Simply put: Now confirmed, mergers of neutron stars are factories of heavy chemical elements

After Einstein's prediction and LIGO's detection, 16-nation European Southern Observatory announces first direct visual identification of the source of a gravitational wave.

Written by Pratik Kanjilal | Updated: October 17, 2017 12:33 pm
black holes, supermassive black holes, how are gravitational waves formed, gravitational waves, Andromeda galaxy This is the first time that such an event has been seen, rather than detected, and the output of numerous telescopes, terrestrial and in orbit, compared to identify a single source. (Photo: NASA)

Space miners of the distant future may find a profitable business model in vacuuming up precious metals like gold and platinum from the vicinity of colliding neutron stars. However, they would have to weather a metalstorm travelling at one-fifth the speed of light, and are unlikely to live to collect their profits.

This much science fiction can be inferred by the layperson from the findings of the European Southern Observatory (ESO), which has announced the first direct, visual identification of the source of a gravitational wave — the very one detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) on August 17. A gravitational wave is a ripple distorting the fabric of spacetime — literally, the universe itself – created when heavy bodies accelerate rapidly.

This is the first time that such an event has been seen, rather than detected, and the output of numerous telescopes, terrestrial and in orbit, compared to identify a single source. Detailed findings will appear in several papers in Nature, Nature Astronomy and Astrophysical Journal Letters.

Elena Pian, an Italian astrophysicist and lead author of one of the forthcoming Nature papers, counts herself fortunate at witnessing the birth of a “new age” in the space sciences. “For the first time we have detected radiations in optical/near-infrared wavelengths as the counterpart of a gravitational wave source. This carries first and unambiguous proof that neutron star mergers are factories of heavy chemical elements.”

On October 3, LIGO founders Rainer Weiss, Barry C Barish and Kip S Thorne were awarded the Nobel Prize in Physics for detecting gravitational waves in 2015, generated by the collision of two black holes. In the interim, in August this year LIGO, together with the Virgo observatory in Italy, detected waves from the cataclysmic merger of two neutron stars, designated GW170817. Soon after, the Fermi space telescope detected a burst of gamma rays from the same region of space, which is a signature of massive stars in collision. It was designated GRB170817A. The question was, were they really the same?

The ESO organised the astrophysical equivalent of a manhunt, marshalling its own resources and those of allied organisations. As night fell in South America, its Vista, VST and La Silla telescopes in Chile, along with the US DECam telescope, located the source of light near the galaxy NEC 4993 in the constellation Hydra. As the wing of night swept over the globe, the signal was picked up in Hawaii and over the following weeks, about 70 observatories watched the event unfold when they were on the dark side of the earth. The Hubble Space Telescope tracked it, too.

The observations place the electromagnetic source at 130 light years away, which matches with the distance to GW170817, confirming that when neutron stars collide into a kilonova — a body 1,000 times brighter than the average nova — it produces a gamma ray burst and gravitational waves. The kilonova was a phenomenon theoretically predicted 30 years ago, and was expected to emit short bursts of gamma rays. Gravitational waves were predicted from general relativity a century ago, and it was surmised that colliding neutron stars would emit them. Both phenomena are now observationally confirmed, and a single event is identified as the source.

Such phenomena are believed to have generated and spewed out into the universe metals heavier than iron, including precious metals like gold, silver and platinum. Spectral observation now confirms the process known as r-process nucleosynthesis, by which kilonovas are believed to have created most of the precious and industrially useful metals (other than iron) of the periodic table.

This enormous exercise has joined the dots between various forms of imaging events and objects in deep space. “We can do astrophysics with different telescopes, instruments and tools: gravitational interferometers (advanced LIGO-Virgo) and electromagnetic facilities (optical and radio telescopes and gamma and X-ray satellites),” explains Pian. “The same source, the neutron star merger, was detected by LIGO-Virgo, a short gamma ray burst was simultaneously detected by instruments on board the Fermi satellite, and an optical/near-infrared source detected via large field-of-view imagers.

It was then identified and characterised as a kilonova with spectroscopy, notably with the VLT and X-shooter spectrograph, that takes spectra simultaneously over the whole ultraviolet-to-near-infrared spectrum.”

Apparently, the metallic spectra from the kilonova’s emissions can be fully mapped to the periodic table only after better theoretical models are developed, but ESO’s experiment has established a much more important principle — that it is now possible to correlate the findings of instruments looking into the sky in different spectra. Even if the ’49ers of the future find that they can’t mine gold in space, we can look forward to many more certainties about the universe we live in.

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