Nikhef press release
LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars
Discovery marks first cosmic event observed in both gravitational waves and light.
For the first time, scientists have directly detected gravitational waves — ripples in space and time — in addition to light from the spectacular collision of two neutron stars. This marks the first time that a cosmic event has been viewed in both gravitational waves and light. The discovery was made using the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO); the Europe-based Virgo detector; and some 70 ground- and space-based observatories.
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas. As these neutron stars spiraled together, they emitted gravitational waves that were detectable for about 100 seconds; when they collided, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves. In the days and weeks following the smashup, other forms of light, or electromagnetic radiation — including X-ray, ultraviolet, optical, infrared, and radio waves — were detected.
The observations have given astronomers an unprecedented opportunity to probe a collision of two neutron stars. For example, observations made by the U.S. Gemini Observatory, the European Very Large Telescope, and NASA’s Hubble Space Telescope reveal signatures of recently synthesized material, including gold and platinum, solving a decades-long mystery of where about half of all elements heavier than iron are produced.
The LIGO-Virgo results are published today in the journal Physical Review Letters; additional papers from the LIGO and Virgo collaborations and the astronomical community have been either submitted or accepted for publication in various journals.
Nikhef director Stan Bentvelsen says the discovery is “incredibly exciting”. “Detecting the merging of neutron stars, first through gravitational waves and immediately afterwards through observations with optical telescopes provides an entirely new picture of these cosmic events. As far as I am concerned, this is the event of the century, which marks a new acquaintance with our universe.”
Physicists from the National Institute for Subatomic Physics (Nikhef), VU University Amsterdam, University of Groningen and Maastricht University, as well as astronomers from Radboud University and NOVA, are directly involved in this research. More details about the Dutch contribution can be found further on in this press release.
Detection and follow-up
The gravitational signal, named GW170817, was first detected on Aug. 17 at 8:41 a.m. Eastern Daylight Time; the detection was made by the two identical LIGO detectors, located in Hanford, Washington, and Livingston, Louisiana. The information provided by the third detector, Virgo, situated near Pisa, Italy, enabled an improvement in localizing the cosmic event.
LIGO’s real-time data analysis software caught a strong signal of gravitational waves from space in one of the two LIGO detectors. At nearly the same time, the Gamma-ray Burst Monitor on NASA’s Fermi space telescope had detected a burst of gamma rays. The signal was confirmed by ESA’s space observatory INTEGRAL. LIGO-Virgo analysis software put the two signals together and saw it was highly unlikely to be a chance coincidence, and another automated LIGO analysis indicated that there was a coincident gravitational wave signal in the other LIGO detector. Rapid gravitational-wave detection by the LIGO-Virgo team, coupled with Fermi’s gamma-ray detection, enabled the launch of follow-up by telescopes around the world.
Virgo and localization
Though the LIGO detectors first picked up the gravitational wave in the United States, Virgo, in Italy, played a key role in the story. Due to its orientation with respect to the source at the time of detection, Virgo recovered a small signal; combined with the signal sizes and timing in the LIGO detectors, this allowed scientists to precisely triangulate the position in the sky. After performing a thorough vetting to make sure the signals were not an artifact of instrumentation, scientists concluded that a gravitational wave came from a relatively small patch in the southern sky.
“This event has the most precise sky localization of all detected gravitational waves so far,” says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration. “This record precision enabled astronomers to perform follow-up observations that led to a plethora of breathtaking results. This result is a great example of the effectiveness of teamwork, of the importance of coordinating, and of the value of scientific collaboration. We are delighted to have played our relevant part in this extraordinary scientific challenge.”
With the help of the LIGO-Virgo coordinates, in the hours following, optical telescopes found a new point of light in the sky. Ultimately, around 70 telescopes and instruments on the ground and in space have looked for the event related to gravitational wave GW170817 in the electromagnetic spectrum.
The LIGO data indicated that two astrophysical objects located at the relatively close distance of about 130 million light-years from Earth had been spiraling in toward each other. It appeared that the objects were not as massive as binary black holes — objects that LIGO and Virgo have previously detected. Instead, the inspiraling objects were estimated to be in a range from around 1.1 to 1.6 times the mass of the sun, in the mass range of neutron stars.
While binary black holes produce “chirps” lasting a fraction of a second in the LIGO detector’s sensitive band, the Aug. 17 chirp lasted approximately 100 seconds and was seen through the entire frequency range of LIGO.
“The gravitational wave signal contains information on how the neutron stars deformed each other due to the tidal forces and this tells us a lot about what they look like inside”, says Chris Van Den Broeck at Nikhef, who helped to develop the first concrete analysis methods for such measurements. “In addition, the gravitational waves from binary neutron stars provide an entirely new way of determining distances in the universe. And it has been established for the first time that the speed of gravitational waves does not significantly deviate from the speed of light. These observations have therefore provided a wealth of information.”
The gamma ray that was detected by Fermi and INTEGRAL shortly after the gravitational waves, is a so called short gamma ray. Erik Kuulkers, INTEGRAL Project Scientist at ESA/ESTEC in Noordwijk, says: “Astronomers had always thought that brief gamma-ray bursts were produced by two neutron stars merging. This time we saw such gamma-ray bursts with ESA’s INTEGRAL satellite and NASA’s Fermi satellite. Thanks to the simultaneous detection of the gravitational waves, we are now certain this is due to neutron stars and the mystery has been solved at last!”
