Optical characterization of the Advanced Virgo gravitational wave detector for the O4 observing run

The Advanced Virgo Plus detector, an upgrade of the Advanced Virgo Detector, is a dual-recycled Fabry–Perot Michelson interferometer characterized by 3 km long arm cavities. The main upgrades in view of the fourth observing run (O4) were the implementation of the signal recycling cavity and the installation of the frequency-dependent squeezing system. Another upgrade was the increasing of the laser power at the input of the detector, which could lead to more severe thermal aberrations impacting the achievement of the interferometer optimal working point. Therefore, the fine-tuning of the thermal compensation system, optimized with respect to the one implemented for the O3 run, was also challenging. In order to achieve the best performance of such a sophisticated optical system, having a clear knowledge of all its optical parameters is crucial. The optical characterization of the detector in different working conditions could help in understanding its behavior and optimizing the global control system. Moreover, the characterization in different thermal conditions, i.e., different values of the input laser power or different configurations of the thermal compensation system, could provide significant guidance for the optimization of the thermal tuning. In this paper, we will describe all the methodologies adopted for the optical characterization activities performed in Advanced Virgo Plus, presenting the experimental results for all the relevant parameters obtained during the preparation of the O4 run

Search for Subsolar Mass Ultracompact Binaries in Advanced LIGO’s Second Observing Run

International audienceWe present a search for subsolar mass ultracompact objects in data obtained during Advanced LIGO’s second observing run. In contrast to a previous search of Advanced LIGO data from the first observing run, this search includes the effects of component spin on the gravitational waveform. We identify no viable gravitational-wave candidates consistent with subsolar mass ultracompact binaries with at least one component between 0.2  M⊙–1.0  M⊙. We use the null result to constrain the binary merger rate of (0.2  M⊙, 0.2  M⊙) binaries to be less than 3.7×105  Gpc-3 yr-1 and the binary merger rate of (1.0  M⊙, 1.0  M⊙) binaries to be less than 5.2×103  Gpc-3 yr-1. Subsolar mass ultracompact objects are not expected to form via known stellar evolution channels, though it has been suggested that primordial density fluctuations or particle dark matter with cooling mechanisms and/or nuclear interactions could form black holes with subsolar masses. Assuming a particular primordial black hole (PBH) formation model, we constrain a population of merging 0.2  M⊙ black holes to account for less than 16% of the dark matter density and a population of merging 1.0  M⊙ black holes to account for less than 2% of the dark matter density. We discuss how constraints on the merger rate and dark matter fraction may be extended to arbitrary black hole population models that predict subsolar mass binaries

Model comparison from LIGO-Virgo data on GW170817's binary components and consequences for the merger remnant

GW170817 is the very first observation of gravitational waves originating from the coalescence of two compact objects in the mass range of neutron stars, accompanied by electromagnetic counterparts, and offers an opportunity to directly probe the internal structure of neutron stars. We perform Bayesian model selection on a wide range of theoretical predictions for the neutron star equation of state. For the binary neutron star hypothesis, we find that we cannot rule out the majority of theoretical models considered. In addition, the gravitational-wave data alone does not rule out the possibility that one or both objects were low-mass black holes. We discuss the possible outcomes in the case of a binary neutron star merger, finding that all scenarios from prompt collapse to long-lived or even stable remnants are possible. For long-lived remnants, we place an upper limit of 1.9 kHz on the rotation rate. If a black hole was formed any time after merger and the coalescing stars were slowly rotating, then the maximum baryonic mass of non-rotating neutron stars is at most , and three equations of state considered here can be ruled out. We obtain a tighter limit of for the case that the merger results in a hypermassive neutron star