Dr Erik Kuulkers, the European Space Agency’s project scientist for the INTEGRAL mission, discusses the laboratory and its achievements, including the detection of gamma-rays associated, for the first time, with gravitational waves.
The European Space Agency’s (ESA) INTErnational Gamma-Ray Astrophysics spacecraft (INTEGRAL) is detecting some of the most energetic radiation that comes from space. The spacecraft was launched on 17th October 2002. It is helping to solve some of the biggest mysteries in astronomy. The INTEGRAL mission is dedicated to the study of celestial gamma-ray sources in gamma-rays, as well as in X-rays and in the optical.
In an interview with SciTech Europa, the mission’s project scientist, Dr Erik Kuulkers, outlines why such studies are important, discusses the detection of gamma-rays associated, for the first time, with gravitational waves, and what both an extended INTEGRAL mission and a proposed future ESA mission could go on to achieve.
What are the main reasons why the study of gamma rays is important? What makes the INTEGRAL mission unique in this area of research?
The task of INTEGRAL, ESA International Gamma-Ray Astrophysics Laboratory is to gather some of the most energetic radiation that comes from space, i.e., gamma-rays. Gamma-rays are even more powerful than the X-rays used in medical examinations. Fortunately, the Earth’s atmosphere acts as a shield to protect us from this dangerous cosmic radiation.
However, this means that gamma-rays from astronomical sources can only be detected by satellites, and INTEGRAL is one of the most sensitive gamma-ray observatories.
The INTEGRAL mission is providing a new insight into the most violent and exotic objects of the Universe, such as black holes, neutron stars, active galactic nuclei, and supernovae. INTEGRAL is also helping us to understand processes such as the formation of new chemical elements and the mysterious gamma-ray bursts, the most energetic phenomena in the Universe.
Most of the elements in our bodies and in everything around us come from dead stars, but how can new elements be formed when a star dies? INTEGRAL helps us to find out more about this element-making process.
The events that made the Universe habitable were catastrophic. Violent stellar explosions provided the energy that was needed to form the elements from which planets and living things could be built. Gamma-rays from these supernova explosions and from newly-formed radioactive elements are registered by INTEGRAL. Gamma-rays also appear after a supernova explosion when matter squirms in the intense gravity of the collapsed and very dense star remnant. Studying such compact objects as neutron stars or black holes is a second, very important, task of INTEGRAL.
Even stranger than the energetic radiation coming from the centre of distant galaxies are the flashes of extremely powerful radiation that suddenly appear somewhere in the gamma sky and disappear again after a short time. Observations in different wavelengths confirm that these bursts are the biggest observed explosions in the Universe. However, what is it that is exploding out there? This is another cosmic mystery that INTEGRAL is helping to solve.
The INTEGRAL mission is a truly international mission with the participation of all member states of ESA plus the United States and the Russian Federation.
ESA appointed Alenia Spazio, Italy, as industrial prime contractor, responsible for the design, integration and testing of the satellite. On board, four instruments from teams led by scientists in Italy, France, Germany, Denmark, and Spain are today gathering and analysing the gamma-rays from space. A Russian Proton rocket successfully placed the spacecraft into orbit. ESA and NASA ground stations are keeping in touch with INTEGRAL.
The mission operations centre responsible for satellite control is located at ESA’s operations centre in Darmstadt, Germany. The INTEGRAL Science Operations Centre (ISOC) in Villanueva de la Cañada, Spain, is providing the observation plan, and Switzerland hosts the centre for the scientific data, the INTEGRAL Science Data Centre (ISDC). As the INTEGRAL Project Scientist, I reside at ESA’s technical centre in Noordwijk, the Netherlands.
Amongst the many successes of the mission, which would you say stands out as being the most ground-breaking?
A most ground-breaking highlight is the detection of gamma-rays associated, for the first time, with gravitational waves, which had been detected seconds earlier on 17 August last year by the Laser Interferometer Gravitational-wave Observatory (LIGO) experiment. This historic discovery confirmed that such short gamma-ray bursts are caused by the merger of two neutron stars. The event triggered an immense multi-wavelength follow-up of never-seen proportions; almost every observatory in space and on Earth able to look at the direction of the event observed the source. This has been the 2017 highlight in science overall (see: http://sci.esa.int/integral/59664-integral-sees-blast-travelling-with-gravitational-waves/).
Note that 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. The two LIGO detectors had seen the first in September 2015, followed by two more in late 2015 and early 2017.
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 in early October.
How difficult – and indeed important – is it to work with such a truly international team?
Working with many nationalities is a very interesting exercise, but also indeed a big challenge. Dealing with different cultures is not always without problems, and logistically it is not always easy to get everybody in the same room. But, in the end, we all share a common goal: to do research. And that brings us all together on the same footing.
What do you hope the mission will further achieve before it comes to an end (potentially in December this year)? Where will your interests and priorities lie following the end of the INTEGRAL mission?
Indeed, INTEGRAL’s operations have been firmly secured up to the end of 2018. Last year and earlier this year, operations have been indicatively approved up to the end of 2020. This needs to be confirmed later this year.
Over the years, INTEGRAL’s priorities have shifted. Since the mission’s launch, our knowledge about the cosmos has been expanded by the detection of new kinds of non-electromagnetic signals in the form of gravitational waves and (ultra-)high-energy cosmic neutrinos. INTEGRAL’s two main gamma-ray instruments normally use their anti-coincidence shields (ACS) to reject photons from outside the field of view of the gamma-ray detectors. However, the roles can be reversed and the ACSs also used as sensitive instruments for detecting any high-energy electromagnetic counterparts of the events discovered in the above-mentioned new observational fields.
This was, for example, the case for the gravitational event detected on 17 august 2017, mentioned above.
INTEGRAL has also the capability to rapidly (typically within half a day) repoint and conduct Target of Opportunity (ToO) observations on a large variety of sources, and to search for any remnant emission from the transient sources. In the coming years, we hope to detect many more gravitational wave events and (ultra-)high-energy cosmic neutrino events in the gamma-rays and try to disentangle their nature.
Are there any missions in the pipeline which can build on the work that the INTEGRAL mission has done? How would you like to see gamma ray observatories evolve moving forwards?
More and more observatories are (getting) designed to observe the transient Universe. Not only in gamma-rays, but also at other wavelengths, including the optical such as the Large Synoptic Survey Telescope (LSST) – and radio – such as the Square Kilometre Array (SKA). Many new phenomena are thought to be revealed and it is crucial to get a complete picture and gather information at all wavelengths, including gamma-rays.
Earlier this year, a mission to do a high-energy survey of the early Universe, amongst two other missions, was selected to be considered for ESA’s fifth medium class mission in its Cosmic Vision science programme, with a planned launch date in 2032. This mission is called the Transient High Energy Sky and Early Universe Surveyor (Theseus). The three missions will be studied in parallel and a final decision is expected in 2021.
Theseus is a novel mission to monitor transient events in the high-energy Universe across the whole sky and over the entirety of cosmic history. In particular, it promises to make a complete census of gamma-ray bursts from the Universe’s first billion years, to help shed light on the life cycle of the first stars. In addition, Theseus would also be able to follow up on gravitational wave observations by locating and identifying radiation from sources spotted by other detectors.
Dr Erik Kuulkers
INTEGRAL Project Scientist
European Space Agency (ESA)
This article will appear in SciTech Europa Quarterly issue 28, which will be published in September, 2018.