It was the first event that could be observed with both gravitational-wave observatories and conventional telescopes: the collision and subsequent fusion of two neutron stars in a galaxy 130 million light-years away. The spectacle, discovered in August 2017, received the catalog number "GW 170817" and for the first time gave astronomers the opportunity to study a gravitational wave event in more detail.

Gravitational waves arise when masses are accelerated. Usually they are so weak that attempts to prove it fail. Star collisions trigger measurable gravitational waves.

Now comes out: The two colliding neutron stars whose gravitational waves were received in August 2017, have shot an almost light-fast matter beam into space. This concludes an international team of astronomers from observations of the cosmic crash.

To accelerate matter to nearly the speed of light, unimaginable energy is needed. According to the theory of relativity, the mass increases with increasing speed - the energy required for acceleration must be greater.

The scientists around Giancarlo Ghirlanda of the Italian National Institute of Astrophysics had observed the afterglow of the collision with 32 radio telescopes distributed all over the world and present their analyzes in the science journal "Science".

The afterglow

In the weeks following the collision, the scientists registered increasing X-ray and radio radiation from the fused pair. They attributed this afterglow to an interaction of the neutron star debris with the surrounding interstellar gas.

How exactly it came, however, was initially not clarified. Because of the enormous distance, the extent of the emission region was not measurable with normal telescopes.

Ghirlanda's team now combined radio telescopes on five continents into virtual super telescopes. 207 days after the fusion of the two neutron stars, they determined the size of the radiation region in this way. It was therefore less than 2.5 thousandths of arc seconds.

By comparison, one-thousandth of an arc-second corresponds approximately to the extension of a two-cent coin on the Eiffel Tower in Paris from New York.

The emission region of the neutron stars is therefore too small to start from a uniform cloud of debris, which pushes into the interstellar gas. Instead, the researchers assume that a so-called jet has formed - an energy-rich matter jet that shoots material into the surrounding gas at almost the speed of light.

stellar remnants

The analyzes indicate that at least one-tenth of all neutron star collisions should produce such jets, which are also known from other objects such as the central black holes in so-called Active Galaxies.

Neutron stars are stellar remnants of supernova explosions. They are made up of densely packed neutrons - one of the two atomic-core building blocks - and therefore resemble a kind of huge atomic nucleus.

They are the stars with the largest known density in the universe: A thimble of their matter has a mass of more than half a billion tons. In a collision, in which two neutron stars merge, a lot of energy is released, which is emitted, inter alia, in the form of gravitational waves and electromagnetic radiation.