Each electromagnetic observatory will be releasing its own detailed observations of the astrophysical event. In the meantime, a general picture is emerging among all observatories involved that further confirms that the initial gravitational-wave signal indeed came from a pair of inspiraling neutron stars.
In the weeks and months ahead, telescopes around the world will continue to observe the afterglow of the neutron star merger and gather further evidence about various stages of the merger, its interaction with its surroundings, and the processes that produce the heaviest elements in the universe.
“It is fantastic and simply unbelievable that, with the first detection of a binary neutron star, we have also picked up signals along the entire electromagnetic spectrum”, says Samaya Nissanke, astronomer at Radboud University and one of the editors of the review article about the joint observations. “Thanks to these discoveries, this new field of research has been opened in an incredibly spectacular manner!”
At the time, LIGO was nearing the end of its second observing run since being upgraded in a program called Advanced LIGO, while Virgo had begun its first run after recently completing an upgrade known as Advanced Virgo. Both detectors are currently being prepared for a new upgrade that will again improve their sensitivity.
Frank Linde, leader of the Gravitational Waves program at Nikhef, has high expectations for the future: “Being able to observe a gravitational wave from two colliding neutron stars for over a minute! And then subsequently making an enormous number of observations in the electromagnetic spectrum. You might think: it can’t get much more exciting than that. Nevertheless, I expect that the research will become far more fascinating still: in the coming years, we will work on upgrading the Virgo (and LIGO) detectors, as a result of which we will not only be able to detect sources, but also follow these sources for a longer period of time. And that will also form the launching pad for our next fantastic project: building the Einstein Telescope underground with the location of South Limburg being a possible main prize.”
The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with the University of Valencia; and the European Gravitational Observatory, EGO, the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef.
LIGO is funded by the NSF, and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project.
More than 1,200 scientists and some 100 institutions from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration and the Australian collaboration OzGrav. Additional partners are listed at http://ligo.org/partners.php
Within the LIGO-Virgo Collaboration, Nikhef makes important contributions to both the instrumentation and the date analysis. In particular, this concerns software for the detection and further analysis of gravitational waves from merging black holes and neutron stars, with the aim of testing the general theory of relativity, clarifying the internal structure of neutron stars, and the use of merging objects as a new way to mark distances in the universe, in order to understand the evolution of the universe on a large scale. Efforts are also being made to find continuous gravitational waves, for example from rapidly rotating neutron stars in binary star systems.
For the Advanced Virgo-detector, Nikhef is responsible for the seismic isolation and for the optical sensors that must guarantee the stable functioning of the instrument. This isolation is, in part, responsible for the current sensitivity of Virgo, which is significantly higher than the previous sensitivity record that was achieved in 2011 before the detector was dismantled in order to be upgraded. Virgo is now an entirely new instrument with various new parts. Nikhef played a major role in the commissioning phase in which all parts were fine-tuned with each other within a period of less than one year.
Nikhef also plays an important role within the Einstein Telescope project, a future observatory for gravitational waves.
The astronomers at Radboud University are focusing on the astrophysical interpretation and on combining gravitational wave information with data from traditional telescopes. They play an important role in the coordination between the LIGO-Virgo Collaboration and dozens of teams of astronomers, and they are developing the BlackGEM telescopes within the Netherlands Research School for Astronomy (NOVA).
A lot of computing power is needed for gravitational wave research. The LIGO-Virgo Collaboration therefore makes use of facilities such as the Dutch National e-Infrastructure that is coordinated by SURF and is partly accommodated at Nikhef.
The National institute for subatomic physics (Nikhef) performs research in the area of particle and astro-particle physics. Nikhef is a partnership between the Netherlands Organisation for Scientific Research (NWO) and five universities: Radboud University, University of Amsterdam, University of Groningen, Utrecht University and VU University Amsterdam.
Radboud University is also an independent member of Virgo.
Images, videos, animations, and further background information can be found at:
Paper Physical Review Letters: “GW170817: Observation of gravitational waves from a binary neutron star merger”
For more information, please contact
Science Communications Department Nikhef
Vanessa Mexner – phone 020 592 5075 / 020 592 2075
Nederlandse Onderzoekschool voor Astronomie (NOVA)
Marieke Baan -– phone 020 525 7480
Prof. Jo van den Brand (present at the press conference LIGO-Virgo in Washington, D.C.)
Virgo Spokesperson (Nikhef, VU University Amsterdam)
– phone 06 20539484
Prof. Stan Bentvelsen
– phone 020 5925001 / 06 51111284
Prof.dr. Frank Linde
Programme leader gravitational physics group Nikhef
– phone 020 592 5140 / 06 36170622
Prof. Chris Van Den Broeck
Senior researcher gravitational physics group Nikhef and professor by special appointment of the University of Groningen
– phone 020 592 2053 / 06 25133968
Dr. Samaya Nissanke (present at the press conference ESO in Garching near München)
Assistant professor astronomy, Radboud Universiteit, and leader Radboud Virgo group, and affiliated to Nikhef
Prof. Gijs Nelemans
Professor of astronomy, Radboud University & KU Leuven, and affiliated to Nikhef
– phone 024 365 2983 / 06 45120189