Gravitational waves

Gravitational waves (GWs) are ripples in the fabric of spacetime and are produced by some of the most energetic events in our Universe. Albert Einstein, in his General Theory of Relativity in 1916, predicted the existence of GWs caused by massive accelerating objects. However, it took almost a century after this work for the first direct detection of GWs to occur in 2015. The first signal ever detected, called GW150914, was from the collision of two black holes, each around 30 times the mass of the Sun, located about 1.3 billion light years away. Since then, around 90 GW signals, all from the collisions of neutron stars and black holes, have been detected by the global network of GW detectors.

GW detectors are massive interferometers, and act as antennae for GWs travelling from (almost) any direction. There are currently 4 interferometers in the global network: two LIGO detectors (one in Hanford, WA and one in Livingston, LA in the USA) with 4-km long arms, the Virgo detector (near Pisa in Italy) and the KAGRA detector (located in Japan), each with 3-km long arms. By the time the GWs reach us here on Earth, their amplitude is tiny and so detecting these waves can be a huge challenge.

Optical follow-up of gravitational wave events

Even more challenging still, is the observation of a source in both the GW and electromagnetic (EM) spectrum. This is referred to as a multimessenger observation. Collisions of black holes are expected to be EM dark, however, should the collision happen inside an accretion disc (diffuse material orbiting a massive central body) for example, an EM signal is possible. When a binary collision involves at least one neutron star, an EM signal is also expected, such as a gamma-ray burst or a kilonova in the optical/near-infrared. So far, the only GW signal to also have a confirmed EM counterpart is GW170817. With this one multimessenger observation we have been able to learn more about the nature of neutron star mergers, the origin of some heavy metals and found a new way to measure the expansion rate of the universe.

The fourth observing run of the global GW network began on 24th May 2023, and is expected to continue until January 2025. GWs are expected to be detected every few days. When a detection occurs, alerts are sent to the public and telescopes like GOTO will be hunting for those elusive optical counterparts!

GOTO

The Gravitational-wave Optical Transient Observer (GOTO) consists of an array of wide-field optical telescopes, designed and optimised for the follow-up of EM counterparts to GW sources.

A key design feature of GOTO is scalability, and it was designed using "off-the-shelf" components to minimise cost and enhance reproducibility, whilst also ensuring rapid response time to alert triggers alongside balancing sky coverage and depth. GOTO is currently comprised of two different nodes, GOTO-North at Roque de los Muchachos Observatory on La Palma, Canary Islands, and GOTO-South at Siding Spring Observatory, Australia, each hosting two mount systems holding eight fast 40 cm diameter unit telescopes, operating together. As La Palma and Siding Spring are on opposite sides of the world this allows for constant observation, i.e. as one site is closing for the day the other will take over. These locations and observing timeframes allow the GOTO system to survey the entire sky every 2-3 days.

Closeup of GOTO domes on La Palma, showing prototype instrument (10/09/2021) with Milky Way in background. (c) Krzysztof Ulaczyk 2021.

The key focus of GOTO is the rapid response system, targeting the initial localisation of GW sources from LIGO, Virgo and KAGRA. When the GOTO system receives an alert of a GW event, it is able to pivot all telescopes to point to that location within 30 seconds and begin scanning that area for the predicted EM counterpart. More information about the design of GOTO can be found here.

Alongside this rapid response mode, GOTO operates in a survey mode, observing the entire sky every few days, making it an ideal tool for time-domain or "transient" astronomy, monitoring any changes across the sky, such as the appearance of supernovae, variable stars, active galactic nuclei, and many others!

Once images are taken, they are immediately processed and uploaded to the dedicated GOTO Marshall for visual inspection by the team. At the same time, candidate objects for this citizen science project are sent over to the Zooniverse platform, enabling this project to be run in near-real time. We need your help to identify interesting transients that might otherwise be missed, as we have to prioritise speed over completeness and only look at a small subset - see We need you! for more details.

Kilonovae, supernovae, and the transient zoo

The night sky is far from static -- all around we see things change on timescales of hours, days, weeks, and months. GOTO is particularly interested in finding:

• Kilonovae: radioactive afterglows of the merger between two neutron stars, that we think are the places in the Universe where heavy elements like gold and platinum are forged.
• Supernovae: explosive deaths of massive stars and white dwarf stellar remnants. These events are key to our understanding of stellar evolution, and have been at the heart of projects like measuring the expansion of the Universe because of how bright they are.

We need you!

The data volumes involved are too vast for the GOTO team to be able to look through all candidates themselves, so we're forced to prioritise looking at a small subset that our machine learning classifiers tell us is likely to be interesting. Although we've spent lots of time optimising our classifiers for maximum performance, they still get things wrong -- we're most curious about the objects that our classifiers are highly uncertain about, as these are likely weird and therefore scientifically interesting! Recruiting your keen eyes, we're able to look at more uncertain candidates, thus finding transients that might be missed otherwise. With your help, we can find these new transients quickly enough that we can trigger follow-up observations and learn what we've discovered together.

GOTO-North and GOTO-South continually survey the night sky every clear night, generating thousands of images. Every hour when the project is live (and the weather is good), we'll upload fresh candidates from the past 24h of GOTO operations for you to look through -- with much of it having not been seen by human eyes so far. You could be the first to spot distant cosmic explosions, just a few hours after the data has been taken. You'll also be making huge contributions to our understanding of how well humans can classify transient events, and building one of the largest labelled datasets in time-domain astrophysics. We're excited to see what hidden gems you discover in the GOTO datastream along the way!