Context: Researchers have proposed to combine the observing power of two future ESA missions, Athena and LISA, to study the effects when two supermassive black holes collide. Currently in the study phase, both missions are scheduled for launch in the early 2030s.

Background:

Supermassive black holes, with masses ranging from millions to billions of Suns, sit at the core of most massive galaxies across the Universe. We don’t know exactly how these huge, enormously dense objects took shape, nor what triggers a fraction of them to start devouring the surrounding matter at extremely intense rates, radiating copiously across the electromagnetic spectrum and turning their host galaxies into ‘active galactic nuclei’.

Athena, the Advanced Telescope for High-ENergy Astrophysics:

Athena will be the largest X-ray observatory ever built, investigating some of the hottest and most energetic phenomena in the cosmos with unprecedented accuracy and depth.

It is designed to answer two fundamental questions: how supermassive black holes at the centre of galaxies form and evolve, and how ‘ordinary’ matter assembles, along with the invisible dark matter, to form the wispy ‘cosmic web’ that pervades the Universe.

Objectives: Athena is going to measure several hundreds of thousands of black holes, from relatively nearby to far away, observing the X-ray emission from the million-degree-hot matter in their surroundings.

LISA, the Laser Interferometer Space Antenna:

LISA will be the first space-borne observatory of gravitational waves—fluctuations in the fabric of spacetime produced by the acceleration of cosmic objects with very strong gravity fields, like pairs of merging black holes.

LISA will detect low-frequency gravitational waves, such as the ones released when two supermassive black holes collide during a merger of galaxies.

LISA will detect the gravitational waves emitted by the spiralling black holes about a month before their final coalescence, when they are still separated by a distance equivalent to several times their radii.

Significance: Scientists expect that a fraction of the mergers found by LISA, especially those within distances of a few billion light years from us, will give rise to an X-ray signal that can be eventually seen by Athena.