Gravitational Wave Pipeline to Warn Scientists of Imminent Neutron Star Merger

Every time a gravitational wave (GW) event is detected using the large interferometer instruments, global astronomers rush to observe the region of the sky where the wobble originated from, using Earth’s most powerful telescopes to see if the event produced electromagnetic (EM) radiation at any wavelength. Now, for the first time, a team of Australian astrophysicists and computer scientists have demonstrated a software tool that can detect GW events at least 10 seconds before they reach their peak intensity.

The Summed Parallel Infinite Impulse Response (SPIIR) pipeline is led by a team at UWA and supported by the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav) and the Gravitational Wave Data Centre (GWDC) at Swinburne University of Technology. Until recently, SPIIR enabled astronomers to identify GW signals very shortly after their detection with the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its European counterpart Virgo.  

Since the first GW detection in 2015, astronomers have sought to observe EM counterparts to GW events in order to better understand the astrophysical mechanisms that cause them. This process has already proven remarkably helpful in one famous case, GW170817, in which a binary neutron star system was observed to merge, emitting a multi-messenger signal in both the GW and EM regimes. By studying both these different signals for the same event, a huge range of science has emerged, with the publication of hundreds of papers related to this one event - including confirmation of how fast GWs travel relative to light, and how some of the heaviest elements in the Universe are formed only through the merger of these massive, compact objects. 

The quest to study the relationship between these EM emissions and their associated space-time disturbances has been dubbed Multi-Messenger Astronomy (MMA), and astronomers have been fighting to minimise the delay between getting the LIGO alert and observing the space around the source of the GW detection.

Space isn’t actually nothingness but rather the four-dimensional fabric of our Universe. Actually jelly is probably a better analogy (and very rigid jelly at that) since space-time - with time being the fourth dimension - is in fact susceptible to waves and wobbling if it is disturbed by sufficiently massive objects. When astronomers talk about gravitational waves they’re referring to an astrophysical event like this, which is so energetic that it creates disturbances in the otherwise rigid space-time that can ripple outwards to distant parts of the Universe.   

This phenomenon was predicted over a hundred years ago by Albert Einstein’s General Theory of Relativity - which describes how space-time is warped by mass - but we’ve only recently been able to detect these unique gravitational signals thanks to advancements in our technology. Even still, these instruments need to be incredibly sensitive to observe GWs, with the feeble signal only registering a change as small as 1/10,000th the diameter of a proton. So far, the LIGO-Virgo collaboration has identified 90 occurrences of these ripples emanating from the mergers of dense objects like black holes and neutron stars.

When a pair of black holes, neutron stars, or a mixed pair (a black hole and a neutron star) orbit too closely, they begin to spiral towards one another until their paths meet and they merge into a single object. The cosmic collisions produce powerful space-time disturbances but LIGO-Virgo can also detect the smaller ripples generated by the inspiral itself, which grow in frequency and amplitude as the objects circle closer.  

The work done by the UWA team, which was published in the Astrophysical Journal Letters (ApJL), involved testing the SPIIR detection pipeline on a simulated dataset designed to represent what the fourth upcoming LIGO-Virgo observing run (O4) will look like. They expect that the pipeline will be able to detect at least one binary neutron star merger per year, roughly 12 seconds before the two dense objects collide.

GW signals can be used to obtain important information about physical properties, like the mass, of black holes and neutron stars before and after they merge. The collision of neutron stars specifically is understood to produce heavy elements like gold and platinum, and other heavy metals which then go on to play an important role in other objects like planets. For example, radioactive decay of heavy elements keeps the interior of Earth warm enough to produce dynamical magma, which in turn drives tectonic plate activity, which is vital for the removal of carbon from the atmosphere, thus making Earth habitable. 

Using the SPIIR pipeline will enable astronomers to observe these energetic events at earlier post-merger times than ever before and help them study the production of neutron-rich materials as well as the birth of newly merged black holes.