Session S07: Theory of Electromagnetic Counterparts to Gravitational Wave Events

Cocoons, structured jets, and the non-thermal emission of binary neutron star mergers

The propagation of a relativistic jet through a static ambient medium drives a bow shock that feeds a cocoon around the jet itself. As the jet breaks out, the cocoon is released, adding a mildly relativistic, wide angle component to the outflow. In this talk I will discuss the dynamics and radiative properties of the ensuing structured jet, considering both radiation from the outflow itself (the so-called prompt emission) and radiation produced by the interaction of the outflow with the interstellar medium (the so-called afterglow). The resulting electromagnetic transients will be discussed as possible interpretations for the observations of GW170817 as well as possible counterparts of future binary merger detections.   

Kilonova Emission from a Binary Neutron Star Merger

On August 17 the LIGO/Virgo gravitational wave observatories detected the first binary neutron star merger event, a discovery followed by the most ambitious electromagnetic (EM) follow-up campaign ever conducted. Within 2 seconds of the merger, a weak burst of gamma-rays was discovered by the Fermi and INTEGRAL satellites. Within 11 hours, a bright but rapidly fading thermal optical counterpart was discovered in the galaxy NGC 4993 at a distance of only 40 Mpc. The properties of the optical transient match remarkably well predictions for ``kilonova'' emission powered by the radioactive decay of heavy nuclei synthesized in the merger ejecta by the rapid capture of neutrons onto lighter seed nuclei (r-process nucleosynthesis). I will argue that the end product of the merger was likely a temporarily stable ``hyper-massive'' neutron star, which collapsed to a black hole relatively quickly - within a few hundred milliseconds following the coalescence. This inference, as well as others obtained by the electromagnetic and gravitational wave signals, place strict constraints on the uncertain properties (radii and maximum mass) of neutron stars. I will conclude by previewing the potential diversity of kilonova emission expected in the impending era of multi-messenger astronomy, once at design sensitivity Advanced LIGO/Virgo detect a merger roughly weekly.   

Fingerprints of Heavy Element Synthesis in the Universe

The source of about half of the heaviest elements in the Universe has been a mystery for a long time. Although the general picture of element formation is well understood, many questions about the nuclear physics processes and particularly the astrophysical details remain to be answered. Here I focus on recent advances in our understanding of the origin of the heaviest and rarest elements in the Universe.