WINTER 2019
work in progress
Neutron Star Collisions & Kilonova Modeling Jessica Metzger1, James Annis2 1 The University of Chicago, 2Fermi National Accelerator Laboratory
Abstract Binary neutron star collisions (BNSs) have been theorized as the sources of astrophysical phenomena ranging from gravitational wave emission and gamma ray bursts to high-energy neutrinos, and as the origin of heavyelement nucleosynthesis. On August 17th, 2017 the first BNS in gravitational waves was observed, and soon afterward, the electromagnetic counterpart, the “kilonova,” was located. Models of this electromagnetic emission are interesting and useful in deducing properties of the merger. The history of kilonova theory and a physicallymotivated kilonova model based on the concept of the “neutrinosphere” are presented. The data from GW170817 (Gravitational Wave 170817), and fits to the lightcurves, can be described using this model.
Introduction
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Pre-GW170817 When neutron stars collide, they are expected to eject some sort of matter, the “ejecta,” the properties of which are central to observations. Until 2017, an understanding of BNSs came from simulations. At the moment neutron stars collide, they eject massive tidal arms of neutron-rich matter (demonstrated in the first BNS simulation by Davies et al. 1994) [1]. After the collision, the remnant either collapses promptly to a black hole or goes through an intermediate “hypermassive neutron star” (HMNS) phase as a rapidly rotating object surviving for tens of milliseconds with a mass larger than the maximum neutron star mass [2]. If there is an HMNS remnant, it oscillates, sending shocks that eject some “shockheated” matter a few milliseconds after the collision [3,4]. These first two ejecta components, tidal arms and shock-heated matter, comprise the “dynamical ejecta.” Even neutrino irradiation may be an ejection mechanism, blowing matter away in a “neutrino-driven wind” during the first few (~10) milliseconds after the collision [5]. Lastly, friction within the accretion disk causes much of it to be blown away in powerful “disk winds” [6,7]. These last two components comprise the “wind ejecta.” In the 1970s, theorists at the University of Chicago and elsewhere began pointing out that the ejecta from neutron star collisions may be one of the sources of the
universe’s heavy elements, synthesized through the “r-process” (rapid neutron capture) whereby neutrons in decompressing neutron-rich matter accumulate quickly onto seed nuclei, forming heavy elements like the lanthanides [8,9]. Simulations have since predicted that the r-process will occur in at least the dynamical ejecta from a BNS [10]. These elements form as very unstable isotopes and quickly radioactively decay, powering a supernova-like electromagnetic transient. In 1998, Li and Paczyński devised the first model of this transient, which predicted the lightcurves (its appearance in our telescopes over time). Metzger et al. in 2010 correctly predicted the brightness of a kilonova using r-process heating rates, coining the term “kilonova” (1000 times brighter than a regular nova) [11,12]. In 2013, two groups found that the synthesized elements would have very high opacities (many complex atomic transitions) that would cause photons to take much longer than expected to diffuse through the cloud of ejecta, thus causing the lanthanide-rich part of the signal to be even redder, dimmer, and slower-evolving than predicted [13,14]. Post-GW170817 The observation of GW170817 was monumental, both as a pioneering feat of multi-messenger astronomy and for its revolutionary contribution to kilonova theory. On August 17th, 2018, the Laser