The Astrophysical Journal Letters
The American Astronomical Society
We solve the Riemann problem for the deceleration of an arbitrarily magne-tized relativistic ﬂow injected into a static unmagnetized medium in one dimen-sion. We ﬁnd that for the same initial Lorentz factor, the reverse shock becomes progressively weaker with increasing magnetization σ (the Poynting-to-kinetic energy ﬂux ratio), and the shock becomes a rarefaction wave when σ exceeds a critical value, σc, deﬁned by the balance between the magnetic pressure in the ﬂow and the thermal pressure in the forward shock. In the rarefaction wave regime, we ﬁnd that the rareﬁed region is accelerated to a Lorentz factor that is signiﬁcantly larger than the initial value. This acceleration mechanism is due to the strong magnetic pressure in the ﬂow. We discuss the implications of these results for models of gamma-ray bursts and active galactic nuclei.
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