To neutronstjerner roterer om hinanden, støder sammen og bliver én under frigivelse af energi, der sender gravitationsbølger og gammastråling gennem rummet. (Illustration: ESA)

DTU takes part in huge space discovery with ESA

Monday 16 Oct 17


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

DTU and the INTEGRAL project

INTEGRAL was launched in October 2002. DTU Space - Denmarks National Space Institute - at the Technical University of Denmark (DTU) has been involved in the ESA INTEGRAL gamma-ray observatory project since the beginning.


We have been responsible for the development and manufacture of two JEM-X X-ray monitor instruments. Now we are producing science from the data from INTEGRAL i cooperation with ESA and a number of other partners. We are still responsible for maintaining the two JEM-X instrument units and providing public analysis software for the INTEGRAL Science Data Center (ISDC) in Geneva.


Contacts at ESA:

Erik Kuulkers,  ESA INTEGRAL Project Scientist:
Tel: +31 71 565 8470
Mob: +31 6 30249526

Volodymyr Savchenko,  INTEGRAL Science Data Centre
University of Geneva, Switzerland:

Paul McNamara, LISA Study Scientist
Tel: +31 71 565 8239

The INTEGRAL satellite where DTU Space is a part of the technical and science team sees gamma blast travelling with gravitational waves for the first time.

ESA’s INTEGRAL satellite recently played a crucial role in discovering the flash of gamma rays linked to the gravitational waves released by the collision of two neutron stars.

On 17 August, a burst of gamma rays lit up in space for almost two seconds. It was promptly recorded by INTEGRAL and NASA’s Fermi satellite.

Such short gamma-ray bursts are not uncommon: INTEGRAL catches about 20 every year. But this one was very special: just seconds before the two satellites saw the blast, an entirely different instrument was triggered on Earth. One of the two detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) experiment, in the USA, recorded the passage of gravitational waves – fluctuations in the fabric of spacetime caused by powerful cosmic events.

“This is a ground-breaking discovery, revealing for the first time gravitational waves and highly energetic light released by the same cosmic source,” says Erik Kuulkers, INTEGRAL project scientist at ESA.

Before this finding, gravitational waves had been confirmed on four occasions: in all cases, they were traced back to pairs of merging black holes as they spiralled towards each other.

”This discovery heralds a new era for astrophysics. Due to good cooperation among partners and to improved instruments both on earth and in space we are able to explore the universe in much greater detail. Here we have coinciding measurements from both earth and space that confirms a phenomenon predicted by Einstein,” says Soeren Brandt an astrophysicist from DTU Space at the Tehchnical University of Denmark (DTU) who is part of the INTEGRAL team.

A clear signature from a neutron star merger

The two LIGO detectors had seen the first in September 2015, followed by two more in late 2015 and early 2017. Recently, on 14 August, the fourth observation of gravitational waves also involved Europe’s Virgo instrument in Italy.

These detections won the LIGO founding scientists the Nobel Prize in physics earlier this month. Gravitational waves are the only ‘messenger’ expected when black holes collide. Following these four measurements, scientists across the world began searching with ground and space telescopes for possible luminous bursts linked to the gravitational waves.

"This discovery heralds a new era for astrophysics"
Søren Brandt astrophysicist, DTU Space at the Tehchnical University of Denmark (DTU)

“We had contributed to these earlier searches with INTEGRAL, looking for gamma- or X-ray emission and finding none, as expected from the vast majority of theories,” says Volodymyr Savchenko from the INTEGRAL Science Data Centre in Geneva, Switzerland.

This time, however, the story took a different turn. Other cosmic clashes are suspected to release not only gravitational waves but also light across the electromagnetic spectrum. This can happen, for example, when the collision involves one or more neutron stars – like black holes, they are compact remnants of what were once massive stars.

Merging neutron stars have also been thought to be the long-sought sources of short gamma-ray bursts, though no observational proof had yet been found. Until August.

“We realised that we were witnessing something historic when we saw the notification of Fermi’s and LIGO’s detections appear on our internal network almost at the same time, and soon after we saw the confirmation in the INTEGRAL data, too,” says Carlo Ferrigno, from the INTEGRAL Science Data Centre.

“Nothing like this had happened before: it was clearly the signature of a neutron star merger,” adds Volodymyr.

Ordinarily, an alert from only one of the three gravitational-wave detectors would not awaken curiosity so suddenly, but the coincidence with the gamma-ray blast detected from space prompted the LIGO/Virgo scientists to look again.

It later appeared that both LIGO detectors had recorded the gravitational waves. Owing to its lower sensitivity and different orientation, Virgo produced a smaller response, but combining all three sets of measurements was crucial to locating the source.

A huge international collaboration

After the initial detection of the blast, INTEGRAL observed it for five and a half days.

No further gamma rays were detected, an important fact in understanding how the neutron stars merged. An extensive follow-up campaign revealed signals across the spectrum, first in the ultraviolet, visible and infrared bands, then in X-rays and, eventually, radio wavelengths.

“What we are witnessing is clearly a kilonova: the neutron-rich material released in the merger is impacting its surroundings, forging a wealth of heavy elements in the process,” explains Carlo.
“This amazing discovery was made possible by a terrific collaboration of thousands of people working in different observatories and experiments worldwide,” says Erik.

“We are thrilled that INTEGRAL could provide a crucial contribution to confirming the nature of such a rare phenomenon that scientists have been seeking for decades.”

With high sensitivity to gamma rays and almost full-sky coverage for brief events, INTEGRAL is amongst the best astronomical facilities for keeping an eye on gamma-ray bursts. When the LIGO/Virgo sensors start their observations again, with improved sensitivity, in late 2018, it is crucial that as many gamma-ray satellites as possible are active to check on the gravitational wave detections.

Next generation mission is on it's way

Meanwhile, ESA is working on the next generation of gravitational-wave experiments, taking the quest to space with LISA, the Laser Interferometer Space Antenna.

Planned for launch in 2034, LISA will be sensitive to gravitational waves of lower frequency than those detected with terrestrial instruments.

These are released by the clashes of even more exotic cosmic objects: supermassive black holes, which sit at the centre of galaxies and have masses millions to billions of times larger than that of the stellar-mass black holes detected by LIGO and Virgo.

“LISA will broaden the study of gravitational waves much like the first observations at infrared and radio wavelengths have revolutionised astronomy,” says Paul McNamara, LISA study scientist at ESA. 

This story is based partly on a news release from ESA 16 october 2017 - read the full ESA release her.