Key takeaways

  • A supermassive black hole in galaxy 1ES 1927+654 showed an unexpected flare, 40 times brighter than usual.
  • Over 40 days, the X-ray brightness dropped drastically and then gradually returned, surprising scientists.
  • The fluctuations may result from a star being disrupted and affecting the black hole’s magnetic field.
  • This event provides insights into the mysterious region around black holes known as the corona.
  • The unusual behavior may help uncover new aspects of black hole dynamics and their environments.

Black holes do not glow; in fact, they are known for doing the opposite. However, if they are actively consuming material from the space around them, the material can ignite like a billion X-ray Suns.

And, for the first time, astronomers have watched that blaze suddenly extinguish before gradually returning to life.

The supermassive black hole weighs 19 million solar masses and powers a galactic core in the galaxy 1ES 1927+654, which is located 275 million light-years away.

Over the course of roughly 40 days, astronomers witnessed its corona drop in brightness before rising again to shine brighter than before.

“We expect that luminosity changes this big should vary on timescales of many thousands to millions of years,” said physicist Erin Kara of the Massachusetts Institute of Technology.

“But in this object, we saw it change by [a factor of] 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.”

The area immediately surrounding a black hole consists of multiple components. There’s the event horizon, the well-known “point of no return” at which even light speed is insufficient to achieve escape velocity. An active black hole also features an accretion disc, which is a large disc of material spinning into the object, similar to water circling a drain.

The corona is located immediately outside an active black hole’s event horizon, along the inner edge of the accretion disc.

This is an area of extremely hot electrons considered to be fueled by the black hole’s magnetic field, which acts as a synchrotron, accelerating the electrons to such high energy that they flash brightly in X-ray wavelengths.

Astronomers first noticed something unusual in 1ES 1927+654 in 2018, when the All-Sky Automated Survey for Super-Novae (ASASSN), an automated survey that looks for bright flashes of light across the entire sky, detected an incredibly bright flare from the galaxy that was 40 times its normal brightness.

This piqued astronomers’ interest, and they focused a number of telescopes at the galaxy to learn more. Everything seemed normal for a while, but then, about 160 days after the flare, 1ES 1927+654’s nucleus began to dim. Over the course of 40 days, the X-ray glow was completely extinguished.

“After ASSASN saw it go through this huge crazy outburst, we watched as the corona disappeared,” Kara observed. “It became undetectable, which we have never seen before.”

But then the brightness began to gradually increase again. By 300 days after the original flare, the galaxy’s nucleus was nearly 20 times brighter than before the incident.

“We just don’t normally see variations like this in accreting black holes,” said astrophysicist Claudio Ricci of Diego Portales University in Chile, the study’s principal author.

“It was so unusual that we initially suspected there was something amiss with the data. We were quite excited when we realized it was true. But we also had no notion what we were up against; no one we spoke with had seen anything like it.”

Astronomers do not fully understand how black hole coronae are formed and driven. However, if it is related to the black hole’s magnetic fields, the extreme changes recorded in 1ES 1927+654’s black hole may be produced by anything upsetting those magnetic fields.

We know that black holes can alter quickly when they seize and devour a star that comes too close. The star is ripped apart in a process known as tidal disruption, emitting a burst of intense light before being sucked beyond the event horizon to meet an unknown end.

And if a runaway star managed to collide with 1ES 1927+654’s black hole, the events could explain the observed fluctuations in X-ray emission. First, the star is tidally perturbed, resulting in the initial flare. The debris from the star may have momentarily damaged the black hole’s magnetic field, which was then rebuilt as the space around the black hole returned to normal.

That could be a key indication for determining the radius at which a magnetic field controls a black hole corona.

“What that tells us is that, if all the action is happening within that tidal disruption radius, that means the magnetic field configuration that’s supporting the corona must be within that radius,” Kara told the group.

“Which means that, for any normal corona, the magnetic fields within that radius are what’s responsible for creating a corona.”

If the event was caused by a star, the scientists determined that the tidal disruption required to occur within four light minutes of the black hole’s center, which is roughly half the distance between Earth and the Sun.

However, there could be another cause for the light show.

We know that black hole coronae can vary in brightness, but this usually happens on far longer periods. It’s likely that the extreme behavior witnessed in 1ES 1927+654 is actually rather usual; we just didn’t notice it until now.

“This dataset has a lot of puzzles in it,” Kara informed me. “But this is fascinating because it indicates we’re discovering something new about the cosmos. We think the star hypothesis is correct, but I also believe we will be studying this event for a long time.”

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