arXiv subjects

Hyesung Kang

Publications and source records attributed to Hyesung Kang.

At least 109 records · Page 6Linked to original sources

arXiv2000

Time dependent cosmic-ray shock acceleration with self-consistent injection

One of the key questions to understanding the efficiency of diffusive shock acceleration of the cosmic rays (CRs) is the injection process from thermal particles. A self-consistent injection model based on the interactions of the suprathermal particles with self-generated magneto-hydrodynamic waves has been developed recently by Malkov (1998, Phys. Rev. E 58, 4911). By adopting this analytic solution, a numerical treatment of the plasma-physical injection model at a strong quasi-parallel shock has been devised and incorporated into the combined gas dynamics and the CR diffusion-convection code. In order to investigate self-consistently the injection and acceleration efficiencies, we have applied this code to the CR modified shocks of both high and low Mach numbers (M=30 and M=2.24) with a Bohm type diffusion model. We find the injection process is self-regulated in such a way that the injection rate reaches and stays at a nearly stable value after quick initial adjustment. For both shocks about 0.1% of the incoming thermal particles are injected into the CRs. For the weak shock, the shock has reached a steady state within our integration time and about 10% of the total available shock energy is transfered into the CR energy density. The strong shock has achieved a higher acceleration efficiency of about 20% by the end of our simulation, but has not yet reached a steady-state.

astro-ph↗

arXiv2000

Formation of Proto-Globular Cluster Clouds by Thermal Instability

Many models of globular cluster formation assume the presence of cold dense clouds in early universe. Here we re-examine the Fall & Rees (1985) model for formation of proto-globular cluster clouds (PGCCs) via thermal instabilities in a protogalactic halo. We first argue, based on the previous study by others, that under the protogalactic environments only nonlinear density inhomogeneities can condense into PGCCs. We then carry out numerical simulations of the collapse of overdense clouds in one-dimensional spherical geometry, including self-gravity and radiative cooling down to T=10^4 K. Since imprinting of Jeans mass at 10^4 K is essential to this model, here we focus on the cases where external UV background radiation prevents the formation of H2 molecules and so prevent the cloud from cooling below 10^4 K. The quantitative results from these simulations can be summarized as follows: 1) Perturbations smaller than M_min ~ (10^{5.6} M_sun) (n_h/0.05 cm3)^{-2} cool isobarically, while perturbations larger than M_max ~ (10^8 M_sun) (n_h/0.05 cm3)^{-2} cool isochorically. On the other hand, intermediate size perturbations (M_min< M_pgcc < M_max) are compressed supersonically. 2) For supersonically collapsing clouds, the density compression factor after they cool to T_c=10^4 K range 10^{2.5}-10^6. 3) For supersonically collapsing clouds the Jeans mass can be reduced to as small as 10^{5.5} M_sun (n_h/0.05 cm3)^{-1/2} at the maximum compression. 4) The density profile of simulated PGCCs can be approximated by a constant core with a halo of rho ~ r^{-2} rather than a singular isothermal sphere.

astro-ph↗

arXiv2000

Winds and Shocks in Galaxy Clusters: Shock Acceleration on an Intergalactic Scale

We review the possible roles of large scale shocks as particle accelerators in clusters of galaxies. Recent observational and theoretical work has suggested that high energy charged particles may constitute a substantial pressure component in clusters. If true that would alter the expected dynamical evolution of clusters and increase the dynamical masses consistent with hydrostatic equilibrium. Moderately strong shocks are probably common in clusters, through the actions of several agents. The most obvious of these agents include winds from galaxies undergoing intense episodes of starbursts, active galaxies and cosmic inflows, such as accretion and cluster mergers. We describe our own work derived from simulations of large scale structure formation, in which we have, for the first time, explicitly included passive components of high energy particles. We find, indeed that shocks associated with these large scale flows can lead to nonthermal particle pressures big enough to influence cluster dynamics. These same simulations allow us also to compute nonthermal emissions from the clusters. Here we present resulting predictions of gamma-ray fluxes.

astro-ph↗

arXiv2000

Acceleration of CR at Large Scale Shocks and Their Cosmological Role for Structure Formation in the Universe

