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 (approximately 10(-3) s). Energy is assumed to be released as an outflow with high mean Lorentz factor GAMMA. 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 GAMMA fluctuates by a factor of approximately 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 GAMMA approximately 10(2), 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 GAMMA-values, freedom from baryonic contamination, etc.) than earlier proposals.