Free Astronomy Magazine November-December 2017

NOVEMBER-DECEMBER 2017 T his composite shows images of the galaxy NGC 4993 from several differ- ent ESO telescopes and instruments. They all reveal a faint source of light close to the centre. This is a kilonova, the explosion resulting from the merger of two neutron stars. This merger produced both gravitational waves, detected by LIGO–Virgo, and gamma rays, detected by Fermi and INTEGRAL in space. [VLT/VIMOS. VLT/MUSE, MPG/ESO 2.2-metre telescope/GROND, VISTA/VIRCAM, VST/OmegaCAM] The Swope 1-metre telescope was the first to announce a new point of light. It appeared very close to NGC 4993, a lenticular galaxy in the con- stellation of Hydra, and VISTA obser- vations pinpointed this source at infrared wavelengths almost at the same time. As night marched west across the globe, the Hawaiian island telescopes Pan-STARRS and Subaru also picked it up and watched it evolve rapidly. “There are rare occasions when a sci- entist has the chance to witness a new era at its beginning,” said Elena Pian, astronomer with INAF, Italy, and lead author of one of the Na- ture papers. “This is one such time!” ESO launched one of the biggest ever “target of opportunity” observ- ing campaigns and many ESO and ESO-partnered telescopes observed the object over the weeks following the detection. ESO’s Very Large Tele- scope (VLT), New Technology Tele- scope (NTT), VST, the MPG/ESO 2.2- metre telescope, and the Atacama Large Millimeter/submillimeter Ar- ray (ALMA) all observed the event and its after-effects over a wide range of wavelengths. About 70 observa- tories around the world also ob- served the event, including the NASA/ESA Hubble Space Telescope. Distance estimates from both the gravitational wave data and other observations agree that GW170817 was at the same distance as NGC 4993, about 130 million light-years from Earth. This makes the source both the closest gravitational wave event detected so far and also one of the closest gamma-ray burst sources ever seen. The ripples in spacetime known as gravitational waves are created by moving masses, but only the most intense, created by rapid changes in the speed of very massive objects, can currently be detected. One such event is the merging of neutron stars, the extremely dense, collapsed cores of high-mass stars left behind