X-ray emissions from black hole jets vary unexpectedly, challenging leading model of particle acceleration

Black hole jets have just recently been found to generate X-rays, and it is still unknown how the jets accelerate particles to this high-energy state. Unexpected new results published in Nature Astronomy seem to disprove one popular explanation, which leaves room for rethinking particle acceleration in the jets and maybe elsewhere in the cosmos.

The X-ray emissions from jets are predicted to be steady over large time scales (millions of years) by a prominent model of how jets produce X-rays. The X-ray emissions of a statistically significant number of jets, however, fluctuated within a short period of time, according to the current study.

As the primary author and an astronomer at the University of Maryland, Baltimore County, Eileen Meyer says, "One of the reasons we're intrigued by the variability is because there are two main ideas for how X-rays are created in these jets, and they're very different. "One model makes use of extremely low-energy electrons, whereas the other does it with very high-energy electrons. One of those models, in particular, is totally incompatible with any sort of variation.

The highest resolution X-ray observatory now in use, the Chandra X-ray Observatory, provided the scientists with archival data for the study. The study team examined 155 distinct locations inside 53 black hole jets, or almost all of the black hole jets for which Chandra had multiple observations.

In the context of these jets, seeing very frequent variability on such brief time scales "is revolutionary because that was absolutely not expected," Meyer claims.

Particle Acceleration Reconsidered

The simplest hypothesis for how jets create X-rays implies particle acceleration happens in the core of the galaxy in the black hole "engine" that powers the jet, as well as stability in X-ray emissions throughout time. The latest study, however, discovered rapid variations in X-ray emissions all along the length of the jets. This indicates that particle acceleration is happening all along the jet, a long way from where the jet first started at the black hole.

There are suggestions for how this may operate, but Meyer notes that most of what they have been working with is now obviously incompatible with their findings.

Intriguingly, the findings also suggested that jets closer to Earth exhibited higher variability than those located at great distances. These final objects are so far away that, by the time their light enters the telescope, it feels as though one is staring back in time. Meyer can understand why older jets would be less variable. Researchers think that earlier in the history of the cosmos, the universe was smaller and ambient radiation was higher, which may have increased the stability of X-rays in the jets.

Crucial cooperation

Despite Chandra's superb imaging resolution, the data collection presented several difficulties. With only a small number of X-ray photons, Chandra was able to see some of the pockets of fluctuation. Additionally, a particular jet's X-ray output often varied by a few tens of percent. Meyer worked with statisticians from the University of Toronto and the Imperial College of London to prevent accidentally counting randomness as true variability.

Since the observations were not made to find it, Meyer thinks that getting this finding from the data was akin to a miracle. According to the team's findings, between 30 and 100 percent of the study's jets exhibited variability over short time intervals. The variability is noticeably not zero, she notes, "even though we would like better constraints."

Meyer expects that the publication will inspire more research because the new findings seriously undermine one of the leading hypotheses for the X-ray emission from black hole jets. She hopes that this will serve as a genuine call to action for theorists to examine the data and develop jet models that are in line with their findings.