X-rays from black holes challenge the particle acceleration model

X-ray emissions from black hole jets are a recent finding. In fact, how the particles accelerate to their high-energy state remains a mystery. Today, a study published in Nature Astronomy dismisses one of the main proposed theories and opens the door to a new understanding of how particle acceleration works in jets (and possibly elsewhere in the universe as well).

One of the main models on the generation of X-rays in the jets predicts that their emissions remain stable for millions of years. But the new work has found that the X-ray emissions from a statistically significant number of jets vary over a few years. One of the reasons we’re excited about the variability is that there are two main models for how X-rays are produced in these jets, and they’re complete opposites.

One model invokes very low-energy electrons and another has very high-energy electrons. And one of those models is completely incompatible with any kind of variability. The team analyzed almost all of the black hole jets for which the Chandra X-ray Observatory (the highest-resolution X-ray observatory available) had observations. Along with the stability of X-ray emissions over time, the simplest theory for how jets generate X-rays assumes that particle acceleration occurs at the center of the galaxy, in the “engine” of the black hole. that propels the jet.

But the new study has detected rapid changes in X-ray emissions throughout the jets, suggesting that particle acceleration occurs throughout the jet, far from its black hole origin.

There are theories about how this might work, but many of the theories we’ve been working with are now clearly incompatible with our observations. Interestingly, the results also suggest that the jets closest to Earth show more variability than those farther away. These are so far away that by the time the light from them reaches the telescope, it’s like looking back in time and that older jets have less variability since earlier in the universe’s history the universe was smaller and ambient radiation was higher, which could lead to greater X-ray stability in the jets.

source: https://www.nature.com/natastron/