In Strange Brightness In Distant Galaxy, Study Deciphers Lessons About Black Holes

In late 2017, a supermassive black hole underwent an identity crisis. The bright object grew a lot brighter over a span of months, & started glowing nearly 100 times more than normal in visible light.

The black hole considered in the study is so luminous that it belongs to a class of black holes known as active galactic nuclei.

Source : NASA

New Delhi: A few years back, scientists observed the occurrence of something strange in a galaxy known as 1ES 1927+654. In late 2017, a supermassive black hole located at the heart of the galaxy underwent an identity crisis. The already bright object grew a lot brighter over a span of months, and started glowing nearly 100 times more than normal in visible light. The black hole is so luminous that it belongs to a class of black holes known as active galactic nuclei. At that time, the reason behind this bizarre occurrence was not known. 

However, astrophysicists, including scientists from the University of Colorado at Boulder, have given an explanation behind the unusual behaviour of the black hole. The findings were published May 5 in The Astrophysical Journal. 

Magnetic Fields ‘Flipped’ Upside Down

According to the study, the magnetic field lines threading through the black hole appear to have flipped upside down, causing a rapid but short-lived change in the cosmic object's properties. It was as though compasses on Earth suddenly started pointing south instead of north, a statement issued by University of Colorado at Boulder said. 

The study, in strange brightness in a distant galaxy, is helping scientists decipher lessons about black holes. According to Nicolas Scepi, a researcher at the University of Colorado at Boulder, the findings could change how scientists look at supermassive black holes. He said normally one would expect black holes to evolve over millions of years. But objects such as the supermassive black hole considered in the study, which are called changing-look active galactic nuclei, evolve over very short time scales. Scientists can understand the rapid evolution by studying the magnetic fields of these black holes. 

In 2021, Scepi and his colleagues at JILA, a joint research institute between University of Colorado at Boulder and the National Institute of Standards and Technology (NIST), first theorised that such a magnetic flip-flop could be possible. A team of researchers at NASA's Goddard Space Flight Center collected the most comprehensive data yet on the far-away object. They drew on observations from seven telescope arrays on the ground and in space, tracing the flow of radiation from IES 1927+654 as the active galactic nucleus blazed bright then dimmed back down.

How Active Galactic Nuclei Are Monsters

The magnetic fields of supermassive black holes may be a lot more dynamic than scientists once believed, the observations suggest. Mitchell Begelman, a JILA Fellow, noted that the active galactic nucleus taken into consideration in the study is probably not alone. He explained that active galactic nuclei are borne out of some of the most extreme physics in the known universe, and are "monsters" which arise when supermassive black holes begin to pull in huge amounts of gas from the galaxies around them. The material will spin faster and faster the closer it gets to the black hole, similar to water circling a drain. This results in the formation of a bright "accretion disk" that generates intense and varied radiation which scientists can view from billions of light-years away. 

The accretion disk generates strong magnetic fields around the central black hole, which is a curious feature. Like Earth's own magnetic field, the black hole's magnetic fields point in a distinct direction, such as north or south. 

How Magnetic Fields Influence Black Hole Dynamics

Jason Dexter, another JILA fellow, said there is increasing evidence from the Event Horizon Telescope and other observations that magnetic fields might play a key role in influencing how gas falls onto black holes. 

The magnetic fields could also influence how bright an active galactic nucleus, like the one at heart of the galaxy studied, looks through telescopes. 

Disconnect Between X-rays And UV Rays

The object's surge in energy had reached a peak by May 2018, the statement said. It was ejecting more visible light and also many times ultraviolet radiation than usual. The active galactic nucleus's emissions of X-ray radiation began to dim around the same time.

Scepi said that normally, if the ultraviolet rises, the X-rays will also rise. But in the case of this active galactic nucleus, the ultraviolet rose, while the X-ray decreased by a lot, which is very unusual, he said.

Why Did The Magnetic Field Of The Active Galactic Nucleus Flip?

In a study published last year in the Monthly Notices of the Royal Astronomical Society: Letters, the researchers at JILA proposed a possible answer for that unusual behaviour. The features of the black hole are constantly pulling in gas from outside space, and some of that gas also carries magnetic fields, Begelman explained. The magnetic field of the active galactic nucleus weakens if it pulls in magnetic fields that point in an opposite direction to its own. For instance, if the magnetic field of the active galactic nucleus points north, and it pulls magnetic fields pointing south, its own magnetic field will weaken. 

Begelman said it is a bit like how a tug-of-war team tugging on a rope in one direction can nullify the efforts of their opponents pulling the other way.

The JILA researchers theorised that with this active galactic nucleus, the black hole's magnetic field got so weak that it flipped upside down. Begelman said this was basically wiping out the magnetic field entirely. 

How A Weak Magnetic Field Changes The Physics Of An Active Galactic Nucleus

The team of researchers led by NASA collected as many observations as it could of 1ES 1927+654, as part of the new study. According to astrophysicists, a weakening magnetic field would cause a change in the physics of an active galactic nucleus, such as a disconnect between ultraviolet and X-ray radiation. The weakening of the magnetic field shifted the black hole's accretion disk so that it ejected more ultraviolet and visible light, and paradoxically, less X-ray radiation. According to the study, no other theory could explain this unusual phenomenon. 

By summer 2021, the active galactic nucleus quieted down, and returned to normal. For Scepi and Begelman, the event is a natural experiment, and a way of probing close to the black hole to learn more about how these objects fuel bright beams of radiation. These lessons about black holes could help scientists know exactly what kinds of signals they should look for to find more strange active galactic nuclei in the night sky.