121801818
Theory of (p,2p) reactions at medium energies
A new and effective means of calculating matrix elements relevant to direct interactions is introduced and applied to a distorted-wave t matrix approximation analysis of the /sup 12/C(p,2p9/sup 11/B and /sup 16/O(p,2p)/sup 15/N reactions at 45-50 MeV incident energy. A realistic separable two-body t matrix has been employed and core polarization, whereby the final state is generated after excitation of giant resonance target states, has been included. The integrals involved in the matrix elements were calculated analytically by making a parametrization of the distorted scattering wave functions, the nucleon-nucleon interaction, and the bound state wave function, the parameters being chosen in accordance with elastic (p,p) and inelastic (p,p') data and realistic shell model wave functions. The results indicate the inadequacy of a distorted-wave t-matrix approximation analysis at these energies.
120845803
SCATTERING OF ENERGY-TIME WAVE PACKETS FROM MANY-BODY SYSTEMS
121255065
Simple Realistic Treatment of Nuclear Direct-Interaction Processes
Physical arguments are used to predict qualitatively the effect on direct-interaction differential cross sections of the distortion of the wave functions of the scattered particle. These qualitative predictions are confirmed by calculations using a simple but fairly realistic model for the wave function distortion in ($\ensuremath{\alpha}, {\ensuremath{\alpha}}^{\ensuremath{'}}$) scattering. The model used is based on examination of the properties of optical model wave functions. Good fits to experimental data are found using the model for ($\ensuremath{\alpha}, {\ensuremath{\alpha}}^{\ensuremath{'}}$) scattering in the energy range 20-40 Mev for scattering angles less than 90\ifmmode^\circ\else\textdegree\fi{}. Features of direct-interaction processes involving nucleons are interpreted in terms of a focus in the optical model wave functions for these particles, but detailed calculations are not presented.
123224123
Distorted wave theory of the 50 MeV (p, 2p) reaction
Abstract An attempt is made to answer the question whether distorting potentials that give really good fits to elastic scattering cross sections and total reaction cross sections for experiments in the relevant two-body subsystems can correctly give positions of maxima and minima in the (p, 2p) angular correlation. The 50 MeV energy range is chosen because the angular correlation is very sensitive to the distortion. It is found that the positions are correctly reproduced by good elastic scattering parameters for the reaction 89 Y(p, 2p) 88 Sr, Q = 7.44 MeV, at 46 MeV. The shortcoming of the distorted wave theory at this energy, as at higher energies, is that the calculated ratio of the cross section at small angles to that at large angles is too large. Apart from this serious defect, the shape of the angular correlation for 12 C(p, 2p) 11 B, Q = 16 MeV, at 50 MeV is also encouraging. Previous studies suggest that the defect can be corrected if a better description of the p-p t -matrix is used in the distorted wave theory.
119722071
Off-shell effects in the (p, 2p) reaction
Abstract It is shown that, within the limitations of the distorted-wave off-shell impulse approximation, future accurate (p, 2p) experiments near 150 MeV incident energy can be helpful in distinguishing between some of six phenomenological nuclear potentials that are examined. On-shell differences between the Reid soft-core and five different one-boson-exchange potentials are amplified in the (p, 2p) context in such a way that an accurately measurable feature, the peak height ratio for 1p knockout, is sensitive to them.
121500029
Time-Dependent Scattering Experiment
Theory is developed for a time-dependenit scatternng experiment that consists in following the time development of a photon wave packet scattered resonantly from the metastable state of an appropriate nucleus. (C.E.S.)
