The cosmic racer SS 433 tears through the Galaxy, ejecting two narrow jets of superheated gas at a quarter of the speed of light. This is a microquasar — a binary system where a black hole siphons matter from a normal star, and some of it escapes as jets. Recently, telescopes detected gamma-ray emission from these jets with energies above 100 TeV, originating from the region where the jet slams into an interstellar cloud. But where exactly these record-breaking particles are born remained a mystery.
An international team of astronomers analyzed archival observations of SS 433 using the Very Long Baseline Array (VLBA). They measured how the magnetic field changes along the inner part of the jet, at distances of tens of astronomical units from the central object. It turned out that the field decays as B ∝ H⁻⁰·⁵, where H is the distance from the compact source. This is a key result: in a conical jet, such a dependence means that the magnetic rigidity (the product of the field and the size of the acceleration region) does not decrease with distance but increases. In other words, the farther from the black hole, the more energetic particles this natural "cannon" can confine and accelerate.
The increase in rigidity means that the Hillas limit (the maximum energy available to the accelerator) at a distance of a few hundred astronomical units goes far beyond the petaelectronvolt (PeV). This is exactly what is needed — about 2.6 PeV — to produce the gamma-ray photons detected by the LHAASO observatory. Thus, a hidden Pevatron has been found in the SS 433 jet — an accelerator of protons to petaelectronvolt energies, located much closer to the black hole than the extended "bubbles" of TeV glow. The giant particles are born in the very heart of the outflow, long before it collides with the cloud.
Now it is clear: the SS 433 jet is not just a pipe for venting gas, but a multi-stage cannon where the magnetic field methodically, step by step, accelerates protons to energies unattainable on Earth. And the hidden Pevatron inside it is just the first stage of this cosmic accelerator.
The energy of 2.6 PeV is equivalent to the energy of a flying mosquito, but concentrated in a single proton. If such a particle were fired at a target, it would trigger a shower of secondary particles, like the impact of a microscopic meteorite.
In Alastair Reynolds's novel "Revelation Space," alien artifacts use relativistic jets for interstellar travel — much like SS 433 accelerates matter to near-light speeds.