This dissertation describes the current version of GenASiS and reports recent progress in its development. GenASiS is a new computational astrophysics code built for large-scale and multi-dimensional computer simulations of astrophysical phenomena, with primary emphasis on the simulations of neutron star mergers and core-collapse supernovae. Neutron star mergers are of high interest to the astrophysics community because they should be the prodigious source of gravitation waves and the most promising candidates for gravitational wave detection. Neutron star mergers are also thought to be associated with the production of short-duration, hard-spectral gamma-ray bursts, though the mechanism is not well understood. In contrast, core-collapse supernovae with massive progenitors are associated with long-duration, soft-spectral gamma-ray bursts, with the `collapsar' hypothesis as the favored mechanism. Of equal interest is the mechanism of core-collapse supernovae themselves, which has been in the forefront of many research efforts for the better half of a century but remains a partially-solved mystery. In addition supernovae, and possibly neutron star mergers, are thought to be sites for the emph{r}-process nucleosynthesis responsible for producing many of the heavy elements. Until we have a proper understanding of these events, we will have only a limited understanding of the origin of the elements. These questions provide some of the scientific motivations and guidelines for the development of GenASiS. In this document the equations and numerical scheme for Newtonian and relativistic magnetohydrodynamics are presented. A new FFT-based parallel solver for Poisson's equation in GenASiS are described. Adaptive mesh refinement in GenASiS, and a novel way to solve Poisson's equation on a mesh with refinement based on a multigrid algorithm, are also presented. Following these descriptions, results of simulations of neutron star mergers with GenASiS such as their evolution and the gravitational wave signals and spectra that they generate are shown. In the context of core-collapse supernovae, we explore the capacity of the stationary shock instability to generate magnetic fields starting from a weak, stationary, and radial magnetic field in an initially spherically symmetric fluid configuration that models the stalled shock in the post-bounce supernova environment. Our results show that the magnetic energy can be amplified by almost 4 orders of magnitude. The amplification mechanisms for the magnetic fields are then explained.

Chemical evolution addresses the problem of the formation of the chemical elements and their evolution throughout the history of the universe. This thesis discusses in particular the chemical evolution in the young universe and what we may learn from the observations of the oldest stars. The present day production of carbon in the Galaxy is also discussed. Interstellar media of young, metal-poor, star-forming systems are expected to show large chemical abundance inhomogeneities due to local supernova explosions. These inhomogeneities are reflected in the surface abundances of the population of longlived, low-mass stars. A stochastic model of the chemical evolution in such systems is presented and used to study the metallicity distribution and the scatter in chemical abundance ratios. The model takes into account mixing of the enriched material by turbulent motions and cloud collisions in the interstellar medium as well as infall of pristine matter. The predicted metallicity distribution shows, in accordance with observations of extreme Pop II strars in the Galactic halo, a distinct cut-off at [Fe/H]~-4. However, the fraction of stars below [Fe/H]=-4 agrees with observatrion only if a population of metal-free stars (Pop III) was never able to form. The predicted scatter in abundance ratios is demonstrated to be crucially dependent on the as yet uncertain supernova yields and the relatively small star-to-star scatter is tentatively explained by the averaging of a large number of contributing supernovae and by the selection effects favouring contributions from supernovae in a certain mass range for the most metal-poor stars. Furthermore, stars enriched by one single supernova are predicted to be found in very narrow sequences in the abundance ratio diagrams (so called A/A diagrams). Verification of the existence of such features, called single supernova sequences, is observationally challenging. Abundance analysis of carbon was performed in a large sample of solar-type stars in the Galactic disk using the forbidden [C I] line at 8727 Å. A comparison between the relation of [C/O] with metallicity for the Galactic stars and that of dwarf irregular galaxies suggests that large amounts of carbon are produced today by massive, so called Wolf-Rayet stars. Low-mass stars are less important. This was also demonstrated by modelling the chemical evolution of carbon.

