Theoretical astrophysics and plasma physics at RPC

Abstract:

The adiabatic equation of state upper P proportional to n Superscript upper Gamma $P\propto n^{\Gamma }$ $P \propto n^{\varGamma }$ describes the pressure evolution of highly collisional, isotropic plasmas in terms of their density, providing a possible closure of the fluid moment hierarchy in the absence of heat fluxes and dissipation. An analogous closure exists for collisionless, magnetised plasmas, whose pressure tensor is anisotropic with respect to the magnetic field, and the closure is therefore double adiabatic, prescribing the evolution of the parallel and perpendicular pressures in terms of the magnetic-field strength and density. Here, we present a general first-principles formalism to derive adiabatic laws using symmetries and phase-space conservation properties of the distribution function. With this theory we recover the adiabatic equation of state upper P proportional to n Superscript upper Gamma $P\propto n^{\Gamma }$ $P \propto n^{\varGamma }$ for isotropic plasmas and the double-adiabatic equations of state for collisionless, magnetised plasmas. We extend the latter to the relativistic regime, finding that their exact functional form depends on the pressure anisotropy and is not a simple power law. Our double-adiabatic equations of state describe simple geometries, like magnetic mirrors or compressed homogeneous plasmas, as well as complex high-energy astrophysical processes, such as the evolution of plasmoid structures formed during magnetic reconnection.

Abstract:

Abstract Gyrokinetic simulations of tokamak turbulence indicate that fluctuation levels increase abruptly and dramatically when the plasma beta exceeds a certain critical value. This increase in fluctuation levels coincides with a transition from a state dominated by zonal flow to one in which turbulent eddies form radially-elongated 'streamers'. Here we derive from gyrokinetics a minimal fluid model for electromagnetic ion-temperature-gradient (ITG) turbulence that captures the key features of this transition. Due to the relative simplicity of the model, we are able to conduct a detailed numerical study of the interplay between the turbulence and the zonal flow across a broad range of values of the plasma beta and the ITG. We find that the transition occurs when the Reynolds stress, which tends to strengthen zonal flows, is overwhelmed by the Maxwell and diamagnetic stresses, which tend to weaken them. Power-law scalings of the stress ratios with plasma beta and ITG are obtained, indicating a possible means by which the location of the transition could be predicted with minimal computational cost.

Abstract:

Eccentric binary black holes (BBHs) formed through dynamical interactions can significantly contribute to gravitational wave (GW) detections. In this work, we present a simulated catalog of dynamically formed, stellar-mass BBHs in the local universe, incorporating contributions from the Galactic field (flyby interactions), Galactic nucleus (eccentric Kozai-Lidov evolution), and globular clusters (N-body interactions). Our results predict a wide, highly eccentric BBH population in the Milky Way, with source counts of ∼25, 9, 3, 1, and <1 for a 4-year LISA mission (∼36, 13, 5, 2, and 1 for a 10-year extended mission), for SNR>1, 3, 8, 20, and 50, respectively. Extending this model to cosmological populations, we show that different dynamical channels can produce distinct eccentricity distributions in the LVK band and can contribute hundreds of additional low-SNR mHz sources. Specifically, our model yields a merger rate of Γ∼9 Gpc-3 yr-1 and ∼150 extragalactic mHz BBHs with SNR>1. However, due to the lower mass and weaker GW signals of stellar-mass BBHs, this number declines sharply at higher detection thresholds (e.g., ∼1 for SNR>8). We further highlight the impact of eccentric BBH signals on the LISA global fit, showing that their individual harmonics can be independently detected in the Milky Way, and may mimic circular binaries with systematically biased chirp masses. Lastly, we show that post-Newtonian waveforms converge reliably for eccentric BBHs with masses of ≲103M⊙ in the mHz band. Overall, eccentric BBHs represent a prevalent and promising target for future space-based GW observatories.