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.
