A University of Michigan-led study has analyzed over 20 years of data from NASA's Chandra X-Ray Observatory, uncovering new complexities around black holes.
The Chandra X-ray Observatory reveals the jet of Centaurus A, extending into the upper left corner of the image. Researchers have found new insights in the jet by focusing on the motion of the bright spots, or knots, within the jet. Image Credit: D. Bogensberger et al. Astrophys. J. (2024) DOI: 10.3847/1538-4357/ad73a1
The study primarily examines the high-energy jet of particles blasted through space by the supermassive black hole at the center of the galaxy Centaurus A. These jets are visible across various wavelengths, from radio waves to X-rays, with Chandra's data attracting particular interest since its 1999 launch due to unusually bright X-ray emissions from jets.
However, previous observations suggested that X-rays were essentially capturing the same features as seen in radio waves, which limited deeper understanding.
Jets are massive cosmic structures—some are larger than their host galaxies—that still harbor many mysteries. If a jet looks the same to different instruments, that does not do any favors for those working to unravel these astrophysical puzzles.
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.
David Bogensberger, Study Lead Author and Postdoctoral Fellow, University of Michigan
The new study represents an important contribution to a growing area of research focused on identifying subtle, significant differences between radio and X-ray jet observations.
Bogensberger added, “The jet in X-rays is different from the jet in radio waves. The X-ray data traces a unique picture that you can’t see in any other wavelength.”
In their study, the team analyzed Chandra’s data on Centaurus A, spanning from 2000 to 2022. To accomplish this, lead researcher David Bogensberger developed a specialized computer algorithm that could track bright, irregular features within the jet, known as "knots." By monitoring the movement of these knots over time, the team was able to calculate their speeds.
One knot’s speed was especially impressive. It appeared to exceed the speed of light when viewed from Chandra’s perspective near Earth, due to an effect where the distance between the knot and Chandra decreases nearly as quickly as light can travel.
The researchers concluded that the knot’s actual speed was at least 94 % the speed of light. A knot in a similar location had previously had its speed measured using radio observations. That result clocked the knot with a significantly slower speed, about 80 % of the speed of light.
“What this means is that radio and X-ray jet knots move differently,” Bogensberger stated.
This wasn’t the only surprising finding in the data.
For instance, previous radio observations indicated that the knots nearest to the black hole tended to move the fastest. However, in this new analysis, Bogensberger and his team identified the fastest-moving knot in a middle region—neither the closest to the black hole nor the farthest from it.
Bogensberger further added, “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. We have shown a new approach to studying jets and I think there’s a lot of interesting work to be done.”
Bogensberger plans to apply the team’s approach to studying other jets.
The jet in Centaurus A holds particular significance because, at roughly 12 million light-years away, it’s the closest known jet to Earth. This proximity made Centaurus A an ideal initial target for testing and validating their methodology. In more distant jets, resolving specific features like knots becomes increasingly challenging.
Bogensberger concluded, “But there are other galaxies where this analysis can be done. And that’s what I plan to do next.”
Journal Reference:
Bogensberger, D. et. al. (2024) Superluminal Proper Motion in the X-Ray Jet of Centaurus A. The Astrophysical Journal. doi.org/10.3847/1538-4357/ad73a1