitations for Venice.

“What a great coincidence of the annual conference, the celebration of the INTEGRAL mission’s success and some new major scientific discoveries that are ready to be revealed, all in one week,” says Søren Brandt.

“So I look forward to some exciting days.”

Kurven viser, at der i forbindelse med neutronstjernekollision udsendes en gravitationsbølge samt gammaglimt - målt på jorden af LIGO-detektoren i USA samt af ESA-satellitten Integral. (Kilde: ESA, NASA, DTU Space, LIGO-konsortiet mf.l.)

The figure shows that during the collision of two neutron stars, a gravitational wave is formed, which is then recorded on Earth by the LIGO detector in the USA (bottom curve) and, two seconds later, gamma-ray bursts, recorded by both ESA’s INTEGRAL satellite and NASA’s Fermi satellite. (Source: ESA, NASA, DTU Space, the LIGO Scientific Collaboration and more)

LIGO and gravitational waves

In 1916, Albert Einstein predicted that gravitational waves had to exist. But back then, they did not have the technology to measure them.

On 14 September 2015, they were observed in real life for the first time.

Gravitational waves are generated and spread at the speed of light in three situations: When two black holes—which can have a mass 30 times that of the sun—rotate around each other and become one. When neutron stars collide. Or when a black hole and a neutron star collide.

This event affects space and time—space time—and makes space vibrate briefly. The changes are difficult to measure, but may be registered as small changes in distances.

In 2015, this happened for the first time ever with the LIGO detectors in the USA. The change measured corresponds to detecting a change in distance of a few thousands of the diameter of an atomic nucleus on a four-kilometre stretch. This is done by means of laser beams that measure the impact from the gravitational waves when they pass Earth.

With the discoveries by the LIGO system, the similar Virgo system in Italy, INTEGRAL, and NASA’s Fermi satellite, this is now the fifth time that gravitational waves have been observed. And the discovery of the first gravitational waves was awarded this year’s Noble Prize in physics. It went to the three American physicists who were the main driving forces behind the development of LIGO.