Systematic Uncertainty of Standard Sirens from the Viewing Angle of Binary Neutron Star Inspirals

The independent measurement of the Hubble constant with gravitational-wave standard sirens will potentially shed light on the tension between the local distance ladders and Planck experiments. Therefore, thorough understanding of the sources of systematic uncertainty for the standard siren method is crucial. In this Letter, we focus on two scenarios that will potentially dominate the systematic uncertainty of standard sirens. First, simulations of electromagnetic counterparts of binary neutron star mergers suggest aspherical emissions, so the binaries available for the standard siren method can be selected by their viewing angles. This selection effect can lead to ≳2% bias in Hubble constant measurement even with mild selection. Second, if the binary viewing angles are constrained by the electromagnetic counterpart observations but the bias of the constraints is not controlled under ∼10°, the resulting systematic uncertainty in the Hubble constant will be >3%. In addition, we find that both of the systematics cannot be properly removed by the viewing angle measurement from gravitational-wave observations. Comparing to the known dominant systematic uncertainty for standard sirens, the ≤2% gravitational-wave calibration uncertainty, the effects from the viewing angle appear to be more significant. Therefore, the systematic uncertainty from the viewing angle might be a major challenge before the standard sirens can resolve the tension in the Hubble constant, which is currently ∼9%.

Kilonovae

The coalescence of double neutron star (NS-NS) and black hole (BH)-NS binaries are prime sources of gravitational waves (GW) for Advanced LIGO/Virgo and future ground-based detectors. Neutron-rich matter released from such events undergoes rapid neutron capture (r-process) nucleosynthesis as it decompresses into space, enriching our universe with rare heavy elements like gold and platinum. Radioactive decay of these unstable nuclei powers a rapidly evolving, approximately isotropic thermal transient known as a "kilonova", which probes the physical conditions during the merger and its aftermath. Here I review the history and physics of kilonovae, leading to the current paradigm of day-timescale emission at optical wavelengths from lanthanide-free components of the ejecta, followed by week-long emission with a spectral peak in the near-infrared (NIR). These theoretical predictions, as compiled in the original version of this review, were largely confirmed by the transient optical/NIR counterpart discovered to the first NS-NS merger, GW170817, discovered by LIGO/Virgo. Using a simple light curve model to illustrate the essential physical processes and their application to GW170817, I then introduce important variations about the standard picture which may be observable in future mergers. These include ∼ hour-long UV precursor emission, powered by the decay of free neutrons in the outermost ejecta layers or shock-heating of the ejecta by a delayed ultra-relativistic outflow; and enhancement of the luminosity from a long-lived central engine, such as an accreting BH or millisecond magnetar. Joint GW and kilonova observations of GW170817 and future events provide a new avenue to constrain the astrophysical origin of the r-process elements and the equation of state of dense nuclear matter.

Multimessenger Bayesian parameter inference of a binary neutron star merger

The combined detection of a binary neutron star merger in both gravitational waves (GWs) and electromagnetic (EM) radiation spanning the entire spectrum – GW170817/AT2017gfo/GRB170817A – marks a breakthrough in the field of multimessenger astronomy. Between the plethora of modelling and observations, the rich synergy that exists among the available data sets creates a unique opportunity to constrain the binary parameters, the equation of state of supranuclear density matter, and the physical processes at work during the kilonova and gamma-ray burst. We report, for the first time, Bayesian parameter estimation combining information from GW170817, AT2017gfo, and GRB170817 to obtain truly multimessenger constraints on the tidal deformability $\tilde{\Lambda } \in [302,860]$, total binary mass M ∈ [2.722, 2.751] M⊙, the radius of a 1.4 solar mass neutron star $R \in [11.3,13.5] \,\,\rm km$ (with additional $0.2\ \rm km$ systematic uncertainty), and an upper bound on the mass ratio of q ≤ 1.27, all at 90 per cent confidence. Our joint novel analysis uses new phenomenological descriptions of the dynamical ejecta, debris disc mass, and remnant black hole properties, all derived from a large suite of numerical relativity simulations.