In a pair of papers in Physical Review Letters and Physical Review D, researchers led by Jordy Davelaar describe a strange object found by its changing light using the Kepler space telescope. Designed to look for the tiny dips in starlight that are seen when a planet orbits its star and blocks the star’s light, Kepler was the perfect tool for finding all kinds of things that flicker and flare in the sky. 

One particular object was spotted getting brighter, dipping slightly, going back to bright, and then fading away. In a delightful case of excellent naming, the researchers dubbed this object ‘Spiky’ because a plot of the object’s light over time appears to spike. 

Through detailed analysis, they figured out that they were watching a pair of massive black holes orbiting one another and taking turns using their gravity to magnify the light and the darkness of the object behind them. As the Event Horizon Telescope (EHT) team highlighted so well this week, the region of black holes has a donut of light surrounding the shadow of the black hole’s dark event horizon. It was that shadow that was causing the dip during the eclipse. Paper co-author Zoltan Haiman explains: That dark spot tells us about the size of the black hole, the shape of the space-time around it, and how matter falls into the black hole near its horizon.

While projects like the Event Horizon Telescope can only see the nearest supermassive black holes and resolve their light and shadow, this technique can be used to study any system with two black holes aligned so that they appear to eclipse one another as they orbit. While there aren’t going to be a high percentage of systems that meet both criteria, the universe is vast. As Davelaar explains: Even if only a tiny fraction of these black hole binaries has the right conditions to measure our proposed effect, we could find many of these black hole dips.

Kepler was an amazing mission, and I’m not sure when new science will stop coming out of its data.

More Information

Columbia University press release

“Self-Lensing Flares from Black Hole Binaries: Observing Black Hole Shadows via Light Curve Tomography,” Jordy Davelaar and Zoltán Haiman, 2022 May 9, Physical Review Letters

“Self-lensing flares from black hole binaries: General-relativistic ray tracing of black hole binaries,” Jordy Davelaar and Zoltán Haiman, 2022 May 9, Physical Review D