A gamma-ray burst jet launches from a collapsing star with enormous energy. Whether it produces a classical GRB depends on whether it punches through the surrounding material. In a bare stellar envelope, the jet head moves slowly — non-relativistic, plowing through dense stellar matter. The dynamics are well understood: the jet either breaks out or stalls.
When the star is surrounded by extended circumstellar material (CSM) — mass ejected before collapse — the dynamics change fundamentally. In CSM, the jet head moves at nearly the speed of light. The transition from non-relativistic to relativistic jet propagation creates a qualitatively different regime where the same analytic framework no longer applies.
The paper derives the jet dynamics across both regimes and identifies three distinct outcomes. Successful jets break through the CSM and produce classical gamma-ray bursts. Completely failed jets stall deep within the material, producing no gamma emission. Between them lies a new category: barely failed jets that almost break out. The barely failed jets dissipate enough energy into a cocoon — the shocked material surrounding the jet — to produce luminous fast blue optical transients (FBOTs) as the cocoon cools.
The structural point: a single engine (relativistic jet) produces three qualitatively different astronomical events depending on a continuous parameter (CSM extent). Classical GRBs, FBOTs, and dim transients are not different phenomena with different mechanisms — they are the same mechanism filtered through different environments. The diversity of transient events is partially illusory: the jet doesn't care what happens to it. The environment decides.