When modelling the collapse of massive stars leading to supernova explosions and the cooling of neutron stars, understanding the microphysical processes, such as the interaction of neutrinos within a dense medium are of vital importance. The interaction of neutrinos with nucleons (neutrons and protons) is altered by the presence of the medium, compared to the same process with free nucleons. Neutrino scattering and production processes may be characterized in terms of the excitations that are created or destroyed in the nuclear medium. One way to analyse the effects of the medium is by using Landau's theory of normal Fermi liquids. This theory gives simple relationships between physical quantities such as the spin susceptibility or the response to a weak interaction probe in terms of Landau parameters, that are measures of the interaction between quasiparticles. One problem when using Landau Fermi liquid theory for nucleon matter is that the interaction has a tensor component. The tensor interaction does not conserve the total spin and, as a consequence, there are generally contributions to long-wavelength response functions from states that have more than one quasiparticle-quasihole pair in the intermediate state. Such contributions cannot be calculated in terms of Landau parameters alone, since in the usual formulation of Landau theory, only singlepair excitations are considered. In this thesis three problems are addressed. First, we obtain bounds on the contributions from more than one quasiparticle-quasihole pair by using sum-rule arguments. Second, we derive expressions for static response functions allowing for the tensor components of the interaction. We analyse which the most important effects are on the static response of nucleon matter, and find that the major contributions comes from renormalization of coupling constants and transitions to states with more than one quasiparticle-quasihole pair. Third, we show how contributions to the dynamical response coming from states containing two quasiparticle-quasihole pairs may be evaluated in terms of Landau theory if one allows for the effect of collisions in the Landau kinetic equation. We consider the case of asymmetric nuclear matter, and our work goes beyond earlier works in that they contain the effects of collisions in addition to those of the mean field.

The propagation of light beams originating from synthetic ‘Type Ia’ supernovae, through an inhomogeneous universe with simplified dynamics, is simulated using a Monte-Carlo Ray-Tracing method. The accumulated statistical (redshift-magnitude) distribution for these synthetic supernovae observations, which is illustrated in the form of a Hubble diagram, produces a luminosity profile similar to the form predicted for a Dark-Energy dominated universe. Further, the amount of mimicked Dark-Energy is found to increase along with the variance in the matter distribution in the universe, converging at a value of ΩX ≈ 0.7. It can be thus postulated that at least under the assumption of simplified dynamics, it is possible to replicate the observed supernovae data in a universe with inhomogeneous matter distribution. This also implies that it is demonstrably not possible to make a direct correspondence between the observed luminosity and redshift with the distance of a cosmological source and the expansion rate of the universe, respectively, at a particular epoch in an inhomogeneous universe. Such a correspondences feigns an apparent variation in dynamics, which creates the illusion of Dark-Energy.

Cette these s'inscrit dans le cadre de l'experience SuperNova factory, une collaboration franco-americaine associant une vingtaine de chercheurs. Le but de cette experience est d'ameliorer l'utilisation des supernovae de type Ia en cosmologie en affinant notre comprehension de la diversite des SNIa et en augmentant la statistique disponible a bas redshift (entre 0.03 et 0.08).<br /><br />Apres avoir presente ces aspects, j'ai presente plus specifiquement l'experience SNfactory. Cette experience se compose de deux versants : l'un d'eux est la recherche de supernovae avec les donnees de la camera QUEST-II installee sur un telescope du Mont Palomar (Californie) et l'autre est le suivi grace a un instrument dedie nomme SNIFS installe en permanence sur le telescope de l'Universite de Hawaii au sommet du Mauna Kea.<br /><br />Mon travail a porte sur les voies spectroscopiques de SNIFS : une simulation d'un cube de donnees et une methode d'extraction de spectres par minimisation des moindres carres sont presentees ici, ainsi qu'une discussion sur le choix de la PSF a adopter. L'objectif de precision de la calibration en flux de 1 % lors de nuits photometriques peut etre atteint a l'aide d'une telle extraction.<br /><br />J'ai enfin presente une analyse d'un echantillon de dix supernovae, notamment en mesurant la vitesse d'expansion calculee a partir du decalage vers le bleu de 4 raies du spectre a l'aide d'une methode novatrice. Ces mesures montrent que 20 % de l'echantillon presente un comportement notablement different du comportement moyen.