We investigate the dynamical importance of a newly recognized possible source of significant feedback generated during structure formation; namely cosmic ray (CR) pressure. We present evidence for the existence of numerous shocks in the hot gas of galaxy clusters (GCs). We employ for the first time an explicit numerical treatment of CR acceleration and transport in hydro simulations of structure formation. According to our results, CRs provide an important fraction of the total pressure inside GCs, up to several tenths. This was true even at high redshift (z=2), meaning that such non-thermal component could affect the evolution of structure formation.

astro-ph↗

arXiv2000

Properties of Cosmic Shock Waves in Large Scale Structure Formation

We have examined the properties of shock waves in simulations of large scale structure formation for two cosmological scenarios (a SCDM and a LCDM with Omega =1). Large-scale shocks result from accretion onto sheets, filaments and Galaxy Clusters (GCs) on a scale of circa 5 Mpc/h in both cases. Energetic motions, both residual of past accretion history and due to current asymmetric inflow along filaments, generate additional, common shocks on a scale of about 1 Mpc/h, which penetrate deep inside GCs. Also collisions between substructures inside GCs form merger shocks. Consequently, the topology of the shocks is very complex and highly connected. During cosmic evolution the comoving shock surface density decreases, reflecting the ongoing structure merger process in both scenarios. Accretion shocks have very high Mach numbers (10-10^3), when photo-heating of the pre-shock gas is not included. The typical shock speed is of order v_{sh}(z) =H(z)lambda_{NL}(z), with lambda_{NL}(z) the wavelength scale of the nonlinear perturbation at the given epoch. However, the Mach number for shocks occuring within clusters is usually smaller (3-10), due to the fact that the intracluster gas is already hot. Statistical fits of shock speed around GCs as a function of GCs temperature give power-law's in accord with 1-D predictions. However, a very different result is obtained for fits of the shock radius, reflecting the very complex shock structures forming in 3-D simulations. The in-flowing kinetic energy across such shocks, giving the power available for cosmic-ray acceleration, is comparable to the cluster X-ray luminosity emitted from a central region of radius 0.5 Mpc/h. Considering their large size and long lifetimes, those shocks are potentially interesting sites for cosmic-ray acceleration, if modest magnetic fields exist within them.

astro-ph↗

arXiv1999

Cosmic Rays and Their Radiative Processes in Numerical Cosmology

A cosmological hydrodynamic code is described, which includes a routine to compute cosmic ray acceleration and transport in a simplified way. The routine was designed to follow explicitly diffusive acceleration at shocks, and second-order Fermi acceleration and adiabatic loss in smooth flows. Synchrotron cooling of the electron population can also be followed. The updated code is intended to be used to study the properties of nonthermal synchrotron emission and inverse Compton scattering from electron cosmic rays in clusters of galaxies, in addition to the properties of thermal bremsstrahlung emission from hot gas. The results of a test simulation using a grid of $128^3$ cells are presented, where cosmic rays and magnetic field have been treated passively and synchrotron cooling of cosmic ray electrons has not been included.

astro-ph↗

arXiv1999

Large Cosmic Shock Waves as Sites for Particle Acceleration

The properties of cosmic shock waves are studied through numerical simulations in two cosmological scenarios (SCDM and LCDM). The scaling relations for the average radius and velocity associated with the accretion shocks are somewhat different, yet qualitatively similar to the self similar solutions for a flat Omega_M=1 universe. The energy supplied by infalling gas at accretion shock waves is large enough to sustain production of abundant cosmic ray populations if a viable acceleration mechanism can take place there. Finally, in addition to shocks created by the encounter of the merging ICMs of two clusters of galaxies, accretion shocks associated with the merging clusters generate strong ``relic'' shocks which propagate through the ICM producing additional heating of the ICM, and associated CR acceleration.

astro-ph↗

arXiv1999

Ion injection and acceleration at modified shocks

The theory of diffusive particle acceleration explains the spectral properties of the cosmic rays below energies of approx. 10^6 GeV as produced at strong shocks in supernova remnants (SNR's). To supply the observed flux of cosmic rays, a significant fraction of the energy released by a supernova has to be transfered to cosmic rays. The key to the question of the efficiency of SNR's in producing cosmic rays is the injection process from thermal energies. A self-consistent model has to take into account the interaction of the accelerated particles with magneto-hydrodynamic waves, which generate the particle diffusion, a requisite for the acceleration process. Such a nonlinear model of the turbulent background plasma has been developed recently (Malkov, 1998, Phys. Rev. E 58, 4911). We use this model for the first numerical treatment of the gas dynamics and the diffusion-convection equation at a quasi-parallel strong shock, which incorporates a plasma-physical injection model to investigate the cosmic ray production.

