Synchrotron aging and the radio spectrum of SN 1993J

We combine the GMRT low frequency radio observations of SN 1993J with the VLA high frequency radio data to get a near simultaneous spectrum around day 3200 since explosion. The low frequency measurements of the supernova determine the turnover frequency and flux scale of the composite spectrum and help reveal a steepening in the spectral index, $\Delta \alpha \sim 0.6$, in the optically thin part of the spectrum. This is the first observational evidence of a break in the radio spectrum of a young supernova. We associate this break with the phenomenon of synchrotron aging of radiating electrons. From the break in the spectrum we calculate the magnetic field in the shocked region independent of the equipartition assumption between energy density of relativistic particles and magnetic energy density. We determine the ratio of these two energy densities and find that this ratio is in the range: $8\times 10^{-6}-5\times 10^{-4}$. We also predict the nature of the evolution of the synchrotron break frequency with time, with competing effects due to diffusive Fermi acceleration and adiabatic expansion of the radiative electron plasma.Comment: 12 pages, 2 figures. Accepted for publication in ApJ

Formation and Fragmentation of Gaseous Spurs in Spiral Galaxies

Intermediate-scale spurs are common in spiral galaxies, but perhaps most distinctively evident in a recent HST image of M51 (Scoville & Rector 2001). We investigate, using time-dependent numerical MHD simulations, how such spurs could form (and subsequently fragment) from the interaction of a gaseous ISM with a stellar spiral arm. We model the gaseous medium as a self-gravitating, magnetized, differentially-rotating, razor-thin disk. The basic flow shocks and compresses as it passes through a local segment of a tightly-wound, trailing stellar spiral arm, modeled as a rigidly-rotating gravitational potential. We first construct 1D profiles for flows with spiral shocks. When the post-shock Toomre parameter Q_sp is sufficiently small, self-gravity is too large for one-dimensional steady solutions to exist. The critical values of Q_sp are 0.8, 0.5, and 0.4 for our models with zero, sub-equipartition, and equipartition magnetic fields, respectively. We then study the growth of self-gravitating perturbations in fully 2D flows, and find that spur-like structures rapidly emerge in our magnetized models. We associate this gravitational instability with the magneto-Jeans mechanism, in which magnetic tension forces oppose the Coriolis forces. The shearing and expanding velocity field shapes the condensed material into spurs as it flows downstream from the arms. Although we find swing amplification can help form spurs when the arm-interarm contrast is moderate, unmagnetized systems that are quasi-axisymmetrically stable are generally also stable to nonaxisymmetric perturbations, suggesting that magnetic effects are essential. In nonlinear stages, the spurs in our models undergo fragmentation to form 4\times 10^6 solar mass clumps, which we suggest could evolve into bright arm/interarm HII regions as seen in spiral galaxies.Comment: 32 pages, 14 figures, Accepted for publication in ApJ; better postscript figures available from http://www.astro.umd.edu/~kimwt/FIGURE2/ ; for associated Animated GIF movies, see http://www.astro.umd.edu/~kimwt/MOVIES

The Boltzmann equation for colourless plasmons in hot QCD plasma. Semiclassical approximation

Within the framework of the semiclassical approximation, we derive the Boltzmann equation describing the dynamics of colorless plasmons in a hot QCD plasma. The probability of the plasmon-plasmon scattering at the leading order in the coupling constant is obtained. This probability is gauge-independent at least in the class of the covariant and temporal gauges. It is noted that the structure of the scattering kernel possesses important qualitative difference from the corresponding one in the Abelian plasma, in spite of the fact that we focused our study on the colorless soft excitations. It is shown that four-plasmon decay is suppressed by the power of $g$ relative to the process of nonlinear scattering of plasmons by thermal particles at the soft momentum scale. It is stated that the former process becomes important in going to the ultrasoft region of the momentum scale.Comment: 41, LaTeX, minor changes, identical to published versio