What is a kilonova, and why do we care?

A kilonova is an astronomical explosion that occurs when two compact objects (two neutron stars or a neutron star and a black hole) collide.

Among the many reasons they are important to astronomers, one that's particularly compelling is that we do not know much about them. In fact, there has only been one confirmed kilonova event ever observed, GW170817, in August of 2017. Finding more could unlock new insights into compact objects, gravitational waves, and more.

Kilonovae are also interesting because, as we discovered from viewing GW170817, they are a source of the long-confusing short gamma-ray bursts as well as the possible birthplace of all of the universe's heavy elements. That means that the gold in your watch or the silver in your earring was likely created billions of years ago in a kilonova far away, before traveling through space and finding itself in your home!

Observing kilonovae with multi-messenger astronomy

Kilonovae are very rare and faint, making them impossible to find with just a conventional telescope. They do, however, produce intense gravitational waves due to the compact objects' extreme density and high acceleration immediately before collision. Therefore, they can be detected with gravitational wave (GW) detectors.

This is called multi-messenger astronomy, and it represents the cutting-edge in observational astrophysics. First, gravitational wave (GW) detectors discover and locate a kilonova. Then, a conventional telescope can directly observe it in the electromagnetic (EM) spectrum.

On the left is a photograph of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Washington state. There are currently three active GW detectors worldwide, including those in Louisiana and Italy. By 2025, two more detectors in Japan and India are expected to come online.

On the right is the Large Synoptic Survey Telescope (LSST), set to begin full operation in 2023. LSST will dominate optical viewing during its decade of use due to its unparalleled combination of width, speed, and depth. About 90% of its time will be dedicated to a survey mode in which it will observe the entire night sky visible from Cerro Pachon in Chile every three nights, but it will likely be available for target of opportunity (ToO) observation as well, such as kilonova follow-up.

Developing strategies for EM follow-up

Efficient use of LSST will require an intelligent strategy for tiling the GW localization area, which can span hundreds of square degrees, with LSST observations, which cover only 10 square degrees and take about 40 seconds each.

Efforts to construct these strategies have been largely impressive, but have failed to consider differing viewing conditions at different locations in the sky.

For this reason, I set out to create a tool that could predict the viewing conditions at any sky location and any time in LSST's ten-year run. Among the tool’s myriad applications, it is my hope that it will facilitate improvements of tiling strategies by providing a more comprehensive set of information for designing EM kilonova follow-up.