When Two Stars Collide

130 million years ago, two neutron stars collided in a galaxy called NGC4993. This collision created gravitational waves which travelled through the Universe and reached earth, where they were detected on August 17th 2017 by the LIGO/Virgo interferometers. Although there have been 4 detections of gravitational waves before, this detection was different as it is the first time astronomers caught a glimpse of light, or electromagnetic radiation, from this collision in their telescopes. A new era of astronomy is born – Gravitational wave astronomy, and UCD astronomers had a part to play in this historic discovery.

Gravitational waves were first detected in 2015. Einstein predicted as part of his general theory of relativity that when heavy bodies such as black holes or neutron stars accelerate, they create a disturbance which stretches space and time and travels at the speed of light. These waves only move space a tiny fraction – a ruler 13km long would only be distorted by the width of an atom. Therefore the most sensitive detectors ever built are used to detect these waves. These are the LIGO-Virgo interferometers – a team of 3 detectors containing long arms along which a laser is reflected off mirrors. The gravitational waves affect the length of the arms, therefore distorting the laser signals. Any outside movement, even a car passing by, must be taken into account.

We cannot see collisions of black holes as there is no matter to emit light, however it is thought that the collision of neutron stars could be detected by telescopes here on earth. The world has waited for LIGO-Virgo to detect colliding neutron stars so we can begin the search for the light from these collisions. On August 17th, this finally happened and UCD astronomers were on hand to glimpse this collision using telescopes on the ground and satellites like NASA’s Fermi telescope and ESA’s INTEGRAL satellite, which both glimpsed a gamma-ray burst 1.7 seconds after the gravitational waves were detected.

Astronomers in the UCD Space Science Group in the UCD School of Physics, where I am a first year PhD student, had a part to play in these discoveries. UCD Professor Lorraine Hanlon and Dr Antonio Martin-Carrillo are members of the INTEGRAL team and Dr Sheila McBreen is a member of the Fermi team. The observations of a gamma-ray burst soon after the gravitational waves by these satellites proves that short gamma ray bursts, which are bursts of energetic gamma-rays lasting less than 2 seconds, can be created by collisions of neutron stars.

As well as detecting these signals in space, UCD astronomers used telescopes on the ground to observe this event at optical wavelengths. Dr Morgan Fraser is a Royal Society-Science Foundation Ireland University research fellow who analysed the first data from the source obtained from the European Southern Observatories ‘New Technology Telescope’ in Chile. This data is obtained as part of the ePESSTO collaboration, where scientists all over Europe conduct research on astrophysical transients.

UCD’s Watcher robotic telescope in South Africa was undergoing maintenance when this event occurred, however a telescope nearby in Boyden Observatory was used by the UCD Space Science group to obtain data from this event. Combining this data with that obtained by the ePESSTO collaboration resulted in a comprehensive analysis of the event, with staggering results. The data was found to match theoretical models of a kilonova – this occurs when the neutron rich material that is blown out of a neutron star merger comes together to create unstable atoms, which then radioactively decay, emitting a large amount of energy. Analysis of the spectra of the event reveals that some of the heaviest elements like Cesium and Tellurium can be created in these mergers. This discovery helps us to understand the origin of the heaviest elements in the Universe.

With the 2017 Nobel Prize in Physics being awarded to Kip Thorne, Barry Barish and Rainer Weiss for their work with the LIGO interferometers, gravitational wave physics is an exciting and upcoming field to be working in. One year ago myself and my supervisors Prof. Lorraine Hanlon and Dr Antonio Martin-Carrillo proposed my PhD project – to search for optical counterparts to gravitational wave sources. We knew we were taking a risk, and we certainly didn’t expect that in the first week of my PhD that this event would occur. It is an incredibly exciting time to be beginning research in this field.

Now that we have detected electromagnetic radiation from a gravitational wave event, what next? When LIGO/Virgo detect gravitational waves, it is hard to pinpoint where in the sky they came from. LIGO/Virgo send out a sky map containing an ‘error box’, giving the probability that the source is within this box. The error box is quite large and optical telescopes like UCD’s Watcher telescope have small fields of view, meaning it takes a long time to image the whole error box. Therefore these telescopes need a good strategy to find the optical event as quickly as possible. I am very lucky to be an Irish Research Council funded PhD student and my project is to develop a strategy to detect these events in the future using the Watcher telescope. It is a tough task with many complications, but the motivation is clear – to understand more about some of the most energetic processes in our Universe.

Over 3500 scientists on six continents worked together on this discovery. If anything, this discovery showcases that Ireland can compete on an international scale as a country of world-class research. With Ireland joining the European Southern Observatory and Irish scientists involved in collaborations like ePESSTO, Fermi and INTEGRAL, this discovery highlights the benefits of Ireland being involved in international organisations in order to perform cutting edge research. Discoveries like these only come about due to investment in Irish and international research and the motivation to push the limits and look for answers to the most compelling questions. LIGO is now offline to conduct maintenance, however I know that astrophysicists like myself all over the world are getting ready for another one of these events when it comes back online. I can’t wait to see what is discovered next!


Lana Salmon

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