You may have heard about yet another breakthrough in gravitational-wave astronomy. LIGO and Virgo have detected more gravitational waves, but this time from an entirely different source. This new source also meant scientists at LCO were able to use the telescope network to contribute follow-up observations to this exciting new discovery.

Gravitational waves are what we call ripples in space-time, much like when we drop a pebble into water and ripples spread outwards from the point of entry. Gravitational waves were predicted by Albert Einstein in 1915 as part of his theory of general relativity, but it wasn’t until last year that it was confirmed we had directly detected these waves.

Gravitational waves are extremely hard to detect, they require extremely sensitive instrumentation and even then we are only able to detect extremely massive accelerating objects. We know that more massive objects have greater gravitational fields, therefore the more massive the object and the more rapid the acceleration, the greater the ripples in space-time. The ideal situation would be two black holes spiralling in towards each other, churning up space-time as the approach the point of impact.

There are two main detectors searching for gravitational waves, Virgo and LIGO. These are interferometers each comprising of two tunnels, several kilometres long that form a giant ‘L’ shape. Laser beams are generated and sent down each of the tunnels, where they are reflected back and forth from mirrors, causing them to recombine at the initial start point. If there are distortions in space-time due to gravitational waves, the time it takes for the laser beam to reach the end of the tunnel and back will change. It is these absolutely tiny differences that scientists look for as evidence of gravitational waves.

LIGO was responsible for the first detection of gravitational waves last year that resulted from the collision of two black holes. These weighed in at approximately 29 and 36 times the mass of our Sun!

However, black holes are so-called because their incredible masses mean nothing can escape their gravitational pull, not even light. As a result, we couldn’t really make any follow-up observations of the event.

That’s what makes the most recent discovery so ground breaking. For the first time, scientists have observed a “kilonova”, the result of the collision of two neutron stars. The image below is an illustration of this process. Neutron stars are the collapsed cores of massive stars (i.e. stars more than 8x the mass of the Sun). Although they are very small in size
(~10 km in diameter) they are extremely dense, packing in around the mass of 1.5 suns – in fact a teaspoon of material from a neutron star would weigh approximately 10 million tonnes!

But most importantly, neutron stars explosions are observable with normal telescopes, so the kilonova (given the catchy name of AT 2017gfo) provided an optical counterpart that we could observe. Scientists at LIGO and Virgo were able to localise the source of these gravitational waves and figure out roughly where in the sky the kilonova would be – but they were not sure exactly where.

Astronomers at LCO then used the GLADE catalog (Galaxy List for the Advanced Detector Era) to identify galaxies in this region of the sky and then applied an algorithm to identify which of these galaxies would be most likely to host a kilonova based on properties such as their distance and luminosity.

This identified 182 possible host galaxies, of which the 5^th in the list turned out to be the correct source. As a result, some of the earliest observations of the first ever optical counterpart of a gravitational wave detection source were taken using the LCO network! In addition, due to LCO’s global network, there is always at least one telescope in the dark somewhere on the planet at all times. This allowed astronomers to take multiple observations of the kilonova. The optical counterpart is shown in the image below as seen by Hubble.

LCO scientists were also able to take spectra of the kilonova, and using the spectrograph on Faulkes Telescope South in Australia they were able to observe its evolution over time. They were able to combine their data with observations from other telescopes and revealed an almost featureless spectrum that could not be completely explained by a blackbody model (blackbodies are opaque objects that emit thermal radiation). It also displayed a very rapid cooling, changing from blue (hot) to red (cool) in just three days.

The Faulkes Telescopes and LCO network are contributing to some of today’s most significant breakthroughs in astronomy, resulting in several exciting observations.

The papers that have been published as a result can be accessed at the following links:

Optical follow-up of gravitational-wave events with Las Cumbres Observatory – https://arxiv.org/abs/1710.05842

The rapid reddening and featureless  optical spectra of the optical counterpart of GW170817, AT 2017gfo, during the first four days – https://arxiv.org/abs/1710.05853

Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger – http://www.nature.com/nature/journal/vaop/ncurrent/full/nature24291.html?WT.feed_name=subjects_physics

To hear more from some of the astronomers working on this research at LCO you can check out their video here: https://www.youtube.com/watch?v=aLCl2PpV-wo

For more on gravitational waves, what they are and how we detect them, see here: https://www.youtube.com/watch?v=4GbWfNHtHRg

Image Credits:

Optical Counterpart – Hubble Space Telescope, NASA and ESA –
https://www.nasa.gov/press-release/nasa-missions-catch-first-light-from-…, Public Domain, https://commons.wikimedia.org/w/index.php?curid=63409545

Artist impression of neutron binary star system: NASA