A rising glow in a distant galaxy could change the way we view black holes | CU Boulder today
Banner Image: Artist’s rendering of a distant galaxy experiencing a sudden increase in brightness. (Credit: NASA/Sonoma State University, Aurore Simonnet)
Something strange is brewing in the galaxy known as 1ES 1927+654: In late 2017, and for reasons scientists couldn’t explain, the supermassive black hole at the heart of this galaxy underwent a massive identity crisis. Within a few months, the already bright object, which is so bright that it belongs to a class of black holes known as active galactic nuclei (AGNs), suddenly became much brighter – glowing nearly 100 times more than normal in visible light.
Now an international team of astrophysicists, including scientists from CU Boulder, can have identified the cause of this change. The magnetic field lines crossing the black hole appear to have reversed, causing a rapid but short-lived change in the properties of the object. It was as if compasses on Earth suddenly started pointing south instead of north.
The conclusions, published on May 5 in The Astrophysical Journalcould change the way scientists look at supermassive black holes, said study co-author Nicolas Scepi.
“Normally, we would expect black holes to evolve over millions of years,” said Scepi, a postdoctoral researcher at JILA, a joint research institute between CU Boulder and the National Institute of Standards and Technology (NIST). “But these objects, which we call shifting AGNs, evolve on very short time scales. Their magnetic fields may be key to understanding this rapid evolution.
Scepi, alongside JILA Fellows Mitchell Begelman and Jason Dexter, first theorized that such a magnetic flip could be possible in 2021.
The new study supports the idea. In it, a team led by Sibasish Laha of NASA’s Goddard Space Flight Center collected the most comprehensive data to date on this distant object. The group relied on observations from seven telescope arrays on the ground and in space, tracing the radiation flux of 1ES 1927+654 as the AGN flared and then waned.
Observations suggest that the magnetic fields of supermassive black holes may be much more dynamic than scientists once believed. And, Begelman noted, this AGN is probably not alone.
“If we’ve seen this in one instance, we’ll definitely see it again,” said Begelman, a professor in the Department of Astrophysical and Planetary Sciences (APS). “Now we know what to look for.”
An unusual black hole
Begelman explained that AGNs stem from some of the most extreme physics in the known universe.
These monsters appear when supermassive black holes start sucking in huge amounts of gas from the galaxies around them. Like water surrounding a drain, this material will spin faster and faster the closer it gets to the black hole, forming a glowing “accretion disk” that generates intense and varied radiation that scientists can see at billions of light years.
These accretion disks also give rise to a curious feature: they generate powerful magnetic fields that wrap around the central black hole and, like Earth’s magnetic field, point in a distinct direction, such as north or south.
“There is growing evidence from the Event Horizon Telescope and other observations that magnetic fields could play a key role in influencing how gas falls on black holes,” said Dexter, an assistant professor at the ‘APS.
What could also influence the brightness of an AGN, like the one at the heart of 1ES 1927+654, is looking through telescopes.
By May 2018, this object’s energy surge had peaked, ejecting more visible light but also much more ultraviolet radiation than usual. Around the same time, X-ray emissions from the AGN began to decrease.
“Normally, if the ultraviolet increases, your X-rays will also increase,” Scepi said. “But here, the ultraviolet has increased, while the X-rays have decreased a lot. It’s very unusual.
Researchers at JILA offered a possible response to this unusual behavior an article published last year.
Begelman explained that these features are constantly pulling in gas from outside space, and some of that gas also carries magnetic fields. If the AGN attracts magnetic fields that point in a direction opposite to its own – they point south, let’s say north – then its own field will weaken. It’s kind of like how a tug of war team pulling a rope in one direction can negate the efforts of their opponents pulling in the other direction.
With this AGN, the JILA team hypothesized that the black hole’s magnetic field became so weak that it flipped over.
“You’re completely eliminating the magnetic field,” Begelman said.
In the new study, NASA-led researchers set out to collect as many observations of 1ES 1927+654 as possible.
The disconnect between ultraviolet rays and X-rays turned out to be the irrefutable proof. Astrophysicists suspect that a weakening of the magnetic field would cause such a change in the physics of an AGN – shifting the black hole’s accretion disk so that it ejects more ultraviolet and visible light and, paradoxically, less X-ray radiation. No other theory could explain what the researchers were seeing.
The AGN itself quieted down and returned to normal in the summer of 2021. But Scepi and Begelman see the event as a natural experiment – a way to probe near the black hole to learn more about the how these objects feed beams of light radiation. This information, in turn, can help scientists know exactly what kinds of signals to look for to find stranger AGNs in the night sky.
“Perhaps there are similar events that have already been observed – we don’t know about them yet,” Scepi said.
Other co-authors of the new study included researchers from the University of Maryland, Baltimore County in the United States; Instituto de Astrofísica de Canarias in Spain; Inter-University Center for Astronomy and Astrophysics in India; Technion in Israel; Space Telescope Science Institute in the United States; National Institute of Astrophysics in Italy; Roma Tre University in Italy; National Autonomous University of Mexico; University of Florence in Italy; and the University of Birmingham in the UK.