arXiv subjects

Hyesung Kang

Publications and source records attributed to Hyesung Kang.

113 records · Page 7Linked to original sources

arXiv1995

Cluster Accretion Shocks as Possible Acceleration Sites for Ultra High Energy Protons below the Greisen Cutoff

Three-dimensional hydrodynamic simulations of large scale structure in the Universe have shown that accretion shocks form during the gravitational collapse of one-dimensional caustics, and that clusters of galaxies formed at intersections of the caustics are surrounded by these accretion shocks. Estimated speed and curvature radius of the shocks are 1000-3000 \kms and about 5 Mpc, respectively, in the $Ω=1$ CDM universe. Assuming that energetic protons are accelerated by these accretion shocks via the first-order Fermi process and modeling particle transport around the shocks through Bohm diffusion, we suggest that protons can be accelerated up to the {\it Greisen cutoff energy} near $6\times 10^{19}$ eV, provided the mean magnetic field strength in the region around the shocks is at least of order a microgauss. We have also estimated the proton flux at earth from the Virgo cluster. Assuming a few (1-10) \% of the ram pressure of the infalling matter would be transferred to the cosmic-rays, the estimated flux for $ E \sim 10^{19}$eV is consistent with observations, so that such clusters could be plausible sources of the UHE CRs.

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arXiv1995

Background X-ray Emission from Hot Gas in CDM and CDM+Lambda Universes: Spectral Signatures

We present a new treatment of two popular models for the growth of structure, examining the X-ray emission from hot gas with allowance for spectral line emission from various atomic species, primarily ``metals". The X-ray emission from the bright cluster sources is not significantly changed from prior work and, as noted earlier, shows the CDM$+Λ$ model (LCDM) to be consistent but the standard, COBE normalized model (SCDM) to be inconsistent with existing observations --- after allowance for still the considerable numerical modelling uncertainties. But we find one important new result. Radiation in the softer band 0.5-1.0keV is predominantly emitted by gas far from cluster centers (hence ``background"). This background emission dominates over the cluster emission below 1keV and observations of it should show clear spectral signatures indicating its origin. In particular the ``iron blend" should be seen prominantly in this spectral bin from cosmic background hot gas at high galactic latitudes and should show shadowing against the SMC indicating its extragalactic origin. Certain OVII lines also provide a signature of this gas which emits a spectrum characteristic of $10^{6.6\pm 0.6}$K gas. Recent ASCA observations of the X-ray background tentatively indicate the presence of component with exactly the spectral features we predict here.

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arXiv1995

Diffusive Shock Acceleration Simulations: Comparison with Particle Methods and Bow Shock Measurements

Direct comparisons of diffusive particle acceleration numerical simulations have been made against Monte Carlo and hybrid plasma simulations by Ellison {\it et. al.} (1993) and against observations at the earth's bow shock presented by Ellison {\it et. al.} (1990). Toward this end we have introduced a new numerical scheme for injection of cosmic-ray particles out of the thermal plasma, modeled by way of the diffusive scattering process itself; that is, the diffusion and acceleration across the shock front of particles out of the suprathermal tail of the Maxwellian distribution. We find that all of these computational methods (diffusion-advection, two-fluid, Monte Carlo and hybrid) are in substantial agreement on the issues they can simultaneously address, so that the essential physics of diffusive particle acceleration is adequately contained within each. This is despite the fact that each makes what appear to be very different assumptions or approximations.

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arXiv1994

Hot Gas in the CDM Scenario: X-Ray Clusters from a High Resolution Numerical Simulation

We exmaine the distribution of hot gas in a standard CDM model of the universe using high resolution hydrodynamic simulations. Adopting standard parameters determined from COBE and light element nucleosynthesis, $σ_8=1.05$, $Ω_b=0.06$ and assuming $h=0.5$, we find the X-ray emitting clusters, compute the luminosity function at several wavelengths, the temperature distribution and estimated sizes as well as the evolution of these quantities with redshift. This standard CDM model, normalized to COBE, produces approximately 5 times too much emission from clusters having $L_x>10^{43}$erg/s, a not unexpected result. If all other parameters were unchanged, we would expect adequate agreement for $σ_8=0.6$. This provides a new and independent argument for lower small scale power than standard CDM at the $8h^{-1}$Mpc scale. The background radiation field at 1keV due to clusters in this model is approximately $1/3$ of the observed background which, after correction for numerical effects, again indicates approximately 5 times too much emission and the appropriateness of $σ_8=0.6$. If we had used the observed ratio of gas to total mass in clusters, rather than basing the mean density on light element nucleosynthesis, then the computed luminosity of each cluster would have increased still further, by a factor of approximately ten. Examining the ratio of gas to total mass in the clusters (which we find to be anti-biased by a factor of approximately 0.6), normalized to $Ω_b h^2=0.015$, and comparing to observations, we conclude, in agreement with S. White, that the cluster observations argue for an open universe.

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arXiv1994

A Comparison of Cosmological Hydrodynamic Codes

We present a detailed comparison of the simulation results of various cosmological hydrodynamic codes. Starting with identical initial conditions based on the Cold Dark Matter scenario for the growth of structure, we integrate from redshift $z=20$ to $z=0$ to determine the physical state within a representative volume of size $L^3$ where $L=64 h^{-1} {\rm Mpc}$. Five independent codes are compared: three of them Eulerian mesh based and two variants of the Smooth Particle Hydrodynamics "SPH" Lagrangian approach. The Eulerian codes were run at $N^3=(32^3,~64^3,~128^3,~{\rm and},~256^3)$ cells, the SPH codes at $N^3= 32^3$ and $64^3$ particles. Results were then rebinned to a $16^3$ grid with the expectation that the rebinned data should converge, by all techniques, to a common and correct result as $N \rightarrow \infty$. We find that global averages of various physical quantities do, as expected, tend to converge in the rebinned model, but that uncertainties in even primitive quantities such as $\langle T \rangle$, $\langle ρ^2\rangle^{1/2}$ persists at the 3\%-17\% level after completion of very large simulations. The two SPH codes and the two shock capturing Eulerian codes achieve comparable and satisfactory accuracy for comparable computer time in their treatment of the high density, high temperature regions as measured in the rebinned data; the variance among the five codes (at highest resolution) for the mean temperature (as weighted by $ρ^2$) is only 4.5\%. Overall the comparison allows us to better estimate errors, it points to ways of improving this current generation of hydrodynamic codes and of suiting their use to problems which exploit their individually best features.

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