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A cosmic event observed in both gravitational waves and light for the first time

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 is the first time that astronomers have been able to study the same event with 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. This breakthrough is the result of 40 years of work, and marks the beginning of a truly new era in observational astronomy.

At 13:41:04 BST on August 17, 2017 the Advanced LIGO and Advanced Virgo detectors recorded the gravitational-wave signal (GW170817) from a binary neutron star coalescence. Two seconds later, NASA's Fermi Gamma-ray Burst Monitor independently detected a short burst of gamma-rays (GRB170817A).

Scientists from the University of Birmingham’s Institute of Gravitational Wave Astronomy, who have worked on the Advanced LIGO project since its inception, have been celebrating the announcement.

Professor Alberto Vecchio said, “Neutron stars are quite remarkable objects: one and a half times the mass of the Sun, primarily in the form of neutrons is packed in a region of the size of Birmingham. I still find it hard to believe that we could observe, almost in real time, the final two minutes of life of a binary composed by two such bodies that eventually collided travelling at a third of the speed of light."

Professor Andreas Freise said, "The LIGO gravitational wave observatories have once again achieved a spectacular discovery. With precision laser interferometers that push boundaries of science and technology we record the faintest vibrations in the fabric of space and time. Detecting for the first time ever the signal of two neutron stars merging has been immensely exiting. We have just seen a glimpse of what gravitational wave detectors can reveal. I am looking forward to exciting times in astronomy."

From the gravitational-wave signal, the neutron star binary was initially localised to a sky region of approximately 30 square degrees (about 150 times the size of the full moon) at a distance of roughly 40 Mpc (130 million light-years). The precise localisation and the short distance (cosmologically speaking) made it the perfect target for follow-up observations with optical telescopes.

Dr Christopher Berry said, “This source is the pot of gold at the end of the rainbow! It's been a long road figuring out how to locate our gravitational-wave sources for astronomers. Now we have the first counterpart---data from across the entire spectrum of light, confirming that neutron stars are the Universe's factory for forging gold."

These joint observations confirm the hypothesis that colliding neutron stars are the engine of at least some short bursts of gamma rays, and are the likely source of rare heavier elements, such as gold and platinum, which cannot be made by regular stars.

Professor Ilya Mandel said, "Our detailed observations of the optical transient tell us that tens of thousands of Earth masses worth of matter were ejected during the merger at the velocity of about 10% of the speed of light. They also provide hints about the fusion of heavy elements that happens within the neutron-rich ejecta."

The combination of optical and gravitational-wave observations enable new science, such as measuring the Hubble parameter, which describes the expansion of the Universe.

Dr Will Farr said, “What I find so exciting about this event is that it has begun the merger of the new field of gravitational wave astronomy with the old field of traditional (electromagnetic) astronomy. By combining gravitational wave and traditional observations of the event, we have been able to measure the Hubble constant---the expansion rate of the Universe, a number of vital importance for cosmology---in a completely new way. I'm incredibly excited to think about the possibilities for this sort of work as the field of gravitational-wave astronomy accelerates."

This is only the beginning for astronomy with both gravitational waves and light. Physicists at the University of Birmingham are already looking forward to a new generation of gravitational-wave detectors.

Dr Haixing Miao said, “It is amazing that we have a coherent story about the diverse physics (gravity, nuclear, and electromagnetism) in such an extreme system! In the future, we want to study the detailed physics right at the moment that the neutron stars merge. This requires a much better detector sensitivity at high frequencies, which turns out to be limited by the quantum nature of light. At the University of Birmingham, we are already looking into a new technique -- active quantum filtering -- to enhance the high-frequency sensitivity."

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.

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