GW170817

GW170817 was a GW detection by LIGO and Virgo for which a corresponding gamma-ray burst (GRB) was detected (a short gamma-ray burst, or SGRB, occurring 1.7 seconds later, GRB 170817A), and for which a corresponding optical transient was found (recorded by the Swope Supernova Survey as AT2017gfo, which matched the sky location of GW170817, given triangulation using observation data from the three gravitational-wave detectors). As such, it was the first case of a new type of multi-messenger astronomy, observation via EMR as well as other media. Virgo's detection was weak (i.e., a low signal-to-noise ratio) and the fact that detector sensitivity depends upon the direction of the source was a factor in reducing the region of the sky for searching. The optical transient was in the galaxy, NGC 4993.

The event has been interpreted as a neutron star merger due to the pattern of gravitational waves (the detected signal lasts longer because the lower mass of neutron stars results in a slower orbital decay). The term kilonova (suggesting a transient with more energy than a nova but less than a supernova) had been coined for such events. The event has been the subject of much study, being the first such observation in which the underlying event is so certain. Among the revelations:

• Confirms that kilonovae host the r-process.
• Information regarding the equation of state of neutron stars.
• A more direct detection of the r-process.
• A more direct observation of a tidal tail from a neutron-star impact.

The r-process is expected since the shock of impact sends neutrons flying free, and is detected through the EMR produced by hot elements, heated for a while by radioactivity (e.g., beta decay), all consistent with the products of r-process nucleosynthesis. Among the effects is high opacity soon after the merger, due to the presence of elements heavier than iron. Early EMR observation was to a degree indistinct, which is attributed to the effects of relativistic speed of ejected material. Interpretation suggests the two neutron stars first merged into a hypermassive neutron star (HMNS), and about a second later, collapsed into a black hole. Given the scenario most likely to form a binary neutron star, the decay of its orbit would take a very long time, and estimates are that this system's orbit decayed over 11 gigayears leading up to the merger.