Short Gamma-Ray Bursts and the Decompression of Neutron Star Matter in Tidal Streams

Short gamma-ray bursts (sGRBs) are generally thought to result from the merger of two neutron stars or the merger of a neutron star with a black hole. It is becoming standard practice to model these mergers with hydrodynamical simulations that employ equations of state that are derived, for example, for determining the behavior of matter in core-collapse supernovae, and which therefore make use of the assumption that the matter is hot and in nuclear statistical equilibrium (NSE). In this Letter we draw attention to the fact that the hydrodynamical timescale (roughly the gravitational timescale of the neutron star) may be several orders of magnitude shorter than the timescale on which such equilibrium can be reestablished in the tidal debris ejected during an sGRB, and that on the initial decompression timescales the unshocked tidal ejecta may remain sufficiently cool that the employed equations of state are not appropriate for modeling the dynamics of this part of the flow. On timescales short compared with the timescale on which NSE can be (re)established, the equation of state can remain relatively stiff and thus the stream of tidal debris can remain narrow and vulnerable to gravitational instability, as has recently been suggested. These findings suggest that estimates of the type and abundances of heavy elements formed in short gamma-ray bursts need to be revisited. We suggest that the most direct method of testing the physical and dynamical properties of tidal ejecta in sGRBs will come from modeling of their lightcurves, which provides the cleanest source of information on the system dynamics.