For the first time ever, a group of scientists has observed the abrupt destruction and rebirth of a supermassive black hole’s corona.
Astronomers have observed how the corona of a supermassive black hole, the ring of high-energy particles that surrounds the event horizon, was abruptly destroyed. However, they also observed the rebirth of the corona soon after. In other words, this is believed to be the very first time that astronomers observed a corona disappear and then rebuild itself.
“This will be really important to understanding how a black hole’s corona is heated and powered in the first place.”
The cause of this dramatic transformation is unclear, although the researchers assume that the origin of this phenomenon may have been a star caught in the gravitational pull of the black hole.
Like a stone thrown into a gearbox, the star may have bounced through the black hole’s swirling disk, causing anything in the vicinity, including the high-energy particles in the corona, to suddenly fall into the black hole.
The result, as astronomers observed, was a dramatic and precipitous drop in the brightness of the black hole, by a factor of 10,000, in less than a year.
“We expect that luminosity changes this big should vary on timescales of many thousands to millions of years,” explained Erin Kara, assistant professor of physics at MIT. “But in this object, we saw it change by 10,000 over a year, and it even changed by a factor of 100 in eight hours, which is just totally unheard of and really mind-boggling.”
Kara and co-authors, including lead author Claudio Ricci of the Diego Portales University in Santiago, Chile, have published their findings in the Astrophysical Journal Letters.
In March 2018, an unexpected explosion lit the view of ASSASN (All-Sky Automated Survey for Super-Novae), which examines supernova activity across the entire night sky.
The survey recorded a flash of 1ES1927+654, an active galactic nucleus, or AGN, which is a type of supermassive black hole with higher-than-normal brightness at the center of a galaxy.
ASSASN observed that the object’s brightness jumped to approximately 40 times its normal luminosity.
“This was an AGN that we sort of knew about, but it wasn’t very special,” Kara revealed.
“Then they noticed that this run-of-the-mill AGN became suddenly bright, which got our attention, and we started pointing lots of other telescopes in lots of other wavelengths to look at it.”
Scientists used multiple telescopes to observe the black hole in the X-ray, optical, and ultraviolet wavebands. Most of these telescopes periodically aimed at the black hole, for example, recording observations for an entire day, every six months.
The team also observed the black hole daily with NASA’s NICER, a much smaller X-ray telescope installed aboard the International Space Station.
Thanks to frequent observations, the researchers were able to observe the black hole when it abruptly dropped in brightness, in virtually all of the wavebands they measured, and especially in the high-energy X-ray band, an observation that noted that the black hole’s corona suddenly and completely disappeared.
Scientists are currently unsure exactly what causes a corona to form, but they believe it has something to do with the configuration of the magnetic field lines running through the accretion disk of a black hole.
In the outer regions of the rotating black hole material disk, the magnetic field lines are more or less straightforward.
However, closer to the event horizon, the circles of material with more energy, in a way that can cause the magnetic field lines to twist and break, then reconnect.
This tangle of magnetic energy could spin swirling particles near the black hole, at the high-energy X-ray level, forming the crown-like corona surrounding the black hole.