Astronomers have discovered a binary system composed of two white dwarfs that are orbiting each other every 1201 seconds, or just over 20 minutes. The system, known as J2322+0509, has the third-shortest known period of all detached binaries and is considered an important target for future gravitational wave observation, making it a truly significant discovery.
"Theories predict that there are many double helium-core white dwarf binaries out there," lead author Dr Warren Brown, from the Harvard & Smithsonian Center for Astronomy, said in a statement. “This detection provides an anchor for those models, and for doing future experiments so that we can find more of these stars and determine their true numbers.”
In 2034, the European Space Agency (ESA) will launch the Laser Interferometer Space Antenna (LISA), a giant gravitational wave detector that will orbit in space. LISA is intended to detect different events than current ground-based observatories, with orbiting white dwarfs expected to be among the most common class of detection.
For this reason, the discovery of J2322+0509 is an important step in preparing for LISA. Knowing the position and properties of the expected detection is crucial for future work. The research will be published in The Astrophysical Journal Letters.
"Verification binaries are important because we know that LISA will see them within a few weeks of turning on the telescopes," added co-author Dr. Mukremin Kilic, from the University of Oklahoma. "There’s only a handful of LISA sources that we know of today. The discovery of the first prototype of a new class of verification binary puts us well ahead of where anyone could have anticipated."
The stars were found thanks to follow-up observations from the ESA Gaia observatory, which is being used to piece together the most comprehensive map of the stars in the Milky Way. Unlike many other binaries discovered, the white dwarfs do not pass in front of each other.
"We're finding that the binaries that might be the hardest to detect may actually be the strongest sources of gravitational waves," said Brown. "This binary was difficult to detect because it is oriented face-on to us, like a bull's eye, rather than edge-on. Remarkably, the binary's gravitational waves are 2.5 times stronger at this orientation than if it were orientated edge-on like an eclipsing binary."
Gravitational waves from these objects will eventually cause the demise of the system. With every emission, the two white dwarfs lose a bit of energy, orbiting closer and closer. In about 6 or 7 million years, the pair will have lost so much energy that they will collide, merging into a more massive white dwarf.