Astronomers have detected one of the biggest black hole jets in the sky

I can see why the images of that jet are reminiscent of the diamond shock patterns in rocket exhausts, but there may be other reasons for that pattern in the black hole jets.

The diamond pattern we have so often seen in videos of rocket exhausts during liftoffs for space launches are caused by a very specific situation. The rocket motors are designed to expand the exhaust gases to a pressure that is below the outside pressure at sea level, so that they will be a little more efficient when they get to a higher altitude where the atmospheric pressure is lower. The diamond pattern occurs because the edge of the exhaust gas needs to change direction as it leaves the solid surface of the rocket nozzle in order to be slightly compressed back to ambient pressure outside the nozzle. But, because it is flowing at supersonic velocity (compared to the speed of sound in that hot gas) it changes direction with a shock wave because it cannot send a "slow-down" signal back up into the gas flow in the nozzle. That shock wave propagates across the exhaust stream, and, at the center of the exhaust stream, it encounters the shock from the other edges of the stream. The combination of those shocks over compresses the exhaust gas, so now it needs to change direction again to accommodate that, and a shock travels from the center of the exhaust back to the edge, where it again meets conditions of the ambient pressure that are now higher than the exhaust gas pressure, again, so the pattern repeats for several visible diamonds before turbulence disrupts the flow patterns.

Trying to translate that to a relativistic velocity jet of some sort of material blasting out of a black hole surroundings (probably not from within the event horizon) requires some serious thinking. For one thing, any confinement is not by a solid surface, but more probably by magnetic forces. And, the "ambient pressure" may also have something todo with magnetism. How that would create shock waves in the relativistic jet is something that I have no experience with. But, I am wondering if any shock waves in the relativistic speed jet could be sonic in form, considering how much slower the speed of sound is in the jets compared to their velocity near the speed of light. Maybe there is some sort of magnetic field oscillation that has similar effects, but that is not something that I know enough about to comment on it.

So, it might also be worth considering other potential causes for the light pattern in those relativistic jets. One might have something to do with the timing of matter in the accretion disk of the black hole getting sucked into the jet. The picture of the jet does seem to show a pretty regular pattern with 5 compressions causing radiation from the jet, so it would have to be some sort of regular oscillation of the accretion disk if the process starts there. Or, it may be some sort of magnetic oscillation in the jet itself.

Without knowing either the ambient conditions outside the jet nor the mechanisms causing the fluctuations inside the jet, it seems to me that trying to use this pattern to understand the intergalactic medium the jet is flowing through might be misleading


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