New calculations reveal what two supermassive black holes about to merge would look like in ultraviolet and X-rays.
Pairs of humongous black holes likely circle each other in the hearts of many galaxies, brought together over the eons by galaxy mergers. As they come within a fraction of a light-year of each other, they create ripples in the fabric of spacetime that undulate across the universe.
But they should also create bizarre light effects. Unlike the smaller black holes that scientists have “seen” merge with LIGO and Virgo (which have long eaten up any nearby gas), supermassive black hole binaries will be surrounded by disks of glowing gas heated by magnetic and gravitational forces. Each black hole will have its own disk; a second, bigger one will ring both, like a playpen around two girls in tutus, their hands linked as they spin around each other.
Simulations by Manuela Campanelli (Rochester Institute of Technology) and her colleagues previously showed that these three disks engage in an almost constant trading scheme, with gas sloshing between the two mini disks and filaments peeling off the circumbinary disk, either to latch onto a mini disk or be flung back to slam into the big ring. All these changes should create distinct signals — not in gravitational waves, but in light.
Reporting in the October 1st Astrophysical Journal, Stéphane d’Ascoli (RIT), Campanelli, and colleagues take the first step in understanding what the light emitted from these systems looks like. The researchers used the same simulation data as before, this time to figure out how light would look after traveling through the warped spacetime landscape created by the black holes’ extreme gravity. Unfortunately they had to leave out the sloshing and peeling filaments to make things simpler, but they were able to create the stunning video below.
No technology in the foreseeable future will enable us to see supermassive black hole binaries this way. Instead, astronomers have to search for unique patterns in how the glow looks at different wavelengths, and how that emission changes with time. The new simulations indicate that the circumbinary disk, the accretion streams, and the mini disks all radiate primarily in ultraviolet. Some X-rays will also come from the particle haze around the mini disks when gas pours onto them. In less intense accretion situations, X-rays will come from the streams, too.
These simulations don’t yet tell observers exactly what to look for — it’s too soon for that. But they’re an important (and gorgeous) milestone along the way to that goal. The team will next dive into more details, like how the sloshing, flinging gas affects things. The previous calculations suggested flares that last hours to days might occur.
S. d’Ascoli et al. “Electromagnetic Emission from Supermassive Binary Black Holes Approaching Merger.” Astrophysical Journal. October 1, 2018.
D. B. Bowen et al. “Quasi-periodic Behavior of Mini-disks in Binary Black Holes Approaching Merger.” Astrophysical Journal Letters. January 20, 2018.