Simulation Study on High Energy Cosmic Electron Detection by Shower Image

Many projects have recently been carried out and proposed for observing high energy electrons since it is realized that cosmic ray electrons are very important when studying the dark matter particles and the acceleration mechanism of cosmic rays. An imaging calorimeter, BETS （Balloon-borne Electron Telescope with Scintillator fiber）, has been developed for this purpose. Using pattern analysis of the shower development, the electrons can be selected from those primary cosmic ray proton events with flux heights one-tenth that of the electrons. The Monte-Carlo simulation is indispensable for the instrument design, the signal trigger and the data analysis. We present different shower simulation codes and compare the simulation results with the beam test and the flight data of BETS. We conclude that the code FLUKA2002 gives the most consistent results with the experimental data.     

Is Ursa Major II the progenitor of the Orphan Stream?

Prominent in the 'Field of Streams'– the Sloan Digital Sky Survey map of substructure in the Galactic halo – is an 'Orphan Stream' without obvious progenitor. In this numerical study, we show a possible connection between the newly found dwarf satellite Ursa Major II (UMa II) and the Orphan Stream. We provide numerical simulations of the disruption of UMa II that match the observational data on the position, distance and morphology of the Orphan Stream. We predict the radial velocity of UMa II as −100 km s⁻¹, as well as the existence of strong velocity gradients along the Orphan Stream. The velocity dispersion of UMa II is expected to be high, though this can be caused both by a high dark matter content or by the presence of unbound stars in a disrupted remnant. However, the existence of a gradient in the mean radial velocity across UMa II provides a clear-cut distinction between these possibilities. The simulations support the idea that some of the anomalous, young halo globular clusters like Palomar 1 or Arp 2 or Ruprecht 106 may be physically associated with the Orphan Stream.     

Dynamical friction force exerted on spherical bodies

We present a rigorous calculation of the dynamical friction force exerted on a spherical massive perturber moving through an infinite homogenous system of field stars. By calculating the shape and mass of the polarization cloud induced by the perturber in the background system, which decelerates the motion of the perturber, we recover Chandrasekhar's drag force law with a modified Coulomb logarithm. As concrete examples, we calculate the drag force exerted on a Plummer sphere or a sphere with the density distribution of a Hernquist profile. It is shown that the shape of the perturber only affects the exact form of the Coulomb logarithm. The latter converges on small scales because encounters of the test and field stars with impact parameters less than the size of the massive perturber become inefficient. We confirm in this way the earlier results based on the impulse approximation of small angle scatterings.