Unsteady outflow models for cosmological gamma-ray bursts
Submitted to Ap.J.(Letters): 4/5/94; accepted: 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 seconds). 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 seconds. 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 ∼ 10 2, the resultant dissipation occurs outside the ’photosphere ’ and can convert a substantial fraction of the overall outflow energy into non-thermal 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. 1.