Astronomers discover light associated with a gravitational wave source


17 October 2017

International teams of researchers including astronomers from the Space Science Group in UCD’s School of Physics, have discovered electromagnetic radiation associated with a gravitational wave source.

The Advanced LIGO and Advanced VIRGO teams reported the detection of a gravitational wave source, consistent with the merger of two neutron stars for the first time. Gamma-rays, followed by optical and infrared radiation, were also detected from the same region of the sky.

Gravitational waves, ripples in the fabric of spacetime, were first anticipated by Albert Einstein a century ago in his general theory of relativity.

This theory predicts that the presence of a mass causes a curvature in spacetime and that when massive objects, such as black holes, merge, this curvature can be altered, sending ripples, or gravitational waves, out across the Universe. By the time these ripples reach Earth, they are however almost imperceptible.

In September 2015 gravitational waves were observed for the first time after scientists from over 20 countries collaborated to develop a detector, the Advanced Laser Interferometer Gravitational Wave Observatory (aLIGO), located in the USA, that was sensitive enough to confirm their existence. The 2017 Nobel Prize in Physics was recently awarded to Kip Thorne, Barry Barish and Rainer Weiss for this discovery.

Since the first gravitational wave detection, caused by a collision between two black holes 1.3 billion years ago, aLIGO, in conjunction with the Advanced VIRGO (aVIRGO) detector, located in Pisa, Italy, has observed gravitational waves from several other sources.

In all cases up to now, the gravitational waves detected were from the merger of black holes about 30 times the mass of our Sun and, despite searches by astronomers, no counterparts were found by telescopes sensitive to gamma-rays, X-rays, optical, infra-red or radio waves.

On 17 August 2017 gravitational waves from the merger of two compact stellar remnants, called neutron stars, were detected by two out of three aLIGO/aVIRGO systems.

Less than two seconds later, gamma-rays were detected from the same region of the sky by NASA’s Fermi Gamma-ray Space Telescope and ESA’s INTEGRAL satellite. Both satellites detected a gamma-ray burst, which proves that some gamma-ray bursts are caused by the merger of neutron stars.

Neutron stars are the ultra-dense remnants that massive stars can produce at the end of their lives, and if such stars merge, can produce gravitational waves and gamma-rays, such as those observed by aLIGO/VIRGO, Fermi and INTEGRAL respectively.

Details of the neutron star merger, its association with the gamma-ray burst, and the physical implications of this association, were published today in four papers by the aLIGO/VIRGO, Fermi and INTEGRAL teams in Astrophysical Journal Letters.

In addition, these discoveries sparked an unprecedented follow-up observational campaign using telescopes on the ground and in space, involving over 3,500 scientists on six continents. The goal of this campaign was to establish if there was any sign of the merger event at other wavelengths.

In a Nature paper also published today, a team led by Prof Stephen Smartt, Queen’s University Belfast, has announced the discovery of the first visible and infrared light associated with the new gravitational wave source detected on 17 August 2017.

This observation is of immense significance to astrophysics and may allow scientists to explain the unknown origin of some of the heavy metals found on Earth.

When first observed at optical wavelengths, this new astronomical transient, was around 100 million times more luminous than the Sun, albeit located at a distance of 130 million light years from Earth. However, the transient faded rapidly, and in a matter of weeks could no longer be detected by even the largest telescopes on Earth.

Dr Morgan Fraser, a Royal Society-Science Foundation Ireland University Research Fellow at UCD’s School of Physics, and a co-author on the Nature paper, analysed the first data from the source obtained from the European Southern Observatory’s (ESO) ‘New Technology Telescope’ in Chile.

This data was obtained as part of the ePESSTO project – a Europe-wide collaboration of scientists, including UCD scientists, working on astronomical transients.

Using the ESO data, astronomers, including Dr Fraser, were able to show that the transient’s behaviour matched theoretical models of a ‘kilonova’, a rare type of event that results from the merger of two neutron stars.

“Since the first discovery of gravitational waves two years ago, astronomers have been working hard to find their counterparts at optical wavelengths. Now with today’s announcement we are seeing the dawn of a new field of ‘multimessenger astronomy’, where detections of transients across the electromagnetic spectrum, along with gravitational waves, are opening up a whole new window on the Universe,” said Dr Fraser.

“The data from ESO telescopes also allowed the team to identify possible signatures of Cesium and Tellurium in the debris of the remains after the neutron stars merged. Finding these elements has been a crucial discovery in understanding the source of the heavy elements in the Universe.

“We can now say, with some confidence, that Cesium and Tellurium on Earth, which are for example used in atomic clocks, electronics and solar panels, would have been produced in a merger of two neutron stars in our own galaxy, more than 4.5 billion years ago.”

Associate Professor Sheila McBreen, UCD School of Physics, a Fermi Gamma-ray Burst Monitor team member, said: “The detection of a short gamma-ray burst associated with a binary neutron star merger is a major step forward and the beginning of an entirely new era in astronomy.

“It has long been inferred that short gamma-ray bursts are the result of the merger of compact objects/stars but it could only be proven by an association with a gravitational wave source. The results published today are a result of long term vision and a sustained investment in fundamental research internationally.”

Prof Lorraine Hanlon, UCD School of Physics, and co-author on a paper published in Astrophysical Journal Letters and a Nature paper focused on the independent detection of gamma-rays from GW170817 by the INTEGRAL satellite, said: “Maintaining access to the gamma-ray sky with more sensitive facilities will be essential to progress the emerging field of multi-messenger astrophysics that has been revealed by these stunning discoveries announced today.

“These discoveries give momentum to the case for supporting Irish astronomers’ participation in international consortia that operate at the frontier of fundamental research, on a scale that cannot be achieved by any single institute or country.”

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