NASA’s Bengali scientist discovers Black Hole’s magnetic reversal

Whether it's a rare blood supermoon, a Moon with a pinkish hue, or a near-total lunar eclipse – human beings have always been fascinated by what’s happening in the sky. So why are people so intrigued? According to Nelson-based space scientist Duncan Steel, space is spectacular, continues to surprise people, and there are a lot of unanswered questions out there. Scientists have been studying different aspects of space to understand and unravel its myriad mysteries.

Recently, a team of astrophysicists from UC San Diego’s Center for Astrophysics and Space Sciences (CASS), NASA Goddard Space Flight Center (GSFC), the University of Maryland, and Leibniz Institute for Astrophysics Potsdam, researching what is going on in the centers of “active galaxies,” stumbled upon a rare and enigmatic outburst from a galaxy 236 million light-years away that led the scientists to probe the matter further. They concluded the event was sparked by a magnetic reversal, a spontaneous flip of the magnetic field surrounding its central black hole that triggered changes in the black hole’s environment. The bright burst from the galaxy identified as 1ES 1927+654 is believed to have occurred in late 2017 following which NASA scientists undertook regular observations using a battery of satellite-based and ground-based optical and radio telescopes. 

Rapid changes in visible and ultraviolet light have been seen in a few dozen galaxies similar to this one, but this event marks the first time that the team of researchers under Dr. Sibashish Laha, saw X-rays dropping out completely while the other wavelengths brighten. In early March 2018, the All-Sky Automated Survey for Supernovae alerted astronomers that a galaxy called 1ES 1927+654 had brightened by nearly 100 times in visible light. A search for earlier detections by the NASA-funded Asteroid Terrestrial-impact Last Alert System showed that the eruption had begun months earlier, at the end of 2017.

When the galaxy was first examined in May 2018, its UV emission was elevated by 12 times but steadily declining, indicating an earlier unobserved peak. Then, in June, the galaxy’s higher-energy X-ray emission disappeared. Most big galaxies, including our own Milky Way, host a supermassive black hole weighing millions to billions of times the Sun's mass. When matter falls toward one, it first collects into a vast, flattened structure called an accretion disk. As the material slowly swirls inward, it heats up and emits visible, UV, and lower-energy X-ray light. Near the black hole, a cloud of extremely hot particles – called the corona – produces higher-energy X-rays. The brightness of these emissions depends on how much material streams toward the black hole.  

An earlier interpretation of the eruption suggested that it was triggered by a star that passed so close to the black hole it was torn apart, disrupting the flow of gas, but the new findings show that such an event would fade out more rapidly than this outburst. The unique disappearance of the X-ray emission provides astronomers with an important clue. They suspect the black hole’s magnetic field creates and sustains the corona, so any magnetic change could impact its X-ray properties. The field initially weakens at the outskirts of the accretion disk, leading to greater heating and brightening in visible and UV light, and a magnetic reversal, where the north pole becomes south and vice versa, seems to best fit the observations.

As the flip progresses, the field becomes so weak that it can no longer support the corona – the X-ray emission vanishes. The magnetic field then gradually strengthens in its new orientation. In October 2018, about 4 months after they disappeared, the X-rays came back, indicating that the corona had been fully restored. By the summer of 2021, the galaxy had completely returned to its pre-eruption state. Magnetic reversals are likely to be common events in the cosmos. The geologic record shows that Earth’s field flips unpredictably, averaging a few reversals every million years in the recent past. The Sun, by contrast, undergoes a magnetic reversal as part of its normal cycle of activity, switching north and south poles roughly every 11 years.

The findings were accepted for publication in ‘The Astrophysical Journal’ in May 2022. The research team analyzed new and archival observations across the spectrum. NASA’s Neil Gehrels Swift Observatory and ESA’s (European Space Agency) XMM-Newton satellite provided UV and X-ray measurements. Visible light observations came from Italy’s 3.6-meter Galileo National Telescope and the 10.4-meter Gran Telescopio Canarias, both located on the island of La Palma in the Canary Islands, Spain. Radio measurements were acquired from the Very Long Baseline Array, a network of 10 radio telescopes located across the United States; the Very Large Array in New Mexico; and the European VLBI Network.

With this groundbreaking discovery, Dr. Laha, leader of the research team from the University of Maryland, Baltimore County, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, added another feather to his hat when he was selected by NASA for the prestigious NASA Exceptional Scientific Achievement Medal (abbreviated ESAM). Laha is the first Bengali scientist to receive this honour. Established by NASA on September 15, 1961, the ESAM is awarded for unusually significant scientific contributions toward the achievement of aeronautical or space exploration goals. Some of the stalwart recipients of the honour include Nobel Laureates Mario Molina (1995 Nobel Laureate in Chemistry), George Smoot, and John C. Mather, who is also a senior astrophysicist at the NASA Goddard Space Flight Center (GSFC) and adjunct professor of physics at the University of Maryland College of Computer, Mathematical, and Natural Sciences. In fact, Dr. Mather is Laha’s current mentor and collaborator.

Sibashish has come a long way from the cozy environs of his school, Don Bosco Lilluah, Scottish Church Collegiate School, and Presidency College Kolkata. He currently works at NASA Goddard Space Flight Centre, Maryland and his research areas include multi-wavelength astronomy involving Radio, optical, UV, and X-ray observations of extra-galactic objects. Mostly he is interested in working on the properties of supermassive black holes at the center of active galaxies. 

Nearly every galaxy has a giant black hole at its center, and some are voracious eaters that gobble up nearby gas and dust. All of this moving material heats up and emits a huge amount of light from the accretion disk— the blurry halo of flowing gas and dust that surrounds a black hole. Such systems are called active galactic nuclei (AGN).

The regions around these central black holes appear so small and far away that they can’t be seen directly. So, astronomers like Laha look for signatures in their light as a way to understand what kind of matter is present. By examining how the signatures change over time, scientists can trace gas as it moves across the line of sight, and thereby learn where different structures are near the black hole.

The study also confirmed that gas and dust in the host galaxy itself play a critical role in how scientists study these AGN systems. According to Laha, the significance of this finding is how it helps scientists better understand how the matter around the vicinity of the supermassive black hole is distributed, which is otherwise not observable to current-generation telescopes due to the lack of spatial resolution. According to him, this matter, in the form of high-density clumps and continuous media, is responsible for feeding the 'monster' black hole and thereby keeping the AGN ‘active’ and bright. Laha says the most surprising result of the study was the finding that not all AGN show signatures of moving clouds along the line of sight on timescales of days – years, implying that the clouds may be distant or continuous, hence no variability. This means that the feeding nature of the giant black holes is not the same across different AGN.