key discoveries

Below is a summary of Dr. Nemmen’s most significant scientific discoveries, written in a way that is accessible to everyone, regardless of their background in astrophysics. These descriptions should help lay people understand how his work is advancing our knowledge of black holes.

Black Holes of all sizes produce energy jets in the same way

One of the biggest mysteries in astrophysics is understanding how black holes produce powerful jets of particles and radiation that shoot out into space. These jets are seen in both stellar black holes—those formed when massive stars collapse—and supermassive black holes at the centers of galaxies. A key question has been whether the same physics applies to all black holes, regardless of their size. Prof. Nemmen made significant progress in answering this question.

By analyzing data from NASA’s Swift and Fermi space telescopes, Prof. Nemmen discovered that black hole jets, whether from small or large black holes, follow the same relationship between the energy carried by the jets and the gamma-ray light they emit. This discovery, published in Science, reveals that all black holes, no matter their size, produce jets with similar efficiency. This finding is crucial because it shows that black holes of all sizes operate in the same way, providing a unified understanding of how these cosmic giants generate some of the most powerful and energetic phenomena in the universe.

Measuring black hole spins in a new way

Before Prof. Nemmen’s work, only one measurement of a supermassive black hole’s spin existed, which was determined through a challenging technique involving the observation of X-rays near the black hole. Prof. Nemmen pioneered a new method that measures black hole spins using observations of material much farther away, more accessible to astronomers. His technique involves comparing the energy that goes into the black hole with the energy that comes out in a powerful jet, which is linked to the black hole’s rotation.

By applying this method to observations from NASA’s Chandra X-ray telescope, Prof. Nemmen increased the number of known black hole spin measurements tenfold. His results were surprising to the scientific community because they showed that supermassive black holes are spinning near the maximum speed allowed by Einstein’s theory. These findings have opened new possibilities for understanding black holes and their role in shaping galaxies.

Understanding low-luminosity active galactic nuclei: faint supermassive black holes

Active galactic nuclei (AGN) is the field of astronomy that studies the centers of galaxies where a supermassive black hole is accreting material and emitting intense radiation. Low-luminosity active galactic nuclei (LLAGN) are an important type of AGN in which the supermassive black holes are not currently consuming a lot of material. As a result, they emit very little light. These black holes are found at the centers of nearby galaxies and are much fainter than other types of AGN, such as quasars. The two black holes that were famously imaged in 2019 and 2022 with the Event Horizon Telescope—M87* and Sagittarius A* in Our Galaxy—are examples of LLAGN.

Prior to Prof. Nemmen’s work, it was understood that some galactic nuclei were not as luminous as quasars; however, a systematic framework for understanding these differences was absent. For instance, the role of black holes with low accretion rates within these systems was not well-defined. Prof. Nemmen’s comprehensive spectral energy distribution modeling, particularly of objects like NGC 1097 and many other LLAGN, provided groundbreaking insights. By establishing LLAGN as a distinct category with radiatively inefficient accretion flows and truncated thin accretion disks, his work led to a more nuanced understanding of the central engines of AGNs.

Shedding light on the gamma-ray secrets of quiet black holes

Prof. Nemmen has made significant contributions to understanding how supermassive black holes generate high-energy gamma rays, particularly in less active systems known as low-luminosity active galactic nuclei (LLAGNs). Using data from the Fermi Space Telescope, he and his team conducted the first major survey of these black holes, analyzing over ten years of observations from 200 different sources. They discovered that only a small fraction of LLAGNs emit strong gamma rays and that powerful radio jets are essential for this emission.

Another of Prof. Nemmen’s key achievements was identifying the gamma-ray counterpart of Sagittarius A*, the supermassive black hole at the center of our galaxy. His work has greatly improved our understanding of how black holes produce energy and interact with their environments, particularly in galaxies like our own.

Harnessing AI for black hole research

Prof. Nemmen and his team have made groundbreaking contributions to the study of black holes, particularly through their innovative use of artificial intelligence (AI). His recent work focuses on integrating AI techniques like convolutional neural networks (CNNs) and neural operators to better understand how matter behaves as it falls into black holes. Traditionally, modeling these behaviors required complex and time-consuming simulations, but Prof. Nemmen’s AI-driven models can produce accurate predictions much faster. This breakthrough allows scientists to explore a wider range of physical scenarios around black holes, significantly advancing our understanding of these mysterious objects.

Beyond theoretical research, Prof. Nemmen’s work also has practical applications in observational astronomy. By using deep neural networks, he can quickly predict the radiation emitted by material around black holes, helping astronomers interpret the data from telescopes more efficiently than ever before. His pioneering approach—combining AI with astrophysics—has the potential to revolutionize how we study black holes and could lead to new discoveries about the universe.

Prof. Nemmen’s research is a shining example of how cutting-edge technology can be used to tackle some of the most challenging questions in science.