Researchers at the Harvard and Smithsonian Center for Astrophysics, along with colleagues at the University of Oklahoma and the University of Montreal, discovered a pair of white dwarf stars that orbit each other and produce gravitational waves — the first wave source of this type ever found.
The discovery — published April 6 in The Astrophysical Journal Letters, a journal where astrophysicists can submit short notices of original research — marks the first confirmed source of gravitational waves from a double helium-core, white dwarf system. Named J2322+0509, the pair exhibits an orbital period of just over 20 minutes, the third shortest period of all detached binaries detected so far.
Einstein’s century-old theory of general relativity predicted the existence of gravitational waves – ripples in spacetime – emitted by two masses accelerating around each other, but these waves were only verified experimentally a few years ago.
“Astronomy was done almost exclusively by collecting light with telescopes,” said Abraham “Avi” Loeb, the chair of the Astronomy department. “But here is a new way of looking at the universe that we now have, which is gravitational waves.”
When stars like the sun burn up all of their fuel, they become white dwarfs, or compact stellar cores, according to Loeb.
Now that gravitational waves have been confirmed to originate from this binary system, this “verification binary” can help train the Laser Interferometer Space Antenna gravitational wave observatory, set to launch in 2034.
“We know this is a system that LISA will detect shortly after it turns on,” according to Warren F. Brown, lead author on the study and an astronomer at the Center for Astrophysics.
Since white dwarfs produce gravitational waves at the same frequencies LISA would be able to detect, they could potentially interfere with the detection of signals from other sources, such as the merging of black holes. This makes studying white dwarf systems all the more important, according to Loeb.
The researchers plan to continue studying the system and monitor the shrinking of the binary’s orbit.
“We are planning to observe, follow the system for the next 10 years, and we can actually see how the period is changing over time,” Mukremin Kilic said. Kilic is a co-author of the study and a physics and astronomy professor at the University of Oklahoma.
Brown said the white dwarf system was initially difficult to detect, due to the absence of a “light curve.” He noted, however, that the binary’s face-on orientation to the Earth resulted in gravitational waves over twice as strong as those that would have resulted from an edge-on alignment.
“It’s a fun fact, some of the most difficult-to-detect binaries may actually be the strongest gravitational wave sources. There might be some among the brightest, nearby stars, we just haven’t detected them,” Brown said.
Though the coronavirus pandemic has closed all telescopes and mandated a shift to remote research, Brown said he and his team remain dedicated to finding similar white dwarf systems.
“But just having one is an interesting anchor on the true number of systems in the galaxy,” Brown said.
“The nice thing about astronomy is that you can discover things that are unusual,” Kilic said.“There are so many unknowns in astronomy still, but there’s a lot of space for new discoveries.”