Microquasars are binary systems where a compact object — a black hole or neutron star — accretes matter from a companion star and launches relativistic jets. The jets accelerate particles to extreme energies. When the jets are active, the system is a known source of high-energy cosmic rays. When accretion stops, the jets shut off. The accelerator dies.
Except the particles don't leave immediately.
Abaroa, Romero, and Bosch-Ramon (arXiv:2603.12337, March 2026) show that dead microquasars — systems where accretion and jet activity ceased thousands of years ago — can still function as PeV cosmic ray sources. The mechanism: the jets, while active, inflated cocoons of magnetized plasma around themselves. When the jets shut off, the cocoons remain. Particles trapped inside continue to bounce between magnetic structures in the cocoon, maintaining their energy for millennia.
The fossil cocoon is a reservoir, not an accelerator. The particles were accelerated when the jets were alive. The cocoon's role is confinement — preventing the particles from escaping and dispersing. The timescale of confinement is set by the cocoon's magnetic field geometry, which can persist for 10⁴ years or longer after the jet dies. The system looks dead from the outside — no accretion, no jets, no X-ray emission — but radiates PeV gamma rays from the trapped particles interacting with ambient photons.
This provides a new explanation for unidentified LHAASO ultrahigh-energy gamma-ray sources. LHAASO detects sources above 100 TeV that have no obvious active counterpart at other wavelengths. A dead microquasar would look exactly like this: a PeV source with no visible engine.
The structural lesson: a system that has stopped producing can continue to emit from stored inventory. The production and the emission are decoupled in time. The reservoir outlasts the engine by orders of magnitude. What looks like an active source at one wavelength may be a fossil at every other.