Theoretical astrophysics and plasma physics at RPC

Abstract:

Compact objects evolving in an astrophysical environment experience a gravitational drag force known as dynamical friction. We present a multipole-frequency decomposition to evaluate the orbit-averaged energy and angular momentum dissipation experienced by point masses on periodic orbits within a homogeneous, fluidlike background. Our focus is on eccentric Keplerian trajectories. Although our approach is currently restricted to linear response theory, it is fully consistent within that framework. We validate our theoretical expressions for the specific case of an ideal fluid, using semi-numerical simulations of the linear response acoustic wake. We demonstrate that, for a finite-time perturbation switched on at t=0, a steady dissipation state is reached after a time bounded by twice the sound crossing time of the apocenter distance. We apply our results to model the secular evolution of compact eccentric binaries in a gaseous medium, assuming low-density conditions where the orbital elements evolve adiabatically. For unequal-mass systems with moderate initial eccentricity, the late-time eccentricity growth is significantly delayed compared to the equal-mass case, due to the binary components becoming transonic at different times along their orbital trajectory. Our approach offers a computationally efficient alternative to full simulations of the linear response wake.

Abstract:

The interaction of gravitational waves (GWs) with matter is thought to be typically negligible in the Universe. We identify an exception in the case of resonant interactions, where GWs emitted by a background binary system, such as an inspiraling supermassive black hole (SMBH) binary, cause a resonant response in a stellar-mass foreground binary and the frequencies of the two systems become, and remain, synchronized. We point out that this previously unexplored dynamical phenomenon is not only possible, but can lead to $\mathcal{O}(30)$ binary systems becoming resonantly locked in the host galaxy of merging SMBHs of mass ${10}^{8.5-11}{M}_{\odot }$ , each of which has a significantly reduced merger time. We predict $\mathcal{O}({10}^{10})$ binary systems have been locked in the Universe’s history. Resonant locking could be detected through anomalous inspiral of binary systems.

Abstract:

Abstract Consider a system of point masses in a spherical potential. In such systems objects execute planar orbits covering two-dimensional rings or annuli, represented by the angular-momentum vectors, which slowly reorient due to the persistent weak gravitational interaction between different rings. This process, called vector resonant relaxation, is much faster than other processes which change the size/shape of the rings. The interaction is strongest between objects with closely aligned angular-momentum vectors. In this paper, we show that nearly parallel angular-momentum vectors may form stable bound pairs in angular-momentum space. We examine the stability of such pairs against an external massive perturber, and determine the critical separation analogous to the Hill radius or tidal radius in the three-body problem, where the angular-momentum pairs are marginally disrupted, as a function of the perturber’s mass, the orbital inclination, and the radial distance. Angular-momentum pairs or multiples closer than the critical inclination will remain bound and evolve together in angular-momentum-direction space under any external influence, such as anisotropic density fluctuations, or massive perturbers. This study has applications in various astrophysical contexts, including galactic nuclei, in particular the Milky Way’s Galactic centre, globular clusters, or planetary systems. In nuclear star clusters with a central super-massive black hole, we apply this criterion to the disc of young, massive stars, and show that clusters in angular-momentum space may be used to constrain the presence of intermediate-mass black holes or the mass of the nearby gaseous torus.

Abstract:

A theory of cosmic-ray transport in multiphase diffusive media is developed, with the specific application to cases in which the cosmic-ray diffusion coefficient has large spatial fluctuations that may be inherently multiscale. We demonstrate that the resulting transport of cosmic rays is diffusive in the long-time limit, with an average diffusion coefficient equal to the harmonic mean of the spatially varying diffusion coefficient. Thus, cosmic-ray transport is dominated by areas of low diffusion even if these areas occupy a relatively small, but not infinitesimal, fraction of the volume. On intermediate time-scales, the cosmic rays experience transient effective subdiffusion, as a result of low-diffusion regions interrupting long flights through high-diffusion regions. In the simplified case of a two-phase medium, we show that the extent and extremity of the subdiffusivity of cosmic-ray transport is controlled by the spectral exponent of the distribution of patch sizes of each of the phases. We finally show that, despite strongly influencing the confinement times, the multiphase medium is only capable of altering the energy dependence of cosmic-ray transport when there is a moderate (but not excessive) level of perpendicular diffusion across magnetic-field lines.