astro-ph↗

arXiv1998

Cosmic Magnetic Fields in Large Scale Filaments and Sheets

We consider the possibility that cosmic magnetic field, instead of being uniformly distributed, is strongly correlated with the large scale structure of the universe. Then, the observed rotational measure of extra-galactic radio sources would be caused mostly by the clumpy magnetic field in cosmological filaments/sheets rather than by a uniform magnetic field, which was often assumed in previous studies. As a model for the inhomogeneity of the cosmological magnetic field, we adopt a cosmological hydrodynamic simulation, where the field is passively included, and can approximately represent the real field distribution with an arbitrary normalization for the field strength. Then, we derive an upper limit of the magnetic field strength by comparing the observed limit of rotational measure with the rotational measure expected from the magnetic field geometry in the simulated model universe. The resulting upper limit to the magnetic field in filaments and sheets is ${\bar B}_{fs} \la 1 μG$ which is $\sim10^3$ times higher than the previously quoted values. This value is close to, but larger than, the equipartition magnetic field strength in filaments and sheets. The amplification mechanism of the magnetic field to the above strength is uncertain. The implications of such a strength of the cosmic magnetic field are discussed.

astro-ph↗

arXiv1997

$10^{51}$ Ergs: The Evolution of Shell Supernova Remnants

This paper reports on a workshop hosted by the University of Minnesota, March 23-26, 1997. It addressed fundamental dynamical issues associated with the evolution of shell supernova remnants and the relationships between supernova remnants and their environments. The workshop considered, in addition to classical shell SNRs, dynamical issues involving X-ray filled composite remnants and pulsar driven shells, such as that in the Crab Nebula. Approximately 75 participants with wide ranging interests attended the workshop. An even larger community helped through extensive on-line debates prior to the meeting. Each of the several sessions, organized mostly around chronological labels, also addressed some underlying, general physical themes: How are SNR dynamics and structures modified by the character of the CSM and the ISM and vice versa? How are magnetic fields generated in SNRs and how do magnetic fields influence SNRs? Where and how are cosmic-rays (electrons and ions) produced in SNRs and how does their presence influence or reveal SNR dynamics? How does SNR blast energy partition into various components over time and what controls conversion between components? In lieu of a proceedings volume, we present here a synopsis of the workshop in the form of brief summaries of the workshop sessions. The sharpest impressions from the workshop were the crucial and under-appreciated roles that environments have on SNR appearance and dynamics and the critical need for broad-based studies to understand these beautiful, but enigmatic objects. \\

astro-ph↗

arXiv1997

The supergalactic structure and the origin of the highest energy cosmic rays

The recent discoveries of several reliable events of high energy cosmic rays at an energy above 10^20 eV raise questions about their path through the nearby universe. The two analyses of the Haverah Park data set and the Akeno data set appear to have an inconsistent pattern. Both data sets showed some measure of a correlation with the supergalactic plane, the locus of cosmologically nearby galaxies, radio galaxies and clusters of galaxies. We need a reasonable model of the true intergalactic magnetic field. Using recent cosmological simulations of structure formation in the universe, we estimate the magnetic fields which correspond to the upper limits in the Rotation Measure to distant radio sources. We estimate that the magnetic field strength in supergalactic sheets and filaments may be in the range of 0.1 to 1 muG. If such strengths are realized inside our Local Supercluster, this opens up the possibility to focus charged particles in the direction perpendicular to the supergalatic plane, analoguously but in the opposite direction to solar wind modulation. If focusing exists, it means that for all particles captured into the sheets, the dilution with distance d is 1/d instead of 1/d^2. This means in effect, that we may see sources to much larger distances than expected sofar. This effect is relevant only for energies for which the possible distances are smaller than the void scale of the cosmological galaxy distribution, in the range possibly up to 100 Mpc.

