Gamma-ray burst GRB 221009A flared so brightly that it was dubbed the 'brightest of all time.' The LHAASO observatory detected gamma rays from it with energies up to 13 TeV — thousands of times more energetic than particles in an X-ray machine. But the spectrum revealed an unexpected dip: at energies of a few TeV, there were fewer particles than expected from a smooth power law.
The authors found an explanation for this break. Inside the fireball, a cloud of hot plasma ejected in the explosion, protons collide with photons to produce neutrons. These neutrons don't stay in the fireball: they escape and slam into the surrounding matter. The collisions create electrons and positrons, which, moving through magnetic fields, emit gamma rays. This 'bounce' off the environment provides an extra flux of photons right in the TeV range and explains the observed decline.
The model also shows that the contribution of gamma rays from intergalactic electromagnetic cascades, triggered by ultra-high-energy primary protons, is limited in the early afterglow phase. For typical magnetic field strengths in the filaments of the cosmic web above 1 nG, such cascades cannot produce a significant flux in the TeV range. This is an important constraint for understanding how ultra-high-energy cosmic rays propagate through the universe.
The energy of 13 TeV in a single gamma quantum is roughly the energy of a flying mosquito, compressed into one elementary particle.
In the novel 'Light of Distant Stars,' neutron echoes from distant gamma-ray bursts serve as beacons for interstellar navigation.