Astronomers continue to search for the ultimate cosmic face-off: a pair of supermassive black holes, each with a mass of millions of Suns, warily circling each other and spiraling toward a titanic clash.
ate cosmic face-off: a pair of supermassive black holes , each with a mass of millions of Suns, warily circling each other and spiraling toward a titanic clash. Such mergers are thought to culminate in the universe’s most energetic blasts of gravitational waves, and they must be common to explain how SMBHs, found at the hearts of most galaxies, grow so big. But despite decades of searching, not a single SMBH binary has been conclusively identified.
Astronomers look instead for odd, periodic behavior in light from SMBH accretion disks. One signature might originate in the cooler gases just beyond a disk’s edge. They emit light at specific wavelengths, which the gases’ swirling motion smears into “broad emission lines” through the Doppler effect.
Another tactic is to look for periodic flaring in the overall brightness of an accretion disk, which could be a sign of a disturbance from an SMBH companion. For example, an SMBH on a close but tilted orbit around a companion with an accretion disk might crash through the disk twice per orbit, causing it to flare.
Greene says she isn’t surprised that the hunt for periodic flares hasn’t paid off. Accretion disks are inherently noisy and can flare from other events, such as the SMBH swallowing stars or gas clouds. “There are many candidates, but nobody believes them,” she says. But a solitary SMBH can also mimic that signature if its accretion disk is tilted compared with the spin of the black hole. Through a process known as frame-dragging, the black hole causes the disk’s axis of rotation to swing round, or “precess.” And because jets are thought to align with the axis of the disk, a precessing disk should also produce a corkscrew jet.
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