X-ray binaries are out in the universe making gravitational waves, and Breanna Binder says we may well be on the verge of being able to detect such waves generated in distant star systems. Binder, a recent University of Washington astronomy Ph.D. who did her dissertation about the evolution of X-ray binary systems, gave a talk on the subject at the August meeting of the Seattle Astronomical Society.
Binder noted that it’s a bit of a longshot for an X-ray binary system to form. They start out as a pair of stars ten times or more massive than our own Sun.
“Almost all massive stars are born in binary systems,” she said. “Not only that, massive stars are more likely to be born with massive companions.”
However, these massive stars live relatively short lives and ultimately explode in supernovae. The more massive the star, the more rapidly it evolves, and so the larger of two massive stars in a binary system will be the first to expand into a blue giant. The more it expands, the weaker its gravitational pull on its outer atmosphere will be, enabling the smaller companion to steal some of its mass.
Eventually the larger of the pair goes supernova and leaves behind a compact object: either a neutron star or a black hole. This is often the end of the binary system, as only about one in 10,000 pairs remain gravitationally bound after the supernova. If they do stick together, that’s when the fireworks really get going. The sibling star, having siphoned off some of its companion’s mass, also begins to grow into a blue giant.
“As this happens, material flows from the giant star onto the compact object,” Binder explained, “and when this happens the system starts to heat up. All that material funneling onto the compact object gets incredibly hot and begins to glow in X-rays.”
These are easy for us to spot from Earth.
“These objects will emit X-rays at levels that are tens of thousands to millions of times above what a normal star like our Sun does,” Binder noted.
This high-mass X-ray binary phase doesn’t last long in astronomical terms, perhaps just 10,000 years or so. Eventually the second star goes supernova.
“If the system survives the second supernova explosion, which is a big if, you end up with two compact objects in orbit around each other,” Binder explained. While two neutron stars is the most likely formation, it can also be two black holes or one of each, she said.
With two neutron stars in a system they spiral rapidly around each other, creating powerful gravitational waves. Eventually the two objects merge, creating a big explosion that we can see as a gamma-ray burst. This is the aftermath of the merger of two neutron stars, and it’s also where the new science comes in.
“In the very near future, we’re hoping to be able to detect neutron stars in the process of spiraling into each other before the gamma-ray burst occurs,” Binder said. We will do that by actually detecting gravitational waves using LIGO—the Laser Interferometer Gravitational-Wave Observatory.
The challenge with LIGO is that there’s a lot of noise out there. Anything that moves through space generates gravitational waves. In its first runs LIGO in Richland was able to detect motion from ocean waves breaking on the Washington coast. So scientists have been busy modeling and tweaking, and expect to make the first science runs of a new version of LIGO some time this fall.
“If we’re going to detect gravitational waves, it’s going to happen as soon as we bring advanced LIGO on,” Binder said. “It could easily be within the next year that we are able for the first time to directly detect gravitational waves from the source.” That will give us some early warning about where to look to spot future gamma-ray bursts.
Ultimately the study of these systems will help us better understand stellar formation and evolution.