The European Southern Observatory (ESO) has announced the unprecedented discovery and observation of the optical counterpart to the gravitational wave event observed on 17 August (GW170817) of this year.
The full announcement and press event can be watched here:
This observation is on par with the historic 1963 observation by Allan Sandage and Thomas A. Matthews of the optical counterpart to Quasar 3C 48 and, theretofore, the first of its kind to be observed. At that time (~1963), a new genre of enigmatic object, Quasars, were discovered and observed as “radio objects” or objects emitting exclusively in the radio part of the electromagnetic spectrum. In fact the term “Quasar” is short for “Quasi-stellar radio source” Now, in an analogous and unprecedented observation, the optical counterpart to an event observed from across the universe by only gravitational whispers propagated through space and time has been observed and thus begins a new era in science and astronomy, Gravity Wave Astronomy!
In a galaxy, far, far away…
at a time when some would argue was the peak of the Age of Dinosaurs; when mammals were small, demure creatures hiding in the shadows of these great creatures who remained the dominant species on the planet for 135 million years, 135 million year ago, 134 million years before man’s distant ancestors walked the plains and jungles of Africa and Europe, two neutron stars, locked together in a veritable death spiral, came together and merged. This merger resulted in a stellar explosion known as a “KiloNova”, an exotic, new classification and genre of object in stellar evolution introduced in 2010 to describe a stellar blast whose output is approximately 1,000 times that of a nova. On 17 August of this year we observed that event as it occurred 135 million years ago in NGC-4993, an elliptical galaxy 135 million light years away in the constellation Hydra.
All stars shine by nuclear fusion, transmuting 4 hydrogen nuclei (protons) into helium in their cores, producing a tremendous amount of energy according to Einstein’s simple but powerful formula E=mC². For the sun and similar stars, this process continues with the production of Carbon and Oxygen through the fusion of helium, the by-product of the first step, hydrogen fusion. This process continues for heavier nuclei with more massive stars, ending with the production of Iron and Nickel as the logical end to stellar nucleosynthesis for the most massive stars. These stars end their lives in spectacular fashion as a Type-II supernova, enriching the interstellar medium with these heavy elements, providing the raw materials for new planets and new life. In one of the most elegant processes in all of nature, stellar nucleosynthesis, the creation of new elements and the subsequent enrichment of the interstellar medium, the death (of a star) provides the seeds for new life, life, much like the Phoenix rising from the ashes.
So what of the remaining elements heavier than Iron and Nickel, elements such as gold, platinum or uranium, how are they formed? Heretofore, two additional methods for the production of heavy elements are well known. First, Rapid Neutron Capture, the capture by lighter elements of high velocity, free neutrons in the furious aftermath of a supernova, forming the heavy elements. Second, Slow Neutron Capture, a much slower, more stable, less energetic process that occurs internally in the late evolutionary stages of high-mass stars. Now, we have confirmation of what had been postulated but never observed, the production of heavy elements through neutron star mergers.
Neutron stars are the end-states, the stellar remnants of high-mass stars, the collapsed cores of those stars compressed to densities comparable to atomic nuclei. At this point in their evolution, the stellar remnant is destined for one of two paths, either continued existence as a neutron star or continued collapse to become a black hole. In the case of GW170817, the event Célèbre, two neutron stars were locked in a decaying orbit, destined for obliteration in a neutron star merger, a heretofore postulated event but one now confirmed.
The answer to another outstanding question, what are the source(s) of Gamma Rays Bursts, has also been answered. Gamma rays are electromagnetic radiation, like visible light, but are of the highest energy and thus, only events that involve changes at the nuclear level can produce them. Gamma ray production is observed in both nuclear fusion and fission reactions; indeed, along with the production of helium, they are the products of the nuclear fusion reactions occurring in the sun’s core, containing the “energy” produced therein. A gamma ray burst was observed by NASA’s Fermi orbiting gamma ray observatory just two seconds after the detection of the gravity wave signal on 17 August from the same source on the sky, thus answering the question. The rapid production of heavy elements such as gold, platinum and uranium following the neutron star merger was the source of the gamma ray burst. Spectroscopic observations of the kilonova’s fading light curve confirmed the presence of these (and other) heavy elements.
These unprecedented discoveries, each warranting a Nobel Prize, are the fruits of a multinational synergy, a collaboration involving teams from around the world that included many European countries, Chile, Brazil, India, South Africa, the US and NASA. Astronomical observing assets from all of these countries along with the astronomers, physicists, scientists and teams that operate them each played an important role. Although the European Southern Observatory (ESO) was a major partner whose formidable observing assets located at Paranal, Chile (the VLT – the Very Large Telescope) were heavily utilized, this was a worldwide effort. The full, peer-reviewed publication that contains all the collaborators, their country of origin and the respective collaborating institutions can be found here. It should also be noted that without the newly commissioned Virgo gravitational wave observatory located near Pisa, Italy, this discovery may not have occurred or we could have missed the initial peak light events associated with supernovae. Using the dual US LIGO sites at Livingston, Louisiana and Hanford, Washington combined with the Virgo site in Italy, astronomers were able to more precisely locate the source on the sky to within 30 square degrees (for reference, the full moon subtends 1/2 degree on the sky). Combined with the Fermi Gamma ray observation occurring less than 2 seconds later, the source was precisely located.
It is impossible to overstate the importance and significance of these discoveries and it would not be a surprise if they lead to the awarding of multiple Nobel prizes.
On August 17, 2017, the Laser Interferometer Gravitational-Wave Observatory detected gravitational waves from a neutron star collision. Within 12 hours, observatories had identified the source of the event within the galaxy NGC 4993, shown in this Hubble Space Telescope image, and located an associated stellar source known as a “kilonova”, a new genre of object introduced in 2010 to describe an event with approximately 1,000 times the energy output of a nova. Hubble observed that flare of light fade over the course of 6 days, as shown in these observations taken on August 22, 26, and 28 (insets). It should be noted that this is a follow-up sequence imaged by the Hubble Space Telescope and is NOT the discovery image. Image credit: NASA/ESA Hubble Space Telescope and StSCI.edu