A radio telescope catches a signal lasting a few milliseconds, and in that instant, energy comparable to what the Sun emits over days is released. This is a fast radio burst, a millisecond radio pulse from a distant galaxy. Since their discovery in 2007, astrophysicists have debated what causes them. A new model offers an unexpected answer: a capacitor discharge.
Around a black hole or neutron star spins an accretion disk, a rotating disk of gas and dust around a compact object. X-ray photons from its inner regions scatter off free electrons, pushing them outward. This creates a charge separation: electrons move to the outer layers, while positive ions remain inside. Thus, a cosmic capacitor is born. The balance between radiation pressure and electrostatic attraction holds the accumulated charge. But if the radiation wavers, a breakdown occurs.
The discharge is described as a simple RC circuit: current flows, charge decays, and the radio pulse fades exponentially. For the burst FRB20180725A, the model gives a characteristic decay time of 5.88 milliseconds. The system's dipole moment reaches 1.3×10³⁰ C·m. The theoretical estimate of the accumulated charge for a stellar-mass black hole at Eddington luminosity, the maximum luminosity when radiation and gravity are in equilibrium, is about 10²⁰ C. A combined analysis of observational data and theory allows determining the electron density in the disk, on the order of 10¹⁸ m⁻³, and the energy of X-ray photons, about 10 keV.
The idea of a "cosmic capacitor" has been quantitatively tested against real data for the first time. Previously, fast radio bursts were mainly linked to magnetic processes: magnetars, magnetic reconnection. The new work shows that ordinary accretion of matter onto a compact object is sufficient. This paves the way for a unified explanation of a wide class of bursts. Further refinements to the model, such as accounting for disk rotation, magnetic fields, and turbulence, will help understand why some bursts repeat and others are one-offs. Predictions can be tested through coordinated observations with radio telescopes and X-ray observatories.
A cosmic lightning bolt, flashing at the edge of a black hole, reaches Earth billions of years later. Now we have a tool to read its traces.
The accumulated charge in the model is comparable to the charge carried by a lightning bolt on Earth, but concentrated in a region the size of a city.