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The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation 1 , 2 . Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole 3 . Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of 8.4 − 1.1 + 0.5 Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects, in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow. Spatially resolved images of M87 obtained at a wavelength of 3.5 mm in 2018 show a ring-like accretion structure, the inner (outer) edge of which connects the black hole (jet).

There is some weak evidence that the black hole merger named GW190521 had a non-zero eccentricity 1 , 2 . In addition, the masses of the component black holes exceeded the limit predicted by stellar evolution 3 . The large masses can be explained by successive mergers 4 , 5 , which may be efficient in gas disks surrounding active galactic nuclei, but it is difficult to maintain an eccentric orbit all the way to the merger, as basic physics would argue for circularization 6 . Here we show that active galactic nuclei disk environments can lead to an excess of eccentric mergers, if the interactions between single and binary black holes are frequent 5 and occur with mutual inclinations of less than a few degrees. We further illustrate that this eccentric population has a different distribution of the inclination between the spin vectors of the black holes and their orbital angular momentum at merger 7 , referred to as the spin–orbit tilt, compared with the remaining circular mergers. The accretion disk environments surrounding active galactic nuclei are potential locations where there is an excess of eccentric mergers of large black holes, which have different spin–orbit tilts compared with circular mergers.

The nearby radio galaxy M87 offers a unique opportunity to explore the connections between the central supermassive black hole and relativistic jets. Previous studies of the inner region of M87 revealed a wide opening angle for the jet originating near the black hole 1 – 4 . The Event Horizon Telescope resolved the central radio source and found an asymmetric ring structure consistent with expectations from general relativity 5 . With a baseline of 17 years of observations, there was a shift in the jet’s transverse position, possibly arising from an 8- to 10-year quasi-periodicity 3 . However, the origin of this sideways shift remains unclear. Here we report an analysis of radio observations over 22 years that suggests a period of about 11 years for the variation in the position angle of the jet. We infer that we are seeing a spinning black hole that induces the Lense–Thirring precession of a misaligned accretion disk. Similar jet precession may commonly occur in other active galactic nuclei but has been challenging to detect owing to the small magnitude and long period of the variation. This study analyses radio observations of the jet in galaxy M87, from which the existence of a spinning black hole that induces Lense–Thirring precession of a misaligned accretion disk is inferred.

While most of the compact-binary mergers detected by LIGO and Virgo are expected to consist of first-generation black holes formed from the collapse of stars, others might instead be of second (or higher) generation, containing the remnants of previous black-hole mergers. We review theoretical findings, astrophysical modelling and current gravitational-wave evidence of hierarchical stellar-mass black-hole mergers. Such a subpopulation of hierarchically assembled black holes presents distinctive gravitational-wave signatures, namely higher masses, possibly within the pair-instability mass gap, and dimensionless spins clustered at the characteristic value of ~0.7. To produce hierarchical mergers, astrophysical environments need to overcome the relativistic recoils imparted to black-hole merger remnants, a condition that prefers hosts with escape speeds of ≳100 km s −1 . Promising locations for efficient production of hierarchical mergers include nuclear star clusters and accretion disks surrounding active galactic nuclei, though environments that are less efficient at retaining merger products such as globular clusters may still contribute significantly to the detectable population of repeated mergers. While GW190521 is the single most promising hierarchical-merger candidate to date, constraints from large population analyses are becoming increasingly more powerful. Theoretical findings and current gravitational-wave evidence of hierarchical stellar-mass black-hole mergers are reviewed. Promising locations for the production of such systems include nuclear star clusters, accretion disks in active galactic nuclei, and potentially globular clusters.

The origin of black hole mergers discovered by the LIGO 1 and Virgo 2 gravitational-wave observatories is currently unknown. GW190521 3 , 4 is the heaviest black hole merger detected so far. Its observed high mass and possible spin-induced orbital precession could arise from the binary having formed following a close encounter. An observational signature of close encounters is eccentric binary orbit 5 – 7 ; however, this feature is currently difficult to identify due to the lack of suitable gravitational waveforms. No eccentric merger has been previously found 8 . Here we report 611 numerical relativity simulations covering the full eccentricity range and an estimation approach to probe the eccentricity of mergers. Our set of simulations corresponds to ~10 5 waveforms, comparable to the number used in gravitational-wave searches, albeit with coarser mass ratio and spin resolution. We applied our approach to GW190521 and found that it is most consistent with a highly eccentric ( e = 0.6 9 − 0.22 + 0.17 ; 90% credible level) merger within our set of waveforms. This interpretation is supported over a non-eccentric merger with >10 odds ratio if ≳10% of GW190521-like mergers are highly eccentric. Detectable orbital eccentricity would be evidence against an isolated binary origin, which is otherwise difficult to rule out on the basis of observed mass and spin 9 , 10 . Massive black holes that are produced dynamically by black hole mergers are thought to involve eccentric orbits, whose imprint may remain in the gravitational waveform detected by the LIGO/Virgo Collaboration.