astro-ph↗

arXiv1997

Cosmic Structure of Magnetic Fields

The simulations of the formation of cosmological structure allows to determine the spatial inhomogeneity of cosmic magnetic fields. Such simulations, however, do not give an absolute number for the strength of the magnetic field due to insufficient spatial resolution. Combining these simulations with observations of the Rotation Measure to distant radio sources allows then to deduce upper limits for the strength of the magnetic field. These upper limits are of order 0.2 - 2 muG along the filaments and sheets of the galaxy distribution. In one case, the sheet outside the Coma cluster, there is a definitive estimate of the strength of the magnetic field consistent with this range. Such estimates are almost three orders of magnitude higher than hitherto assumed usually. High energy cosmic ray particles can be either focussed or strongly scattered in such magnetic filaments and sheets, depending on the initial transverse momentum. The cosmological background in radio and X-ray wavelengths will have contributions from these intergalactic filaments and sheets, should the magnetic fields really be as high as 0.2 - 2 muG.

astro-ph↗

arXiv1997

Cosmic Shock Waves on Large Scales of the Universe

In the standard theory of the large scale structure formation, matter accretes onto high density perturbations via gravitational instability. Collisionless dark matter forms caustics around such structures, while collisional baryonic matter forms accretion shocks which then halt and heat the infalling gas. Here, we discuss the characteristics, roles, and observational consequences of these accretion shocks.

astro-ph↗

arXiv1996

Accreting Matter around Clusters of Galaxies: One-Dimensional Considerations

During the formation of the large scale structure of the Universe, matter accretes onto high density peaks. Accreting collisionless dark matter (DM) forms caustics around them, while accreting collisional baryonic matter (BM) forms accretion shocks. The properties of the accreting matter depend upon the power spectrum of the initial perturbations on a given scale as well as the background expansion in a given cosmological model. In this paper, we have calculated the accretion of DM particles in one-dimensional spherical geometry under various cosmological models including the Einstein-de Sitter universe, the open universe with $Ω_o<1$, and the flat universe with $Ω_Λ=1-Ω_o$. A density parameter in the range $0.1\le Ω_o \le 1$ has been considered. The initial perturbation characterized by a point mass at the origin has been considered. Since the accretion shock of BM is expected to form close to the first caustic of DM, the properties of the accreting BM are common with those of the DM. Hence, the accretion calculations with DM particles have been used to find the position and velocity of the accretion shock and the cluster mass inside it. The average temperature of BM has been estimated by adopting simplifying assumptions. The velocity of the accreting BM around clusters of a given temperature is smaller in a universe with smaller $Ω_o$, but only by up to $\sim24\%$ in the models with $0.1\le Ω_o \le 1$. Thus, it would be difficult to use that quantity to discriminate among the cosmological models. However, the accretion velocity around clusters of a given mass or a given radius depends more sensitively on the cosmological models. It is smaller in a universe with smaller $Ω_o$ by up to $\sim41\%$ and $\sim65\%$, respectively. So, it can provide a better

astro-ph↗

arXiv1996

Contributions to the Cosmic Ray Flux above the Ankle: Clusters of Galaxies

Motivated by the suggestion of Kang, Ryu \& Jones (1996) that particles can be accelerated to high energies via diffusive shock acceleration process at the accretion shocks formed by the infalling flow toward the clusters of galaxies, we have calculated the expected particle flux from a cosmological ensemble of clusters. We use the observed temperature distribution of local clusters and assume a simple power-law evolutionary model for the comoving density of the clusters. The shock parameters such as the shock radius and velocity are deduced from the ICM temperature using the self-similar solutions for secondary infall onto the clusters. The magnetic field strength is assume to be in equipartition with the postshock thermal energy behind the accretion shock. We also assume that the injected energy spectrum is a power-law with the exponential cutoff at the maximum energy which is calculated from the condition that the energy gain rate for diffusive shock acceleration is balanced by the loss rate due to the interactions with the cosmic background radiation. In contrast to the earlier paper we have adopted here the description of the cosmic ray diffusion by Jokipii (1987) which leads to considerably higher particle energies. Finally the injected particle spectrum at the clusters is integrated over the cosmological distance to earth by considering the energy loss due to the interactions with the cosmic background radiation. Our calculations show that the expected spectrum of high-energy protons from the cosmological ensemble of the cluster accretion shocks could match well the observed cosmic ray spectrum near $10^{19}$eV with reasonable parameters and models if about $10^{-4}$ of the infalling kinetic energy can be injected into the intergalactic space as the high energy particles.

