Free Astronomy Magazine November-December 2017

17 NOVEMBER-DECEMBER 2017 SPACE CHRONICLES after supernovae. These mergers have so far been the leading hypothesis to explain short gamma-ray bursts. An explosive event 1000 times brighter than a typical nova — known as a kilonova — is expected to follow this type of event. The almost simul- taneous detections of both gravitational waves and gamma rays from GW170817 raised hopes that this object was in- deed a long-sought kilo- nova and observations with ESO facilities have revealed properties re- markably close to theo- retical predictions. Kilonovae were sugges- ted more than 30 years ago but this marks the first confirmed observa- tion. Following the mer- ger of the two neutron stars, a burst of rapidly expanding radioactive heavy chemical elements left the kilonova, moving as fast as one-fifth of the speed of light. The colour of the kilonova shifted from very blue to very red over the next few days, a faster change than that seen in any other observed stel- lar explosion. “When the spectrum appeared on our screens I realised that this was the most unusual transient event I’d ever seen,” remarked Stephen Smartt, who led observations with ESO’s NTT as part of the extended Public ESO Spectroscopic Survey of Transient Objects (ePESSTO) observ- ing programme. “I had never seen anything like it. Our data, along with data from other groups, proved to everyone that this was not a supernova or a foreground vari- T his image from the VIMOS instrument on ESO’s Very Large Telescope at the Paranal Ob- servatory in Chile shows the galaxy NGC 4993, about 130 million light-years from Earth. The galaxy is not itself unusual, but it contains something never before witnessed, the af- termath of the explosion of a pair of merging neutron stars, a rare event called a kilonova (indicated with the arrow). This merger also produced gravitational waves and gamma rays, both of which were detected by LIGO–Virgo and Fermi/INTEGRAL respectively. [ESO] an amazingly close match to theory. It is a triumph for the theorists, a confirmation that the LIGO–VIRGO events are absolutely real, and an achievement for ESO to have gath- ered such an astonishing data set on the kilonova,” adds Stefano Covino, lead author of one of the Nature As- tronomy papers. “ESO’s great strength is that it has a wide range of telescopes and instru- ments to tackle big and complex as- tronomical projects, and at short notice. We have entered a new era of multi-messenger astronomy!” concludes Andrew Levan, lead au- thor of one of the papers. able star, but was something quite remarkable.” Spectra from ePESSTO and the VLT’s X-shooter instrument suggest the presence of caesium and tellurium ejected from the merging neutron stars. These and other heavy ele- ments, produced during the neutron star merger, would be blown into space by the subsequent kilonova. These observations pin down the formation of elements heavier than iron through nuclear reactions within high-density stellar objects, known as r-process nucleosynthesis, something which was only theorised before. “The data we have so far are !