BRENT: Black holes are paradoxically some of the brightest objects in the universe. It’s not the black holes themselves that are bright; it’s their entourages. Surrounding these spacetime warps are glowing disks of hot gas, which can feed blazing jets and flares.Astronomers regularly watch black holes flare, but they’re not actually sure why flares happen. New X-ray observations suggest that these brilliant surges are a sort of physics-engendered optical effect, created by changes in a mysterious structure called the corona.
The corona is a haze of high-energy electrons that hovers over the black hole and its accretion disk. The electrons whiz around with so much energy that when they collide with ultraviolet photons flying away from the disk, they give a mighty kick that turns those photons into X-rays. These X-rays in turn reflect back off the accretion disk, carrying away spectral fingerprints from the disk that reveal things like how fast the black hole is spinning.
Although astronomers know the corona is there, they don’t know exactly what it looks like — it might be a compact “light bulb” shining on the disk, an extended cloud, or, to steal a metaphor from JPL’s press release, the sandwich bread on either side of the disk’s ham.
Dan Wilkins (Saint Mary’s University, Canada) and colleagues explored what makes a flare happen by observing the active black hole at the center of the galaxy Markarian 335, which lies roughly 350 million light-years away in the constellation Pegasus. Mrk 335 is well known for its fits of brilliance, its X-ray emission rising and falling at whim. Previous observations have hinted that these changes come with changes in the corona’s structure.
