Researchers at the University of Michigan, using over 20 years of data from NASA’s Chandra X-Ray Observatory, have made new strides in understanding the enigmatic nature of black holes and the jets they produce. Their recent study delves into the powerful, high-energy jet emitted by the supermassive black hole at the heart of the galaxy Centaurus A. These jets, massive enough to exceed the size of their host galaxies, remain a source of intrigue for astrophysicists due to their complexity and the mysteries they hold.
Since its launch in 1999, Chandra has allowed astronomers to study X-ray signals from these jets, which are also visible through instruments that detect radio waves. However, prior to this study, it seemed that both X-ray and radio observations revealed similar features of jets, offering limited new insights into their behavior. This overlap posed a challenge for scientists trying to understand the intricacies of these cosmic structures.
David Bogensberger, a postdoctoral fellow at the University of Michigan and lead author of the study, emphasized that uncovering the distinct characteristics of jets in different wavelengths is essential for advancing scientific understanding. He noted that “a key to understanding what’s going on in the jet could be understanding how different wavelength bands trace different parts of the environment. Now we have that possibility.”
The findings from this research, recently published in The Astrophysical Journal, suggest that there are notable differences between the X-ray and radio observations of the jets from Centaurus A. Specifically, Bogensberger and his international team discovered that the X-ray data show unique structures not visible in radio wavelengths. “The jet in X-rays is different from the jet in radio waves,” Bogensberger said. This revelation points to a more complex picture of how these jets behave and interact with their environments.
To conduct the study, the team used Chandra’s observations of Centaurus A spanning from 2000 to 2022. They employed a computer algorithm developed by Bogensberger to track bright, uneven features in the jet known as knots. By observing the movement of these knots over time, the researchers were able to measure their speed and behavior.
One particular knot captured the team’s attention due to its extraordinary speed. Observations suggested that it appeared to be moving faster than the speed of light—an optical illusion caused by the knot’s motion in relation to Chandra’s position near Earth. The effect, known as superluminal motion, occurs when an object travels close to the speed of light in a direction that is not perfectly perpendicular to the observer’s line of sight. The shrinking distance between the knot and the observer makes it seem as if it is exceeding light speed. The team estimated that the knot was actually traveling at 94% of the speed of light. This finding contrasted with previous measurements using radio data, which recorded a slower speed of about 80% of the speed of light for a knot in a similar region.
The disparity in the observed speeds between radio and X-ray data indicated that jet knots do not behave uniformly across different wavelengths. “What this means is that radio and X-ray jet knots move differently,” Bogensberger said. This discovery raises further questions about the mechanisms driving the jets and the distribution of energy within them.
Another intriguing result of the study was the location of the fastest-moving knot. Radio observations had previously suggested that the areas closest to the black hole should contain the fastest-moving structures. However, the new X-ray analysis found that the most rapid knot was positioned in an intermediate region—not the closest to the black hole, nor the farthest from it. This finding challenges existing theories about how jets function and distribute energy.
Bogensberger emphasized the need for continued exploration of jets using X-ray observations. “There’s a lot we still don’t really know about how jets work in the X-ray band. This highlights the need for further research,” he said. The team’s methodology opens the door for similar studies of other galaxies. The proximity of Centaurus A, at approximately 12 million light-years from Earth, made it an ideal candidate for this research, as features such as knots are easier to resolve in nearby jets. Nonetheless, Bogensberger expressed his intent to apply this method to more distant galaxies in future work.
These findings mark a significant step forward in the study of astrophysical jets and highlight the importance of multiwavelength observations. By distinguishing between X-ray and radio data, scientists can gain deeper insights into the complex dynamics of black hole jets and their role in shaping the surrounding cosmic environment.
Source: University of Michigan