Theoretical astrophysics and plasma physics at RPC

Abstract:

With the advent of high power lasers, new opportunities have opened up for simulating astrophysical processes in the laboratory. We show that 2nd-order Fermi acceleration can be directly investigated at the National Ignition Facility, Livermore. This requires measuring the momentumspace diffusion of 3 MeV protons produced within a turbulent plasma generated by a laser. Treating Fermi acceleration as a biased diffusion process, we show analytically that a measurable broadening of the initial proton distribution is then expected for particles exiting the plasma.

Abstract:

We present a semi-analytic model for self-consistently evolving a population of globular clusters (GCs) in a given host galaxy across cosmic time. We compute the fraction of GCs still hosting intermediate-mass black holes (IMBHs) at a given redshift in early and late type galaxies of different masses and sizes, and the corresponding rate of tidal disruption events (TDEs), both main-sequence (MS) and white dwarf (WD) stars. We find that the integrated TDE rate for the entire GC population can exceed the corresponding rate in a given galactic nucleus and that $\sim 90$% of the TDEs reside in GCs within a maximum radius of $\sim 2-15$ kpc from the host galaxy's center. This suggests that observational efforts designed to identify TDEs should not confine themselves to galactic nuclei alone, but should also consider the outer galactic halo where massive old GCs hosting IMBHs would reside. Indeed, such off-centre TDEs as predicted here may already have been observed. MS TDE rates are more common than WD TDE rates by a factor 30 (100) at $z\leq 0.5$ ($z=2$). We also calculate the rate of IMBH-SBH mergers across cosmic time, finding that the typical IMRI rate at low redshift is of the order of $\sim 0.5-3$ Gpc$^{-3}$ yr$^{-1}$, which becomes as high as $\sim 100$ Gpc$^{-3}$ yr$^{-1}$ near the peak of GC formation. Advanced LIGO combined with VIRGO, KAGRA, ET and LISA will be able to observe the bottom-end and top-end of the IMBH population, respectively.

Abstract:

Abstract We apply collisionless particle-in-cell simulations of relativistic pair plasmas to explore whether driven turbulence is a viable high-energy astrophysical particle accelerator. We characterize nonthermal particle distributions for varying system sizes up to L/2πρ e0 = 163, where L/2π is the driving scale and ρ e0 is the initial characteristic Larmor radius. We show that turbulent particle acceleration produces power-law energy distributions that, when compared at a fixed number of large-scale dynamical times, slowly steepen with increasing system size. We demonstrate, however, that convergence is obtained by comparing the distributions at different times that increase with system size (approximately logarithmically). We suggest that the system-size dependence arises from the time required for particles to reach the highest accessible energies via Fermi acceleration. The converged power-law index of the energy distribution, α ≈ 3.0 for magnetization σ = 3/8, makes turbulence a possible explanation for nonthermal spectra observed in systems such as the Crab Nebula.

Abstract:

The high rate of black hole (BH) mergers detected by LIGO/Virgo opened questions on their astrophysical origin. One possibility is the dynamical channel, in which binary formation and hardening is catalyzed by dynamical encounters in globular clusters (GCs). Previous studies have shown that the BH merger rate from the present day GC density in the Universe is lower than the observed rate. In this \textit{Letter}, we study the BH merger rate by accounting for the first time for the evolution of GCs within their host galaxies. The mass in GCs was initially $\sim 8\times$ higher, which decreased to its present value due to evaporation and tidal disruption. Many BH binaries that were ejected long before their merger, originated in GCs that no longer exist. We find that the comoving merger rate in the dynamical channel from GCs varies between $18$ to $35\,{\rm Gpc}^{-3}\,{\rm yr}^{-1}$ between redshift $z=0.5$ to $2$, and the total rate is $1$, $5$, $24$ events per day within $z=0.5$, $1$, and $2$, respectively. The cosmic evolution and disruption of GCs systematically increases the present-day merger rate by a factor $\sim 2$ relative to isolated clusters. Gravitational wave detector networks offer an unique observational probe of the initial number of GC populations and their subsequent evolution across cosmic time.

Abstract:

© 2018. The American Astronomical Society. All rights reserved.. Black hole (BH) mergers detectable with the Laser Interferometer Gravitational-wave Observatory (LIGO) can occur in active galactic nucleus (AGN) disks. Here we parameterize the merger rates, the mass spectrum, and the spin spectrum of BHs in AGN disks. The predicted merger rate spans ∼10-3-104 Gpc-1 yr-1, so upper limits from LIGO (<212 Gpc-1 yr-1) already constrain it. The predicted mass spectrum has the form of a broken power law, consisting of a pre-existing BH power-law mass spectrum and a harder power-law mass spectrum resulting from mergers. The predicted spin spectrum is multipeaked with the evolution of retrograde spin BHs in the gas disk playing a key role. We outline the large uncertainties in each of these LIGO observables for this channel and we discuss ways in which they can be constrained in the future.