117921064
Probing the physics of the intracluster medium
Clusters of galaxies are the largest and most massive virialized objects in the universe. And because structure formation in the universe occurs hierarchically, clusters represent the end points of this process. As a result, these systems contain a wealth of cosmological information including a detailed fossil record of the structure formation process. However, in order to fully exploit clusters for cosmological purposes, it is necessary to model the internal properties of these systems in detail, particularly the baryonic component. Although much progress has been made on this front, there remain several important outstanding issues in our understanding of the properties of the intracluster medium (ICM), a diffuse plasma that fills clusters and dominates (by mass) the baryonic component of these systems. In particular, very little is presently known about the potential role of non-gravitational ICM physics such as radiative cooling, thermal conduction, and heating via outflows from supermassive black holes (i.e., active galactic nuclei AGN) that are often located at the centers of clusters. In this dissertation, we have attempted to shed light on this important issue. In particular, we have developed physically-motivated analytic models of the ICM in order to assess the role that non-gravitational processes, such as radiative cooling and heating from AGN, play in mediating the observed properties of clusters. We have carried out detailed and systematic comparisons between our models and the observed global and structural X-ray and Sunyaev-Zeldovich (SZ) effect properties of clusters. From this comparison we conclude the following. As expected, a pure gravitational model (i.e., the standard self-similar model) fails to match the observed properties of clusters. A model that invokes radiative cooling but no sources of non-gravitational heating also fails, as it has no hope of avoiding the so-called "cooling crisis" or of explaining the origin of "non-cooling flow" clusters. On the other hand, a model that includes non-gravitational heating but that does not take into account the effects of radiative cooling fails to account for the observed global and structure properties of "cooling flow"c1usters and obviously has no hope of explaining galaxy or star formation in clusters. We find that in order to account for the global X-ray and SZ effect scaling relations (including their intrinsic scatter), it is necessary to invoke both radiative cooling and a distribution in the level of non-gravitational heating experienced by clusters. Under this scenario, clusters that were severely heated early on likely evolved into %oncooling flow"clusters, whereas clusters that were heated by only mild amounts likely evolved into "cooling flow" clusters. This conclusion is reinforced by com' parisons to new spatially-resolved entropy, temperature, and surface brightness profiles derived from Chandra and XMM-Newton X-ray data. Supervisor: Dr. A. Babul, (Department of Physics and Astronomy)
10013368
Cluster Sunyaev-Zeldovich Effect Scaling Relations
X-ray observations of an "entropy floor" in nearby groups and clusters of galaxies offer evidence that important nongravitational processes, such as radiative cooling and/or "preheating," have strongly influenced the evolution of the intracluster medium (ICM). We examine how the presence of an entropy floor modifies the thermal Sunyaev-Zeldovich (SZ) effect. A detailed analysis of scaling relations between X-ray and SZ effect observables and also between the two primary SZ effect observables is presented. We find that relationships between the central Compton parameter and the temperature or mass of a cluster are extremely sensitive to the presence of an entropy floor. The same is true for correlations between the integrated Compton parameter and the X-ray luminosity or the central Compton parameter. In fact, if the entropy floor is as high as inferred in recent analyses of X-ray data, a comparison of these correlations with both current and future SZ effect observations should show a clear signature of this excess entropy. Moreover, because the SZ effect is redshift independent, the relations can potentially be used to track the evolution of the cluster gas and possibly discriminate between the possible sources of the excess entropy. To facilitate comparisons with observations, we provide analytic fits to these scaling relations.
118956459
The similarity of observed X-ray coronae associated with L* disc and elliptical galaxies
The existence of hot, X-ray luminous gaseous coronae surrounding present day L* galaxies is a generic prediction of galaxy formation theory in the cold dark matter cosmogony. While extended X-ray emission has been known to exist around elliptical galaxies for a long time, diffuse extra-planar emission has only recently been detected around disc galaxies. We compile samples of elliptical and disc galaxies that have Chandra and XMM-Newton measurements, and compare the scaling of the coronal X-ray luminosity (L_X) with both the K-band luminosity (L_K) and the coronal X-ray temperature (T_X). The X-ray flux measurements are corrected for non-thermal point source contamination by spatial excision and spectral subtraction for resolved and unresolved sources respectively. We find that the properties of the extended X-ray emission from galaxies of different morphological types are similar: for both elliptical and disc galaxies, the L_X - L_K and L_X - T_X relations have similar slope, normalisation and scatter. The observed universality of coronal X-ray properties suggests that the bulk of this emission originates from gas that has been accreted, shock-heated and compressed during the assembly of the galaxy and that outflows triggered by stellar processes make only a minor contribution to the X-ray emission. This reservoir of cooling gas is a potential source of morphological transformation; it provides a fresh supply of material for discs to grow around galaxies of all morphological types.
