Wide black hole binaries (wide-BBHs; ≥103AU) in the field can be perturbed by random stellar flybys that excite their eccentricities. Once a wide binary is driven to a sufficiently small pericentre approach, gravitational wave (GW) emission becomes significant, and the binary inspirals and merges. In our previous study, using simplified models for wide-BBHs, we found that successive flybys lead to significant merger fractions of wide-BBHs in less than Hubble time, making the flyby perturbation mechanism a relevant contributor to the production rate of GW-sources. However, the exact rates and detailed properties of the resulting GW sources depend on the wide binary progenitors. In this paper we use detailed population synthesis models for the initial wide-BBH population, considering several populations corresponding to different natal-kick models and metallicities, and then follow the wide-BBHs evolution due to flyby perturbations and GW-emission. We show that the cumulative effect of flybys is conducive for the production of GW sources in non-negligible rates of 1 − 20 Gpc−3 yr−1, which are sensitive to the natal kicks model. Such rates are relevant to the observationally inferred rate. Our models, now derived from detailed population of binaries, provide detailed properties of the produced GW-sources, including mass-functions and delay times. The observational signatures from this channel include circular orbits in aLIGO band; preference for high velocity dispersion host galaxies (in particular ellipticals); relatively uniform delay-time distribution; and, likely, mild - i.e., less than isolated evolution channels and more than dynamical channels - spin-spin and spin-orbit alignments.

The gravitational-wave (GW) inspirals of stellar-mass compact objects on to a supermassive black hole (SMBH), are some of the most promising GW sources detectable by next-generation space-born GW-detectors. The rates and characteristics of such extreme mass ratio inspirals (EMRIs) sources are highly uncertain. They are determined by the dynamics of stars near MBHs and the rate at which compacts objects are driven to the close proximity of the MBH. Here, we consider weakly and strongly mass-segregated nuclear clusters and the evolution of stars captured into highly eccentric orbits following binary disruptions by the MBH. We make use of a Monte Carlo approach to model the diffusion of both captured objects and compact-objects brought through two-body relaxation processes. We calculate the rates of GW-inspirals resulting from relaxation-driven objects and characterize EMRIs properties. We correct previous studies and show that relaxation-driven sources produce GW-sources with lower-eccentricity than previously found and provide the detailed EMRI eccentricity distribution in the weak and strong mass-segregation regimes. We also show that binary-disruption captured-stars could introduce low-eccentricity GW-sources of stellar BH EMRIs in mass-segregated clusters. The eccentricities of the GW-sources from the capture channel, however, are strongly affected by relaxation processes and are significantly higher than previously suggested. We find that both the rate and eccentricity distribution of EMRIs could probe the dynamics near MBHs, and the contribution of captured stars, characterize the mass-function of stellar compact objects, and verify whether weak or strong mass-segregation processes take place near MBHs.

The hyper-velocity star S5-HVS1, ejected 5 Myr ago from the Galactic Center at 1800 km/s, was most likely produced by tidal break-up of a tight binary by the supermassive black hole SgrA*. Taking a Monte Carlo approach, we show that the former companion of S5-HVS1 was likely a main-sequence star between 1.2 and 6M⊙ and was captured into a highly eccentric orbit with pericenter distance in the range 1–10 AU and semimajor axis about 103 AU. We then explore the fate of the captured star. We find that the heat deposited by tidally excited stellar oscillation modes leads to runaway disruption if the pericenter distance is smaller than about $3\rm \, AU$. Over the past 5 Myr, its angular momentum has been significantly modified by orbital relaxation, which may stochastically drive the pericenter inwards below $3\rm \, AU$ and cause tidal disruption. We find an overall survival probability in the range 5% to 50%, depending on the local relaxation time in the close environment of the captured star, and the initial pericenter at capture. The pericenter distance of the surviving star has migrated to 10–100 AU, making it potentially the most extreme member of the S-star cluster. From the ejection rate of S5-HVS1-like stars, we estimate that there may currently be a few stars in such highly eccentric orbits. They should be detectable (typically Ks ≲ 18.5 mag) by the GRAVITY instrument and by future Extremely Large Telescopes and hence provide an extraordinary probe of the spin of SgrA*.

We investigate the possible detection of the Hypercharge Axion (HCA) in colliders. The HCA is a hypothetical pseudoscalar that couples to weak hypercharge topological density and could potentially explain the dominance of matter over antimatter in the observable universe. If the HCA exists, it can be produced in colliders via vector boson fusion or in association with a photon or with a Z boson, and detected by looking for its decay into photons or Z bosons. We find that for certain values of the HCA mass and coupling scales, both of the order of a TeV, existing data from the Large Hadron Collider (LHC) can already put interesting constraints and in future colliders, such as the High Luminosity LHC, the accessible detection range is increased significantly.

We propose a novel source of gravitational wave (GW) emission: the inspirals of compact fragments inside primordial supermassive stars (SMSs). Such systems are thought to be an essential channel in the as-yet little understood formation of supermassive black holes. One model suggests that high accretion rates of 0.1-1 M⊙/yr attainable in atomically-cooled primordial halos can lead to the formation of a nuclear-burning SMS. This will ultimately undergo collapse through a relativistic instability, leaving a massive black hole remnant. Recent simulations suggest that supermassive stars rarely form in isolation, and that companion stars and even black holes formed may be captured/accreted and inspiral to the SMS core due to gas dynamical friction. Here, we explore the GW emission produced from such inspirals, which could probe the formation and evolution of SMS and seeds of the first supermassive black holes. We use a semi-analytic gas-dynamical friction model of the inspirals in the SMS to characterize their properties. We find such sources could potentially be observable by upcoming space-born GW-detectors at their formation redshifts with the benefit of gravitational lensing. Mergers within closely-related quasi-stars may produce a much stronger signal, though disambiguating such events from other high-z events may prove challenging.