A simulated map of the Milky Way as it might appear in gravitational waves has given a strong impression of what future space-based detectors will observe.
Over 90 gravitational-wave events have been detected to date by the triumvirate of ground-based detectors — the Laser Interferometer Gravitational-Wave Observatory (LIGO) within the U.S., Virgo in Italy and KAGRA in Japan. All these detected events are mergers of stellar-mass black holes and/or neutron stars in distant galaxies; no gravitational-wave events have been found coming from our Milky Way galaxy.
Nevertheless, our galaxy is stuffed with so-called ultracompact binaries, which was binary stars but which have since evolved to turn out to be stellar remnants.
“Binary systems … fill the Milky Way, and we expect a lot of them to contain compact objects like white dwarfs, neutron stars and black holes in tight orbits,” said Cecilia Chirenti, of the University of Maryland and NASA’s Goddard Space Flight Center, in a statement. “But we’d like a space observatory to ‘hear’ them because their gravitational waves hum at frequencies too low for ground-based detectors.”
Earth-bound observatories resembling LIGO are in a position to detect gravitational waves of frequencies between 5 and 20,000 Hertz. Ultracompact binaries in our galaxy, as they spiral around one another and eventually merge, have frequencies within the range of milliHertz.
Several space-based gravitational-wave detectors are within the works. The European Space Agency‘s Laser Interferometer Space Antenna (LISA) is on the forefront, with launch projected to be within the 2030s, while Chinese scientists even have two mission concepts, named TianQin and Taiji, respectively.
Chirenti is a component of a team, led by Kaitlyn Szekerczes of the Gravitational Astrophysics Laboratory at NASA Goddard, who’ve now simulated the intensity and frequency of gravitational waves emitted by ultracompact binaries within the Milky Way. The resulting image shows how observatories resembling LISA will have the ability to review the Milky Way in gravitational waves similar to astronomers study it in X-rays, gamma rays and so forth. The simulated image shows ultracompact binaries concentrated within the plane of the Milky Way’s spiral disc and spilling out into the galactic halo.
“Our image is directly analogous to an all-sky view of the sky in a specific variety of light, resembling visible, infrared or X-rays,” said team-member James Ira Thorpe, who can be based at NASA Goddard. “The promise of gravitational waves is that we will observe the universe in a very different way, and this image really brings that home.”
To this point, astronomers know of only a handful of ultracompact binaries with orbital periods lower than an hour, which might place the compact objects close enough to one another to emit detectable gravitational waves. Finding them is difficult, because neutron stars and black holes don’t emit much light. That is where LISA will are available: ultracompact binaries should radiate brightly in gravitational waves, allowing LISA to find tens of hundreds of them.
The shorter the orbital period of an ultracompact binary, the upper the frequency and the lower the amplitude of the gravitational waves they emit. In the event that they are really close together, there may even be some mass transfer between the 2 objects that astronomers could follow up on with optical, X-ray and gamma-ray telescopes. Scientists consult with this fusing of electromagnetic and gravitational-wave observations as “multi-messenger astronomy.”
Details of the simulated image were published in a paper in The Astronomical Journal this past June.