Abstract
Our existence in the Universe resulted from a rare combination of circumstances. The same must hold for any highly developed extraterrestrial civilisation, and if they have ever existed in the Milky Way, they would likely be scattered over large distances in space and time. However, all technologically advanced species must be aware of the unique property of the galactic centre: it hosts Sagittarius A* (Sgr A*), the closest supermassive black hole to anyone in the Galaxy. A civilisation with sufficient technical know-how may have placed material in orbit around Sgr A* for research, energy extraction, and communication purposes. In either case, its orbital motion will necessarily be a source of gravitational waves. We show that a Jupiter-mass probe on the retrograde innermost stable circular orbit around Sgr A* emits, depending on the black hole spin, at a frequency of fGW = 0.63–1.07 mHz and with a power of PGW = 2.7 × 1036–2.0 × 1037 erg/s. We discuss that the energy output of a single star is sufficient to stabilise the location of an orbiting probe for a billion years against gravitational wave induced orbital decay. Placing and sustaining a device near Sgr A* is therefore astrophysically possible. Such a probe will emit an unambiguously artificial continuous gravitational wave signal that is observable with LISA-type detectors.
Highlights
Our existence in the Universe resulted from a rare combination of circumstances
The “Galactic Centre Messenger” is a thought experiment that originated from the question, what kind of a mass on what kind of an orbit around Sgr A* would produce a measurable gravitational wave signal and what kind of an energy would be required to make that signal continuous
We show by means of a few simple calculations that the energy supply of one solar mass can sustain one Jupiter mass in an orbit for one billion years
Summary
Our existence in the Universe resulted from a rare combination of circumstances. The same must hold for any highly developed extraterrestrial civilisation, and if they have ever existed in the Milky Way, they would likely be scattered over large distances in space and time. We argue below that from fundamental laws of Physics and logical deduction it follows necessarily that such a Messenger should ideally be a Jupiter-mass black hole, orbiting Sgr A* for a few billion years at the retrograde innermost stable circular orbit (ISCO), and naturally emitting gravitational waves with the frequency fGW = 0.63 - 1.07 mHz and power PGW = 2.7 × 1036 - 2.0 × 1037 erg/s, depending on the black hole spin.
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