S4MMA

Neutron Star Mergers

A small fraction of all neutron stars (NS) end their life with a violent collision with another compact object, another NS or a Black Hole (BH). These collisions can be driven by the loss of orbital energy due to the emission of gravitational waves at the end of a complex binary evolution, or they can occur as a result of many-body interactions in dense stellar environments (for example, in globular clusters). Presently, about a dozen double NS systems in and around our galaxy are known. Of these, roughly half will merge within a few billions of years. There are currently no known NS-BH binaries.

The estimated event rates based on these observations and other means are still very uncertain. For double NS binaries, the merger rate estimates range from one every thousand years to one every million years for a galaxy lik ours. The rates for NS-BH binaries are even more uncertain, with one predicted every 10,000 to 100 million years per Milky-Way-type galaxy.

Despite being rare events, these mergers are violent enough to produce photons throughout the entire electromagnetic spectrum (from the radio to hard X-rays and γ-ray) detectable at intergalactic distances. Indeed, these mergers typically result in the formation of a black hole surrounded by thick, massive (roughly solar mass) and hot accretion disk that accrete on a timescale of less than a second. These systems are expected to produce relativistic jets and are considered the most likely candidates for the central engine of short γ-ray bursts.

These mergers are one of the main target of gravitational wave detectors like LIGO, which is expected to detect them at distances as large as hundreds of Megaparsecs. More importantly for TEAMS, NS mergers should result in the dynamical ejection of up to 1% of a solar mass of formerly NS matter due to tidal torques or shocks during the merger. These outflows are expected to be very neutron rich, because they are made of catalyzed neutron star material, and thus robustly produce r-process elements.