astro-ph↗

arXiv1996

Diffusive Shock Acceleration in Oblique MHD Shocks: Comparison with Monte Carlo Methods and Observations

We report simulations of diffusive particle acceleration in oblique magnetohydrodynamical (MHD) shocks. These calculations are based on extension to oblique shocks of a numerical model for ``thermal leakage'' injection of particles at low energy into the cosmic-ray population. That technique, incorporated into a fully dynamical diffusion-convection formalism, was recently introduced for parallel shocks by Kang \& Jones (1995). Here, we have compared results of time dependent numerical simulations using our technique with Monte Carlo simulations by Ellison, Baring \& Jones 1995 and with {\it in situ} observations from the Ulysses spacecraft of oblique interplanetary shocks discussed by Baring \etal (1995). Through the success of these comparisons we have demonstrated that our {diffusion-convection} method and injection techniques provide a practical tool to capture essential physics of the injection process and particle acceleration at oblique MHD shocks. In addition to the diffusion-convection simulations, we have included time dependent two-fluid simulations for a couple of the shocks to demonstrate the basic validity of that formalism in the oblique shock context. Using simple models for the two-fluid closure parameters based on test-particle considerations, we find good agreement with the dynamical properties of the more detailed diffusion-convection results. We emphasize, however, that such two-fluid results can be sensitive to the properties of these closure parameters when the flows are not truly steady. Furthermore, we emphasize through example how the validity of the two-fluid formalism does not necessarily mean that {\it steady-state} two-fluid models provide a reliable tool for predicting the efficiency of particle acceleration in real shocks.

astro-ph↗

arXiv1996

Close Encounters between a Neutron Star and a Main-Sequence Star

We have examined consequences of strong tidal encounters between a neutron star and a normal star using SPH as a possible formation mechanism of isolated recycled pulsars in globular clusters. We have made a number of SPH simulations for close encounters between a main-sequence star of mass ranging from 0.2 to 0.7 Solar masses represented by an n=3/2 polytrope and a neutron star represented by a point mass. The outcomes of the first encounters are found to be dependent only on the dimensionless parameter eta' = (m/(m+M))^(1/2) (r_min/R_MS)^(3/2) (m/M)^(1/6), where m and M are the masses of the main-sequence star and the neutron star, respectively, r_min the minimum separation between two stars, and R_MS the size of the main-sequence star. The material from the (at least partially) disrupted star forms a disk around the neutron star. If all material in the disk is to be acctreted onto the neutron star's surface, the mass of the disk is enough to spin up the neutron star to spin period of 1 ms.

astro-ph↗

arXiv1995

Supersonic Collisions between Two Gas Streams

A star around a massive black hole can be disrupted tidally by the gravity of the black hole. Then, its debris may form a precessing stream which may even collide with itself. In order to understand the dynamical effects of the stream-stream collision on the eventual accretion of the stellar debris onto the black hole, we have studied how gas flow behaves when the outgoing stream collides supersonically with the incoming stream. We have investigated the problem analytically with one-dimensional plane-parallel streams and numerically with more realistic three-dimensional streams. A shock formed around the contact surface converts the bulk of the orbital streaming kinetic energy into thermal energy. In three-dimensional simulations, the accumulated hot post-shock gas then expands adiabatically and drives another shock into the low density ambient region. Through this expansion, thermal energy is converted back to the kinetic energy associated with the expanding motion. Thus, in the end, only a small fraction of the orbital kinetic energy is actually converted to the thermal energy, while most of it is transferred to the kinetic energy of the expanding gas. Nevertheless the collision is effective in circularizing the debris orbit, because the shock efficiently transforms the ordered motion of the streams into the expanding motion in directions perpendicular to the streams. The circularization efficiency decreases, if two colliding streams have a large ratio of cross sections and a large density contrast. But even in such cases, the main shock extends beyond the overlapping contact surface and the high pressure region behind the shock keeps the stream of the larger cross section from passing freely. Thus the stream-stream collisions are still expected to circularize the stellar debris rather efficiently, unless the ratio

astro-ph↗