On Discovering Electromagnetic Emission from Neutron Star Mergers: The Early Years of Two Gravitational Wave Detectors

We present the first simulation addressing the prospects of finding an electromagnetic (EM) counterpart to gravitational wave detections (GW) during the early years of only two advanced interferometers. The perils of such a search may have appeared insurmountable when considering the coarse ring-shaped GW localizations spanning thousands of deg^2 using time-of-arrival information alone. We show that leveraging the amplitude and phase information of the predicted GW signal narrows the localization to arcs with a median area of only ~250 deg^2, thereby making an EM search tractable. Based on the locations and orientations of the two LIGO detectors, we find that the GW sensitivity is limited to one polarization and thus to only two sky quadrants. Thus, the rates of GW events with two interferometers is only ~40% of the rate with three interferometers of similar sensitivity. Another important implication of the sky quadrant bias is that EM observatories in North America and Southern Africa would be able to systematically respond to GW triggers several hours sooner than Russia and Chile. Given the larger sky areas and the relative proximity of detected mergers, 1m-class telescopes with very wide-field cameras are well positioned for the challenge of finding an EM counterpart. Identification of the EM counterpart amidst the even larger numbers of false positives further underscores the importance of building a comprehensive catalog of foreground stellar sources, background AGN and potential host galaxies in the local universe.