Cosmic gold: Portsmouth researchers herald new era of gravitational wave astronomy

The image on the right shows the nearby NGC4993 galaxy before the GW event, while on the left shows a new image of the same galaxy, but with a new object identified. This is the kilonova. CREDIT: Dark Energy Survey

The image on the right shows the nearby NGC4993 galaxy before the GW event, while on the left shows a new image of the same galaxy, but with a new object identified. This is the kilonova. CREDIT: Dark Energy Survey

Researchers at the University of Portsmouth are part of the global announcement today of a new astronomical object, a kilonova – the cosmic explosion of two colliding neutron stars.

The kilonova was initially seen via a burst of gravitational waves (GW) – ripples in the fabric of space and time – on 17th of August 2017, which pinpointed the location of this event for telescopes across the world. Within hours of the GW detection by the LIGO and VIRGO observatories in California and the Italy, astronomers were pointing their telescopes at the possible source and found the new transient object in a nearby galaxy called NGC4993, only 130 million light years away.

“It doesn’t get more exciting that this for an astronomer,” says Bob Nichol, Director of the Institute of Cosmology and Gravitation (ICG) at the University of Portsmouth, and a member of the Dark Energy Survey (DES) that was part of the global search for this new object. “At sunset, the DES team was ready to scan the position of the gravitational waves for a new source”.

A kilonova has been theorised for many years as the merger of neutron stars – the remnants of giant stars that have died in supernovae – but this is the first time such an event has been observed.

In the following days, astronomers studied the kilonova event from across the electromagnetic spectrum from gamma-rays to radio waves. ICG researcher Chris Frohmaier was part of a team led by Caltech and Berkeley who were interested in understanding the rate and light-curves of this kilonova; how many we expect in the local universe, and how the explosion changed with time.

Their work, published today in Science, led to an unexpected solution to a long-standing problem in cosmology; kilonovae could be the production sites for half the heavy elements in the universe.

“I was brought into the team because of my expertise in predicting the rates of such rare explosions,” said Dr Frohmaier, a research fellow at the ICG. “I had literally just arrived at the ICG when the LIGO event happened and got a call from colleagues to join the analysis of the subsequent data. Frankly it was a once in a lifetime opportunity”.

While Chris’s work was important, it was not until the final stages of writing the Science paper that he and others realise the incredible consequence of the measurement he had made.

“Someone in the team noted that if you multiply my rate of kilonova events expected, with the yield of heavy elements like uranium, gold and platinum per explosion, then you obtained a rather large number, basically enough to explain half the abundance of such elements in the Universe.”

The team had accidentally solved the problem regarding the ‘missing heavy elements’. These elements were thought to form in the aftermath of massive supernova explosions, but the predictions for the observed abundance of such elements didn’t produce enough, by roughly a half. A new source of such heavy matter was needed.

“It’s one of those eureka moments” said Chris. “It’s fantastic when such different areas of astronomy just come together. I started out studying the rates of kilonova and we found half the gold in the Universe!”

The discovery of the kilonova and the fact that gravitational waves can lead astronomers to such events opens a new window on the Universe. The combined observing strength of LIGO+Virgo and other telescopes will allow astronomers to discover further strange phenomena and solve remaining mysteries of the Universe.

“In a few years, such gravitational wave events could solve the long-standing problem of Hubble’s Constant” comments Bob Nichol. “These events are precise sirens and allow us to measure distances in the universe to unprecedented accuracy. It’s a new era of astronomy”.

The Science paper, Illuminating gravitational waves: A concordant picture of photons from a neutron star merger,  was led by Caltech’s GROWTH collaboration. The lead author is Mansi Kasliwal.

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