Title: Gravitation and Multimessenger Astrophysics
Description: Gravitational waves originate from the most violent cosmic events, which are often hidden from traditional means of observation. Starting with the first direct observation of gravitational waves in the coming years, astronomy will become richer with a new messenger that can help unravel many of the yet unanswered questions on various cosmic phenomena. The ongoing construction of advanced gravitational wave observatories requires disruptive innovations in many aspects of detector technology in order to achieve the sensitivity that lets us reach cosmic events. We present the development of a component of this technology, the Advanced LIGO Optical Timing Distribution System. This technology aids the detection of relativistic phenomena through ensuring that time, at least for the observatories, is absolute. Gravitational waves will be used to look into the depth of cosmic events and understand the engines behind the observed phenomena. As an example, we examine some of the plausible engines behind the creation of gamma ray bursts.

We anticipate that, by reaching through shrouding blastwaves, efficiently discovering off-axis events, and observing the central engine at work, gravitational wave detectors will soon transform the study of gamma ray bursts. We discuss how the detection of gravitational waves could revolutionize our understanding of the progenitors of gamma ray bursts, as well as related phenomena such as the properties of neutron stars. One of the most intriguing directions in utilizing gravitational waves is their combination with other cosmic messengers such as photons or neutrinos. We discuss the strategies and ongoing efforts in this direction. Further, we present the first observational constraints on joint sources of gravitational waves and high energy neutrinos, the latter of which is created in relativistic plasma outflows, e.g., in gamma ray burst progenitors. High energy neutrinos may be created inside a relativistic outflow burrowing its way out of a massive star from the star's collapsed core.

We demonstrate how the detection of high energy neutrinos can be used to extract important information about the supernova/gamma-ray burst progenitor structure. We show that, under favorable conditions, even a few neutrinos are sufficient to probe the progenitor structure, opening up new possibilities for the first detections, as well for progenitor population studies. We present the science reach and method of an ongoing search for common sources of gravitational waves and high energy neutrinos using the initial LIGO/Virgo detectors and the partially completed IceCube detector. We also present results on the sensitivity of the search. We argue that such searches will open the window onto source populations whose electromagnetic emission is hardly detectable.