In this paper we investigate quasinormal modes (QNM) for a scalar field around a noncommutative Schwarzschild black hole. We verify the effect of noncommutativity on quasinormal frequencies by applying two procedures widely used in the literature. The first is the Wentzel–Kramers–Brillouin (WKB) approximation up to sixth order. In the second case we use the continuous fraction method developed by Leaver. Besides, we also show that due to noncommutativity, the shadow radius is reduced when we increase the noncommutative parameter. In addition, we find that the shadow radius is nonzero even at the zero mass limit for finite noncommutative parameter.

General relativity 1 predicts that short-orbital-period binaries emit considerable amounts of gravitational radiation. The upcoming Laser Interferometer Space Antenna 2 (LISA) is expected to detect tens of thousands of such systems 3 but few have been identified 4 , of which only one 5 is eclipsing—the double-white-dwarf binary SDSS J065133.338+284423.37, which has an orbital period of 12.75 minutes. Here we report the discovery of an eclipsing double-white-dwarf binary system, ZTF J153932.16+502738.8, with an orbital period of 6.91 minutes. This system has an orbit so compact that the entire binary could fit within the diameter of the planet Saturn. The system exhibits a deep eclipse, and a double-lined spectroscopic nature. We see rapid orbital decay, consistent with that expected from general relativity. ZTF J153932.16+502738.8 is a strong source of gravitational radiation close to the peak of LISA’s sensitivity, and we expect it to be detected within the first week of LISA observations, once LISA launches in approximately 2034. Observations of an eclipsing double-white-dwarf binary with an orbital period of 6.91 minutes that is decaying as predicted by general relativity are reported; once launched, the Laser Interferometer Space Antenna (LISA) should swiftly detect this binary.

The binary neutron-star merger GW170817 1 was accompanied by radiation across the electromagnetic spectrum 2 and localized 2 to the galaxy NGC 4993 at a distance 3 of about 41 megaparsecs from Earth. The radio and X-ray afterglows of GW170817 exhibited delayed onset 4 – 7 , a gradual increase 8 in the emission with time (proportional to t 0.8 ) to a peak about 150 days after the merger event 9 , followed by a relatively rapid decline 9 , 10 . So far, various models have been proposed to explain the afterglow emission, including a choked-jet cocoon 4 , 8 , 11 – 13 and a successful-jet cocoon 4 , 8 , 11 – 18 (also called a structured jet). However, the observational data have remained inconclusive 10 , 15 , 19 , 20 as to whether GW170817 launched a successful relativistic jet. Here we report radio observations using very long-baseline interferometry. We find that the compact radio source associated with GW170817 exhibits superluminal apparent motion between 75 days and 230 days after the merger event. This measurement breaks the degeneracy between the choked- and successful-jet cocoon models and indicates that, although the early-time radio emission was powered by a wide-angle outflow 8 (a cocoon), the late-time emission was most probably dominated by an energetic and narrowly collimated jet (with an opening angle of less than five degrees) and observed from a viewing angle of about 20 degrees. The imaging of a collimated relativistic outflow emerging from GW170817 adds substantial weight to the evidence linking binary neutron-star mergers and short γ-ray bursts. Emission from the radio counterpart of the gravitation-wave event GW170817 was powered by a wide-angle outflow at early times, but probably dominated by a narrowly collimated jet at later times.

The afterglow of the binary neutron-star merger GW170817 1 gave evidence for a structured relativistic jet 2 – 6 and a link 3 , 7 , 8 between such mergers and short gamma-ray bursts. Superluminal motion, found using radio very long baseline interferometry 3 (VLBI), together with the afterglow light curve provided constraints on the viewing angle (14–28 degrees), the opening angle of the jet core (less than 5 degrees) and a modest limit on the initial Lorentz factor of the jet core (more than 4). Here we report on another superluminal motion measurement, at seven times the speed of light, leveraging Hubble Space Telescope precision astrometry and previous radio VLBI data for GW170817. We thereby obtain a measurement of the Lorentz factor of the wing of the structured jet, as well as substantially improved constraints on the viewing angle (19–25 degrees) and the initial Lorentz factor of the jet core (more than 40). Optical superluminal motion in the binary neutron-star merger GW170817 is used to constrain the speed and morphology of the structured jet, and improve constraints on the inclination angle of the merging binary system.

The quantization of gravity is widely believed to result in gravitons – particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single graviton exchange can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyze the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental clue of the quantization of gravity. While it has been suggested that low- energy experiments might allow to find evidence for quantization of gravity, direct detection of single gravitons has normally been considered a hopeless task. Here, the authors suggest that a massive body cooled to the ground state in a gravitational wave background should display detectable stimulated single graviton transitions.