Black holes don’t shine; in fact, they are famous for doing the opposite. But if they are actively devouring material from the space around them, that material can burn like a trillion X-ray suns.
And for the first time, astronomers have now seen that mysteriously extinguished fire, before gradually returning to brightness.
The supermassive black hole is a beast that registers 19 million solar masses, feeding a galactic nucleus 275 million light years away, in a galaxy called 1ES 1927 + 654.
Over a period of only 40 days, astronomers watched as its crown absolutely plummeted in brightness, before rising again to shine brighter than before.
“We expect such large changes in luminosity to vary on time scales of thousands to millions of years,” said physicist Erin Kara of the Massachusetts Institute of Technology (MIT).
“But in this object, we saw it change by [a factor of] 10,000 for a year, and it even changed by a factor of 100 in eight hours, which is totally unknown and really amazing. “
There are various components in the area immediately around a black hole. There is the horizon of events; that is the famous “point of no return”, in which even the speed of light is not enough to reach the escape velocity. An active black hole also has an accretion disk, a huge disk of material that swirls around the object, like the water surrounding a drain.
And just outside the event horizon of an active black hole, around the inner edge of the accretion disk, is the crown.
This is a region of scorching electrons believed to be powered by the black hole’s magnetic field, acting as a synchrotron to accelerate the electrons to such high energies that they glow brightly at wavelengths of X-rays.
Astronomers first noticed something strange was happening on 1ES 1927 + 654 in 2018, when the All-Sky Automated Survey for Super-Novae (ASASSN), an automated survey looking for bright flashes of light across the sky, caught a flare. Incredibly bright galaxy, 40 times its normal brightness.
This caught the attention of astronomers, and they pointed a bunch of telescopes in the direction of the galaxy for more information. For a time, everything was quite normal, but then, around 160 days after the eruption, the core of 1ES 1927 + 654 began to dim. Over a period of 40 days, the brightness of the X-rays was completely extinguished.
“After ASSASN saw him go through this huge crazy bang, we saw the crown disappear,” Kara said. “It became undetectable, which we’ve never seen before.”
But then the brightness began to constantly rise again. At 300 days after the initial eruption, the core of the galaxy shone almost 20 times brighter than it had been before the initial event.
“Normally we don’t see variations like this in the accumulation of black holes,” said astrophysicist Claudio Ricci of the Diego Portales University in Chile, and lead author of the study.
“It was so strange that at first we thought maybe there was something wrong with the data. When we saw it was real, it was very exciting. But we also had no idea what we were dealing with; no one we spoke to had seen anything like this.”
Astronomers are not completely sure how the black hole crowns are generated and powered. But if, as theorized, it has something to do with the black hole’s magnetic fields, then the dramatic changes observed in the black hole of 1ES 1927 + 654 could have been caused by something that disrupted those magnetic fields.
We know that black holes can change fairly quickly when they capture and devour a star that ventures too close. The star rips apart in a process called tidal disruption, screaming a blaze of bright light, before being dragged beyond the event horizon to fulfill its mysterious destiny.
And if a runaway star encounters the black hole from 1ES 1927 + 654, the events could adjust to the observed changes in X-ray radiation. First, the star is disrupted by the tide, causing the initial flare. Then the debris from the star could have temporarily disrupted the black hole’s magnetic field, after which it rebuilt itself as the space around the black hole was restored to a more normal state.
And that could be an important clue to understanding the radius within which a magnetic field controls a black hole corona.
“What it tells us is that, if all the action is taking place within that tidal interruption radius, that means that the configuration of the magnetic field that supports the corona must be within that radius,” Kara said.
“Which means that, for any normal corona, the magnetic fields within that radius are responsible for creating a corona.”
If a star was responsible for the event, the team calculated that the tidal disruption must have occurred within four light minutes of the center of the black hole, about half the distance from Earth to the Sun.
But there could be some other reason for the light show.
We know that the crowns of black holes can vary in brightness, although this generally occurs on much longer time scales. It is possible that the extreme behavior observed in 1ES 1927 + 654 is actually quite normal behavior as well, we just haven’t seen it so far.
“This data set has a lot of puzzles,” said Kara. “But that is exciting, because it means that we are learning something new about the universe. We think the star hypothesis is good, but I also think we are going to analyze this event for a long time.”
The research has been published in The letters of the astrophysical journal.
.
