Implications of the search for optical counterparts during the first six months of the Advanced LIGO's and Advanced Virgo's third observing run: possible limits on the ejecta mass and binary properties
The multi-messenger observation of GW170817 has opened a new window into the Universe and proved that neutron star mergers not only emit gravitational waves, but in addition, can give rise to a variety of 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 community doing electromagnetic searches for their optical counterparts, such as kilonovae. Furthermore, the events have not been retracted by the LIGO and Virgo collaboration, however, according to the released classification, there is a high probability that some of these events might not be of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, not due to noise, 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 constraints on the matter ejection consistent with the existing gravitational-wave trigger information but the non-existing kilonova. 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 observations for future gravitational wave events near the horizon limit of the advanced detectors are essential.