Kilohertz quasiperiodic oscillations in short gamma-ray bursts.

Short gamma-ray bursts (GRBs) are associated with binary neutron star mergers, which are multimessenger astronomical events that have been observed both in gravitational waves and in the multiband electromagnetic spectrum1. Depending on the masses of the stars in the binary and on details of their largely unknown equation of state, a dynamically evolving and short-lived neutron star may be formed after the merger, existing for approximately 10-300 ms before collapsing to a black hole2,3. Numerical relativity simulations across different groups consistently show broad power spectral features in the 1-5-kHz range in the post-merger gravitational-wave signal4-14, which is inaccessible by current gravitational-wave detectors but could be seen by future third-generation ground-based detectors in the next decade15-17. This implies the possibility of quasiperiodic modulation of the emitted gamma rays in a subset of events in which a neutron star is formed shortly before the final collapse to a black hole18-21. Here we present two such signals identified in the short bursts GRB 910711 and GRB 931101B from archival Burst and Transient Source Experiment (BATSE) data, which are compatible with the predictions from numerical relativity.

The new frontier of gravitational waves.

In 2015, almost a century after Einstein published the general theory of relativity, one of its most important predictions was verified by direct detection: the production of gravitational waves in spacetime by accelerating objects. Since then, gravitational-wave astronomy has enabled tests of the nature of gravity and the properties of black holes, and in 2017 electromagnetic observations of a double neutron star merger producing gravitational waves led to a focus on multi-messenger astronomy. Here we review the history and accomplishments of gravitational-wave astronomy and look towards the future.

Distinguishing spin-aligned and isotropic black hole populations with gravitational waves.

The direct detection of gravitational waves from merging binary black holes opens up a window into the environments in which binary black holes form. One signature of such environments is the angular distribution of the black hole spins. Binary systems that formed through dynamical interactions between already-compact objects are expected to have isotropic spin orientations (that is, the spins of the black holes are randomly oriented with respect to the orbit of the binary system), whereas those that formed from pairs of stars born together are more likely to have spins that are preferentially aligned with the orbit. The best-measured combination of spin parameters for each of the four likely binary black hole detections GW150914, LVT151012, GW151226 and GW170104 is the 'effective' spin. Here we report that, if the magnitudes of the black hole spins are allowed to extend to high values, the effective spins for these systems indicate a 0.015 odds ratio against an aligned angular distribution compared to an isotropic one. When considering the effect of ten additional detections, this odds ratio decreases to 2.9 × 10-7 against alignment. The existing preference for either an isotropic spin distribution or low spin magnitudes for the observed systems will be confirmed (or overturned) confidently in the near future.

Electromagnetic counterparts to massive black-hole mergers.

The next two decades are expected to open the door to the first coincident detections of electromagnetic (EM) and gravitational-wave (GW) signatures associated with massive black-hole (MBH) binaries heading for coalescence. These detections will launch a new era of multimessenger astrophysics by expanding this growing field to the low-frequency GW regime and will provide an unprecedented understanding of the evolution of MBHs and galaxies. They will also constitute fundamentally new probes of cosmology and would enable unique tests of gravity. The aim of this Living Review is to provide an introduction to this research topic by presenting a summary of key findings, physical processes and ideas pertaining to EM counterparts to MBH mergers as they are known at the time of this writing. We review current observational evidence for close MBH binaries, discuss relevant physical processes and timescales, and summarize the possible EM counterparts to GWs in the precursor, coalescence, and afterglow stages of a MBH merger. We also describe open questions and discuss future prospects in this dynamic and quick-paced research area.

Flows of X-ray gas reveal the disruption of a star by a massive black hole.

Tidal forces close to massive black holes can violently disrupt stars that make a close approach. These extreme events are discovered via bright X-ray and optical/ultraviolet flares in galactic centres. Prior studies based on modelling decaying flux trends have been able to estimate broad properties, such as the mass accretion rate. Here we report the detection of flows of hot, ionized gas in high-resolution X-ray spectra of a nearby tidal disruption event, ASASSN-14li in the galaxy PGC 043234. Variability within the absorption-dominated spectra indicates that the gas is relatively close to the black hole. Narrow linewidths indicate that the gas does not stretch over a large range of radii, giving a low volume filling factor. Modest outflow speeds of a few hundred kilometres per second are observed; these are below the escape speed from the radius set by variability. The gas flow is consistent with a rotating wind from the inner, super-Eddington region of a nascent accretion disk, or with a filament of disrupted stellar gas near to the apocentre of an elliptical orbit. Flows of this sort are predicted by fundamental analytical theory and more recent numerical simulations.

Modeling extreme mass ratio inspirals within the effective-one-body approach.

We present the first models of extreme-mass-ratio inspirals within the effective-one-body (EOB) formalism, focusing on quasicircular orbits into nonrotating black holes. We show that the phase difference and (Newtonian-normalized) amplitude difference between analytical EOB and numerical Teukolsky-based gravitational waveforms can be reduced to less than or approximately 10{-1} rad and less than or approximately 2x10{-3}, respectively, after a 2-year evolution. The inclusion of post-Newtonian self-force terms in the EOB approach leads to a phase disagreement of approximately 6-27 rad after a 2-year evolution. Such inclusion could also allow for the EOB modeling of waveforms from intermediate-mass-ratio, quasicircular inspirals.