Unsteady outflow models for cosmological gamma-ray bursts

The "event" that triggers a gamma-ray burst cannot last for more than a few seconds. This is, however, long compared with the dynamical timescale of a compact stellar-mass object (∼10^-3 s). Energy is assumed to be released as an outflow with high mean Lorentz factor Γ. But a compact stellar-mass collapse or merger is, realistically, likely to generate a mass (or energy) flux that is unsteady on some timescales in the range 10^-3-10 s. If Γ fluctuates by a factor of ∼2 around its mean value, relative motions within the outflowing material will themselves (in the comoving frame) be relativistic, and can give rise to internal shocks. For Γ ∼10², the resultant dissipation occurs outside the "photosphere" and can convert a substantial fraction of the overall outflow energy into nonthermal radiation. This suggests a mechanism for cosmological bursts that demands less extreme assumptions (in respect of Γ-values, freedom from baryonic contamination, etc.) than earlier proposals.