Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence

We analyze the time dependence of fluid variables in general relativistic, magnetohydrodynamic simulations of accretion flows onto a black hole with dimensionless spin parameter a/M=0.9. We consider both the case where the angular momentum of the accretion material is aligned with the black hole spin axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted flow). In comparison to the untilted simulation, the tilted simulation exhibits a clear excess of inertial variability, that is, variability at frequencies below the local radial epicyclic frequency. We further study the radial structure of this inertial-like power by focusing on a radially extended band at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables. The three dimensional density structure at this frequency suggests that the power is a composite oscillation whose dominant components are an over dense clump corotating with the background flow, a low order inertial wave, and a low order inertial-acoustic wave. Our results provide preliminary confirmation of earlier suggestions that disk tilt can be an important excitation mechanism for inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap

Persistent acceleration of positrons in a nonstationary shock wave

Long-time evolution of positrons accelerated in an oblique shock wave in an electron-positron-ion plasma is studied with relativistic, electromagnetic, particle simulations. In the early stage, some positrons move nearly parallel to the external magnetic field in the shock transition region and gain energy from the parallel electric field. The acceleration can become stagnant owing to the deformation of the wave profile. After the recovery of the shock profile, however, the acceleration can start again. By the end of simulation runs, omega_pet=5000, positron Lorentz factors reached values ~2000. In this second stage, three different types of acceleration are found. In the first type, the acceleration process is the same as that in the early stage. In the second type, positrons make gyromotions in the wave frame and gain energy mainly from the perpendicular electric field. In the third type, particle orbits are similar to curtate cycloids. Theoretical estimate for this energy increase is given

Crowdsourcing the Collection of Transportation Behavior Data

Understanding the travel behaviors of individuals who use public transit is essential for enhancing the performance, sustainability and efficiency of public transportation. Contemporary methods for collecting data on transportation behavior are focused on manual or automated procedures for counting the number of individual passengers entering or exiting transit vehicles. While such methods provide useful data for understanding transit demand throughout a network, they ignore the important details of how passengers travel to and within a network as well as their personal experiences during their commute, all of which can enrich the ability of transit agencies to provide sustainable transportation. To address this issue, there has been a proliferation of location-based services (LBS) that allow for new methods of data collection involving passengers volunteering data about their commute. In this light, passengers engage in a crowdsourcing effort to generate data about experiences across the network. This project’s objective is to implement and test specific LBS in a bus transit network to better understand their potential and limitations for improving the crowdsourcing of travel behavior data

Global Radiation-Magnetohydrodynamic Simulations of Black Hole Accretion Flow and Outflow: Unified Model of Three States

Black-hole accretion systems are known to possess several distinct modes (or spectral states), such as low/hard state, high/soft state, and so on. Since the dynamics of the corresponding flows is distinct, theoretical models were separately discussed for each state. We here propose a unified model based on our new, global, two-dimensional radiation-magnetohydrodynamic simulations. By controlling a density normalization we could for the first time reproduce three distinct modes of accretion flow and outflow with one numerical code. When the density is large (model A), a geometrically thick, very luminous disk forms, in which photon trapping takes place. When the density is moderate (model B), the accreting gas can effectively cool by emitting radiation, thus generating a thin disk, i.e., the soft-state disk. When the density is too low for radiative cooling to be important (model C), a disk becomes hot, thick, and faint; i.e., the hard-state disk. The magnetic energy is amplified within the disk up to about twice, 30%, and 20% of the gas energy in models A, B, and C, respectively. Notably, the disk outflows with helical magnetic fields, which are driven either by radiation pressure force or magnetic pressure force, are ubiquitous in any accretion modes. Finally, our simulations are consistent with the phenomenological alpha-viscosity prescription, that is, the disk viscosity is proportional to the pressure.Comment: 5 pages, 2 figures, accepted for publication in PASJ Letter