Bence Kocsis

Abstract:

Abstract Black hole binary systems embedded in AGN discs have been proposed as a source of the observed gravitational waves (GWs) from LIGO-Virgo-KAGRA. Studies have indicated binary-single encounters could be common place within this population, yet we lack a comprehensive understanding of how the ambient gas affects the dynamics of these three-body encounters. We present the first hydrodynamical simulations of black hole binary-single encounters in an AGN disc. We find gas is a non-negligible component of binary-single interactions, leading to unique dynamics, including the formation of quasi-stable hierarchical triples. The gas efficiently and reliably dissipates the energy of the three-body system, hardening the triple provided it remains bound after the initial encounter. The hardening timescale is shorter for higher ambient gas densities. Formed triples can be hardened reliably by 2 − 3 orders of magnitude relative to the initial binary semi-major axis within less than a few AGN orbits, limited only by our resolution. The gas hardening of the triple enhances the probability for a merger by a minimum factor of 3.5 − 8 depending on our assumptions. In several cases, two of the black holes can execute periapses of less than 10 Schwarzschild radii, where the dynamics were fully resolved for previous close approaches. Our results suggest that current timescale estimates (without gas drag) for binary-single induced mergers are an upper bound. The shrinkage of the triple by gas has the prospect of increasing the chance for unique GW phenomena such as residual eccentricity, dephasing from a third object and double GW mergers.

Abstract:

<jats:p><jats:italic>Context</jats:italic>. Many Milky Way globular clusters (GCs) host multiple stellar populations, challenging the traditional view that GCs are single-population systems. It has been suggested that second-generation stars could form in a disk from gas lost by first-generation stars or from external accreted gas. Understanding how these multiple stellar populations evolve under a time-varying Galactic tidal field is crucial for studying internal mixing, the rotational properties, and mass loss of GCs over cosmological timescales.</jats:p> <jats:p><jats:italic>Aims</jats:italic>. We investigated how the introduction of a second stellar generation affects mass loss’ internal mixing, and rotational properties of GCs in a time-varying Galactic tidal field and different orbital configurations.</jats:p> <jats:p><jats:italic>Methods</jats:italic>. We conducted direct <jats:italic>N</jats:italic>-body simulations of GCs on three types of orbits derived from the observed Milky Way GCs using state-of-the-art stellar evolution prescriptions. We evolved the clusters for 8 Gyr in the time-varying Galactic potential of the IllustrisTNG-100 cosmological simulation. After 2 Gyr, we introduced a second stellar generation, comprising 5% of the initial mass of the first generation, as a flattened disk of stars. For comparison, we ran control simulations using a static Galactic potential and isolated clusters.</jats:p> <jats:p><jats:italic>Results</jats:italic>. We present here the mass loss, structural evolution, and kinematic properties of GCs with two stellar generations, focusing on tidal mass’ half-mass radii, velocity distributions, and angular momentum. We also examine the transition of the second generation from a flattened disk to a spherical shape.</jats:p> <jats:p><jats:italic>Conclusions</jats:italic>. Our results show that the mass loss of GCs depends primarily on their orbital parameters, with tighter orbits leading to higher mass loss. The growth of the Galaxy led to tighter orbits’ implying that the GCs lost much less mass than if the Galaxy had always had its current mass. The initially flattened second-generation disk became nearly spherical within one relaxation time. However, whether its distinct rotational signature was retained depends on the orbit: for the long radial orbit, it vanished quickly; for the tube orbit' it lasted several billion years for the circular orbit' rotation persisted until the present day.</jats:p>

Abstract:

Wide, highly eccentric (𝑒 >0.9) compact binaries can naturally arise as progenitors of gravitational wave (GW) mergers. These systems are expected to have a significant population in the mHz band (e.g., ∼3–45 detectable stellar-mass binary black holes with 𝑒 >0.9 in the Milky Way), with their GW signals characterized by “repeated bursts” emitted upon each pericenter passage. In this study, we show that the detection of mHz GW signals from highly eccentric stellar mass binaries in the local universe can strongly constrain their orbital parameters. Specifically, it can achieve a relative measurement error of ∼10−6 for orbital frequency and ∼1% for eccentricity (as 1 −𝑒) in most of the detectable cases. On the other hand, the binary’s mass ratio, distance, and intrinsic orbital orientation may be less precisely determined due to degeneracies in the GW waveform. We also perform mock LISA data analysis to evaluate the realistic detectability of highly eccentric compact binaries. Our results show that highly eccentric systems could be efficiently identified when multiple GW sources and stationary Gaussian instrumental noise are present in the detector output. This work highlights the potential of extracting the signal of “bursting” LISA sources to provide valuable insights into their orbital evolution, surrounding environment, and formation channels.