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Scientists from the University of Leicester have today published the first detections of light accompanying gravitational waves from colliding neutron stars.
Teams led by University astronomers have explored for the first time the detailed aftermath of the collision of two neutron stars: a violent interaction which produces the heaviest chemical elements in the universe.
Today the advanced LIGO and Virgo gravitational wave observatories announced details of the event which occurred on 2017 August 17th. The gravitational wave signal was accompanied by gamma rays seen by the Fermi satellite, the first time that light has been definitively detected from a gravitational wave source.
Astronomers across the world began searching for the precise location of this event, quickly tracking it down to the nearby galaxy NGC 4993. University of Leicester astronomers using the VISTA telescope in Chile were among the first to locate the new source. “Our strategy was to hunt through many galaxies, searching for evidence of an ongoing explosion”, explained Prof Nial Tanvir, who leads a paper in Astrophysical Journal Letters today.
Once pin-pointed, the Swift satellite quickly maneuvered to look at the object with its X-ray and UV/optical telescopes. “We didn’t detect any X-rays from the object, which was surprising given the gamma ray detection,” said Dr Phil Evans, lead-author of a paper published today in Science. “But we did find bright ultra-violet emission, which most people were not expecting. This discovery helped us to pin down what happened after the neutron star collision that LIGO and Virgo saw.”
Neutron stars are the dead remnants of massive stars, they contain the mass of the sun in an object the size of a city. When they collide, some of the neutrons are ripped off and start to interact with each other, forming some of the heaviest elements in the universe. Radioactive decay of these elements then produces light in what is often called a ‘kilonova’. “The exact origin of these heavy elements, including gold and platinum, has been debated for decades” said Prof Tanvir. “To finally observe a kilonova in great detail, to see how it evolves and changes colour allows us to confirm that colliding neutron stars really are an important source of these elements."
The Swift data gave unprecedented insight into how this nucleosynthesis occurs after a neutron star merger. “We found that there were two sites where this took place,” explained Dr Evans. “We expected slowly rising red emission from where the heaviest elements are created. The Swift data showed that there was also early blue emission, in a pulse lasting just one day, which came from material blown away in a wind. This does not make the heaviest elements, so the blue light can escape. The discovery of the neutron wind was only possible using light, which is why combining gravitational waves and light in what we call ‘multi-messenger astronomy’ is so important.”
University of Leicester astronomers also studied the polarisation of the light using the Very Large Telescope in Chile. “We found no sign of polarisation from the kilonova”, said Dr Klaas Wiersema, co-author of a study published in Nature Astronomy. “This suggests that there must have been a roughly spherical outflow of material after the merger, such as the wind that Swift detected.”
“This is a really exciting event” concludes Prof Tanvir, who also used the Hubble Space Telescope to study this object. “For the first time we have detected an event in both gravitational waves and light, which gives us excellent complementary information. This and future events will enable us to learn much more about neutron stars, black holes and physics in really extreme environments and the creation of heavy chemical elements in the universe.”
This film was produced by External Relations, University of Leicester in 2017.
Filmed & Edited by Carl Vivian
Produced by Ellen White
