Black Hole collisions illuminated by X-ray-gravitational wave tag-team

Multimessenger astronomy promises to shed light on collisions between massive black holes within the next ten years.

An artist’s impression of a collision between two black holes. Within the next ten years, Athena and LISA will enable astronomers to ‘see’ these vents in great detail. (Mark Garlick/ Getty Images)

Collisions between massive black holes should be observable by both gravitational wave and X-ray observatories by the start of the next decade, new research from the University of Birmingham reveals. This co-observation represents the fulfilment of the promise of ‘multi-messenger’ astronomy opened up by the first measurement of gravitational waves by LIGO/VIRGO in 2015.

The study, published in Nature Astronomy, points to the coordination of two of the European Space Agency’s (ESA) major space observatories of the 2030s — Athena, the next-generation X-ray space telescope and LISA, the first space-based gravitational wave observatory. The two projects will be timed to launch so they can be coordinated to begin observations within a year of each other. This will kick-start a period of at least four-years of syncronised science operations and the promise of delivering data on events that no-single technique in astronomy could collect.

This should give astronomers an unprecedented opportunity to create ‘multi-messenger maps’ of the universe with objects and events seen in both the electromagnetic and gravitational wave spectrums. As such, this will enable us to ‘see’ violent events in the Universe that have previously been invisible. Events which could hold the key to unlocking long-standing mysteries in our understanding of cosmic evolution.

These events include collisions between supermassive black holes in distant galaxies and massive black holes consuming compact stellar remnants such as neutron stars and even, more diminutive black holes.

LISA and Athena — the ultimate multi-messenger tag-team

The two ESA projects will combine with LISA using gravitational-wave measurements to locate the ripples in spacetime caused by mergers, whilst Athena uses X-ray emissions to detect physical processes that create hot, highly energetic environments. This multi-messenger observation of the same phenomena should lead to a massive leap in our understanding of several outstanding cosmic mysteries that have puzzled astronomers for decades. This includes, but is not limited to, how black holes and galaxies co-evolve, the role of gas and dust around black holes, and how massive black holes accrete material and grow to supermassive status.

An artist’s impression of the LISA interferometer in orbit around Earth collecting gravitational wave data (LISA)

“The prospect of simultaneous observations of these events is uncharted territory and could lead to huge advances,” says Dr Sean McGee, Lecturer in Astrophysics at the University of Birmingham and a member of both the Athena and LISA consortiums, who led the study in question. “This promises to be a revolution in our understanding of supermassive black holes and their growth within galaxies.”

Professor Alberto Vecchio, Director of the Institute for Gravitational Wave Astronomy, University of Birmingham, and a co-author on the study, has worked on the LISA project for twenty years. He sees the double-punch of LISA/Athena as a fulfilment of his and his colleagues’ hard work: “The prospect of combining forces with the most powerful X-ray eyes ever designed to look right at the centre of galaxies promises to make this long haul even more rewarding.”

A conceptual design for the Athena spacecraft derived from the ESA CDF study, designed to be accommodated in an Ariane 5 launcher. (ESA)

Researchers are currently unsure just how many black hole collisions will be available for the LIGO/Athena team-up to observe over its lifetime. Current estimates suggest that there could be as many as 10 mergers of black holes ranging in size from 10⁵ to 10⁷ solar masses — up to 10 million times the mass of our Sun — both the final number of observed events could be more or less than this number. In fact, because we are so unsure of the unlying physics of these events and just how frequent they are, the only way to really learn more is to begin observations.

Vecchio concludes: “It is difficult to predict exactly what we’re going to discover: we should just buckle up because it is going to be quite a ride.”