Title:

Optimization of electromagnetic follow up observations and localization of gravitational wave signals from compact binary coalescences

Many gravitational wave sources will produce electromagnetic signals as they emit gravitational waves. An important example is binary neutron star mergers. The joint observations and discoveries of the electromagnetic signatures of these gravitational wave sources can produce substantial scientific benefits in physics, astrophysics and cosmology. To maximize the scientific outcomes of such gravitational events as much as possible, the detections of their electromagnetic signatures are necessary. The first detection of the inspiral signals from binary neutron stars by LIGO and VIRGO, and the observations of the associated electromagnetic counterparts throughout the electromagnetic spectrum have served an excellent example. These detections and discoveries have also ushered in a new era of both gravitational wave astronomy and multimessenger astronomy. However, using gravitational wave interferometric detectors, the sky location estimates of the gravitational wave signals from binary neutron star can span a few hundreds square degrees, unless there are three or more detectors observing the event simultaneously. The large sky localization error poses a challenge for astronomers scanning the localization error to look for the electromagnetic signals of these gravitational wave events. The electromagnetic counterparts may also not be readily detectable depending on the distance and orientation of the sources, which presents further difficulties in detecting their signals. To alleviate the situation, we develop an algorithm to maximize the detection probability of the electromagnetic counterparts of gravitational wave events. The algorithm we develop is able to generate an observing strategy that optimizes the probability of successful electromagnetic followup observations given limited observational resources. This is achieved by using a greedy algorithm for tiling the sky location error and Lagrange multiplier for assigning observation times to observation fields. The analysis with the algorithm also allows an estimate of the detection probability. In Chapter 3, we present a proofofconcept demonstration of this algorithm to four telescopes SubaruHyperSuprimeCam, CTIODark Energy Camera, Palomar Transient Factory and PanStarrs, for three different simulated binary neutron star events, assuming kilonova to be the target electromagnetic counterpart. By applying the algorithm to telescopes with arbitrary field of view and sensitivity within a range, we provide an insight into the potential of future telescopes and other telescopes not directly included in our analysis. Moreover, the algorithm is applied to the design of a space based mission, the Einstein Probe, to find the optimal combination of the size of field of view and the sensitivity. The localization of gravitational wave sources, which is determined both by the gravitational wave signals and the detectors, is an important factor to the success of electromagnetic followup observations. We investigate the localization of binary neutron star mergers detected with the Einstein Telescope and Cosmic Explorer. Compared to the existing detectors, the improvement in the sensitivity of the Einstein Telescope and Cosmic Explorer in the low frequency band has many important implications. One of them is the considerable increase in the length of the inband of the signals from binary neutron stars, which is useful in localizing the sources. In Chapter 4, using a Fisher matrix approach, we estimate the sky localization error of binary neutron stars as a population and distributed at various distances. As the extended inband duration of signals also increases the possibility of identifying and releasing the presence of a signal prior to merger, known as early warning, we investigate the prospect for early warning of binary neutron star merger events with these detectors. While the Einstein Telescope and Cosmic Explorer hold promising future for gravitational wave astronomy, they are not likely to be operative until the 2030s. In the literature, detectors designed with more advanced technologies than LIGO and VIRGO are proposed to fill the gap in time. We estimate the localization of binary black holes with two such detectors in Australia and China and seconds generation detectors such as LIGO, LIGO India, VIRGO and KAGRA. In chapter 5, we study electromagnetic observations of binary neutron star mergers with the Large Synoptic Survey Telescope. The Large Synoptic Survey Telescope is a telescope designed with large size of field of view and excellent sensitivity in its observing bands. Such a telescope provides a promising prospect for multimessenger astronomy with gravitational waves. With its sensitivity and field of view, the telescope is expected to enable electromagnetic followup observations with shorter exposure time and fewer observation fields than many existing telescopes. We define a simple procedure for electromagnetic followup observations triggered by gravitational waves using the telescope. Taking advantages of the Fisher matrix approach in Chapter 4 for the sky location estimates, we quantify the observation time necessary for the telescope to perform electromagnetic followup observation of binary neutron star mergers detected with different networks of gravitational wave detectors.
