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ABSTRACT Searches for gravitational-wave counterparts have been going in earnest since GW170817 and the discovery of AT2017gfo. Since then, the lack of detection of other optical counterparts connected to binary neutron star or black hole–neutron star candidates has highlighted the need for a better discrimination criterion to support this effort. At the moment, low-latency gravitational-wave alerts contain preliminary information about binary properties and hence whether a detected binary might have an electromagnetic counterpart. The current alert method is a classifier that estimates the probability that there is a debris disc outside the black hole created during the merger as wellmore »as the probability of a signal being a binary neutron star, a black hole–neutron star, a binary black hole, or of terrestrial origin. In this work, we expand upon this approach to both predict the ejecta properties and provide contours of potential light curves for these events, in order to improve the follow-up observation strategy. The various sources of uncertainty are discussed, and we conclude that our ignorance about the ejecta composition and the insufficient constraint of the binary parameters by low-latency pipelines represent the main limitations. To validate the method, we test our approach on real events from the second and third Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)–Virgo observing runs.« less

ABSTRACT Joint multimessenger observations with gravitational waves and electromagnetic (EM) data offer new insights into the astrophysical studies of compact objects. The third Advanced LIGO and Advanced Virgo observing run began on 2019 April 1; during the 11 months of observation, there have been 14 compact binary systems candidates for which at least one component is potentially a neutron star. Although intensive follow-up campaigns involving tens of ground and space-based observatories searched for counterparts, no EM counterpart has been detected. Following on a previous study of the first six months of the campaign, we present in this paper the nextmore »five months of the campaign from 2019 October to 2020 March. We highlight two neutron star–black hole candidates (S191205ah and S200105ae), two binary neutron star candidates (S191213g and S200213t), and a binary merger with a possible neutron star and a ‘MassGap’ component, S200115j. Assuming that the gravitational-wave (GW) candidates are of astrophysical origin and their location was covered by optical telescopes, we derive possible constraints on the matter ejected during the events based on the non-detection of counterparts. We find that the follow-up observations during the second half of the third observing run did not meet the necessary sensitivity to constrain the source properties of the potential GW candidate. Consequently, we suggest that different strategies have to be used to allow a better usage of the available telescope time. We examine different choices for follow-up surveys to optimize sky localization coverage versus observational depth to understand the likelihood of counterpart detection.« less

ABSTRACT The ever-increasing sensitivity of the network of gravitational-wave detectors has resulted in the accelerated rate of detections from compact binary coalescence systems in the third observing run of Advanced LIGO and Advanced Virgo. Not only has the event rate increased, but also the distances to which phenomena can be detected, leading to a rise in the required sky volume coverage to search for counterparts. Additionally, the improvement of the detectors has resulted in the discovery of more compact binary mergers involving neutron stars, revitalizing dedicated follow-up campaigns. While significant effort has been made by the community to optimize singlemore »telescope observations, using both synoptic and galaxy-targeting methods, less effort has been paid to coordinated observations in a network. This is becoming crucial, as the advent of gravitational-wave astronomy has garnered interest around the globe, resulting in abundant networks of telescopes available to search for counterparts. In this paper, we extend some of the techniques developed for single telescopes to a telescope network. We describe simple modifications to these algorithms and demonstrate them on existing network examples. These algorithms are implemented in the open-source software gwemopt, used by some follow-up teams, for ease of use by the broader community.« less

Observations of neutron-star mergers with distinct messengers, including gravitational waves and electromagnetic signals, can be used to study the behavior of matter denser than an atomic nucleus and to measure the expansion rate of the Universe as quantified by the Hubble constant. We performed a joint analysis of the gravitational-wave event GW170817 with its electromagnetic counterparts AT2017gfo and GRB170817A, and the gravitational-wave event GW190425, both originating from neutron-star mergers. We combined these with previous measurements of pulsars using x-ray and radio observations, and nuclear-theory computations using chiral effective field theory, to constrain the neutron-star equation of state. We found thatmore »the radius of a 1.4–solar mass neutron star is${11.75}_{-0.81}^{+0.86}$km at 90% confidence and the Hubble constant is${66.2}_{-4.2}^{+4.4}$at 1σ uncertainty.

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GW170817 showed that neutron star mergers not only emit gravitational waves but also can release electromagnetic signatures in multiple wavelengths. Within the first half of the third observing run of the Advanced LIGO and Virgo detectors, there have been a number of gravitational wave candidates of compact binary systems for which at least one component is potentially a neutron star. In this article, we look at the candidates S190425z, S190426c, S190510g, S190901ap, and S190910h, predicted to have potentially a non-zero remnant mass, in more detail. All these triggers have been followed up with extensive campaigns by the astronomical communitymore »doing electromagnetic searches for their optical counterparts; however, according to the released classification, there is a high probability that some of these events might not be of extraterrestrial origin. Assuming that the triggers are caused by a compact binary coalescence and that the individual source locations have been covered during the EM follow-up campaigns, we employ three different kilonova models and apply them to derive possible constraints on the matter ejection consistent with the publicly available gravitational-wave trigger information and the lack of a kilonova detection. These upper bounds on the ejecta mass can be related to limits on the maximum mass of the binary neutron star candidate S190425z and to constraints on the mass-ratio, spin, and NS compactness for the potential black hole–neutron star candidate S190426c. Our results show that deeper electromagnetic observations for future gravitational wave events near the horizon limit of the advanced detectors are essential.

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