Month: August 2021

All About Space: What are X-ray binaries?

Earlier this summer, I was asked to contribute to the British popular science magazine All About Space, answering a reader question about X-ray binaries. My write-up was published in the August 2021 issue!

Digital magazine page with text overlaying a picture of a low-mass X-ray binary. Transcript in text on blogpost page. Caption: These pairs can be detected thanks to the X-rays emitted as the smaller star is eaten. Did you know? An X-class solar flare occurred on 2 July 2021 and was the most extreme flare observed since 2017.

What are X-ray binaries?

An X-ray binary has a compact object, like a stellar black hole or a neutron star, and a companion star that is normal like our Sun. The compact object and the companion orbit around each other, bound together by gravity, and they’re close enough that the compact object slowly eats its friend! As it drains material from the companion and gathers it together to eat it, this material gets hotter than the surface of the sun and shines very bright X-ray light.

How the compact object eats its companion depends on how massive the companion is. If the star is bigger than our Sun, it will have big clouds of wind coming off it, and as the compact object passes through the wind, it gobbles up the gas in its path (like Pacman). We cleverly named these systems “high-mass X-ray binaries”, since the companion is big. If the companion is smaller than our Sun, then the compact object swirls the star’s gas around itself, as if the compact object is sitting in the middle of its own dinner plate. This swirling plate of gas is called an accretion disk, and these systems with small companions are called “low-mass X-ray binaries”.

(Header image/inset background credit: ESA. Inset credit: All About Space magazine.)