17639376
THE EFFECTS OF RADIATIVE COOLING ON THE ENTROPY DISTRIBUTION OF INTRACLUSTER GAS
High resolution X-ray observations indicate that the entropy profiles in the central regions of some massive cooling flow clusters are well approximated by powerlaws. McCarthy and coworkers recently accounted for this trend with an analytic model that includes the detailed effects of radiative cooling. Interestingly, these authors found that cooling (and subsequent inflow of the gas) naturally establishes approximate steady-state powerlaw entropy profiles in the cores of clusters. However, the origin of this behavior and its dependence on initial conditions have yet to be elucidated. In the present study, we explain this trend in the context of the self-similar cooling wave model developed previously by Bertschinger (1989). It is shown that the logarithmic slope of the entropy profile in the cores of relaxed cooling flow clusters is given by a simple analytic function that depends only on the logarithmic slopes of the local gravitational potential and the cooling function. We discuss a number of potentially interesting uses of the above result, including: (1) a way of measuring the shapes of gravitational potentials of cooling flow clusters (which may, for example, be compared against the standard hydrostatic equilibrium method); (2) a simple method for constructing realistic analytic cluster models that include the effects of radiative cooling; and (3) a test of the reliability of cooling routines implemented in analytic models and hydrodynamic simulations. Subject headings: cosmology: theory — galaxies: clusters: general — X-rays: galaxies: clusters
17658039
Modelling shock heating in cluster mergers – I. Moving beyond the spherical accretion model
The thermal history of the intracluster medium (ICM) is complex. Heat input from cluster mergers, from active galactic nuclei (AGN) and from winds in galaxies offsets and may even prevent the cooling of the ICM. Consequently, the processes that set the temperature and density structure of the ICM play a key role in determining how galaxies form. In this paper, we focus on the heating of the ICM during cluster mergers, with the eventual aim of incorporating this mechanism into semi-analytic models for galaxy formation. 
 
 
 
We generate and examine a suite of non-radiative hydrodynamic simulations of mergers in which the initial temperature and density structure of the systems are set using realistic scaling laws. Our collisions cover a range of mass ratios and impact parameters, and consider both systems composed entirely of gas (these reduce the physical processes involved), and systems comprising a realistic mixture of gas and dark matter. We find that the heating of the ICM can be understood relatively simply by considering evolution of the gas entropy during the mergers. The increase in this quantity in our simulations closely corresponds to that predicted from scaling relations based on the increase in cluster mass. 
 
 
 
We examine the physical processes that succeed in generating the entropy in order to understand why previous analytical approaches failed. We find the following. (i) The energy that is thermalized during the collision greatly exceeds the kinetic energy available when the systems first touch. The smaller system penetrates deep into the gravitational potential before it is disrupted. (ii) For systems with a large mass ratio, most of the energy is thermalized in the massive component. The heating of the smaller system is minor and its gas sinks to the centre of the final system. This contrasts with spherically symmetric analytical models in which accreted material is simply added to the outer radius of the system. (iii) The bulk of the entropy generation occurs in two distinct episodes. The first episode occurs following the collision of the cores, when a large shock wave is generated that propagates outwards from the centre. This causes the combined system to expand rapidly and overshoot hydrostatic equilibrium. The second entropy generation episode occurs as this material is shock heated as it recollapses. Both heating processes play an important role, contributing approximately equally to the final entropy. This revised model for entropy generation improves our physical understanding of cosmological gas simulations.