In this manuscript, some relations between theoretical nuclear physics and compact stars, which are known to be excellent tools to test matter under extreme conditions, are studied. Most of these links are performed within theoretical modelling which are used to describe both nuclei and nuclear systems in astrophysics. Self-consistent approaches unifying the description of isolated nuclei, dilute nuclei in a gas of light particles, and uniform matter, are presented in this manuscript and employed to understand the physics of compact stars. The manuscript is organized as follow: The first chapter is a general introduction to impact stars and supernovae physics, as well as to nuclear physics. In the second chapter, various aspects of the modeling of the in-medium nucleon-nucleon interaction are presented, such as, extension of the Skyrme interaction in the spin and spin-isospin channels, the properties of the V(low k) interaction and the low-density properties of nuclear matter. In the third chapter, the superfluid properties of dense matter are investigated, either through an iso-vector pairing interaction design to reproduce microscopic calculations in nuclear matter, or through the impact of superfluidity on derivatives of the EOS, such as the incompressibility and the symmetry energy, and finally, is presented the use of pairing vibration to study the properties of pairing. The questions of the in-medium effective mass and its impact for the dynamics of core-collapse supernovae, and the level density in nuclei are presented in the chapter 4. The microscopic modelling of the crust of neutron stars, as derived from band theory, and including superfluidity, is shown in chapter 5. In chapter 6, some links between the properties of nuclei, and neutron stars are presented: the properties of overflowing nuclei at the neutron drip, the correlation between the curst-core properties and nuclear empirical quantities, and the stiffness of the nuclear EOS, are shown. Finally, conclusions are presented in chapter 7.

Type Ia supernovae (SNe Ia) are excellent tools in cosmology. Their intrinsic luminosities are found to vary systematically with the light-curve widths, providing an empirical calibration. This property, called the width-luminosity relation (WLR), is the basis of modern SN Ia cosmology and led to the unexpected discovery of the current accelerated rate of cosmic expansion. By examining the spectroscopic diversity of SNe Ia, this thesis aims to improve both the use of SNe Ia in cosmology and the physical understanding of the observed properties. Spectra of SNe Ia contain a wealth of information, but are difficult to organize. In this thesis, new methods are developed to consistently quantify and analyze the spectral features of supernovae. The efficacy of the methods is tested on a large library of observed spectra encompassing a wide range of properties. The spectroscopic diversity of SNe Ia enters cosmology through K-correction calculations. Before this work, K-correction was a major contributor of the systematic errors in cosmology. It is shown here that the systematic errors can be largely diminished by carefully quantifying the mean spectroscopic properties of SNe Ia. The remaining statistical errors are also quantified and shown to be redshift dependent. With the aid of principal component analysis (PCA), the multidimensional spectral information is reduced to a few components describing the largest variations in the spectral library. Using this tool, it is shown here that SN Ia intrinsic luminosity is the main driver of the spectroscopic diversity at maximum light, for every spectral feature from the ultraviolet to the near-infrared. These spectroscopic sequences can potentially account for a large fraction of the K-correction statistical errors and even enable the use of SN Ia spectra as independent indicators of intrinsic luminosity and colors. The established relations will also disentangle the effects of demographic shift and true evolution in high-redshift SN Ia spectra. The temporal evolution of the spectral features is shown to exhibit the persistence of the spectroscopic sequences throughout other epochs. The effect is attributed to the more rapid spectroscopic temporal evolution of fainter SNe Ia. This conclusion supports the theory that WLR is primarily a spectroscopic effect, rather than a bolometric one.

L'expérience H.E.S.S. est un réseau de télescope Tcherenkov, situé en Namibie, qui observe le rayonnement de sources astrophysiques de rayons gamma de très haute énergie. Depuis sa mise en service il y a plus de dix ans, H.E.S.S. a permis d'améliorer considérablement notre vision de l'univers à ces énergies. Depuis 2012, le démarrage de la deuxième phase de l'expérience avec la mise en service d'un cinquième télescope permet de baisser le seuil en énergie de l'expérience.L'optimisation d'une analyse multivariée au sein de l'expérience H.E.S.S. est présentée dans cette thèse. L'analyse Xeff a été amélioré en prenant en compte les conditions d'observations, et en introduisant de nouvelles variables dans l'estimateur. Un gain de sensibilité de l'analyse par rapport aux analyses standards est observé. Cette méthode a été ensuite utilisée pour diverses analyses de sources de rayons gamma.La prise en compte des effets de la focalisation de l'analyse de données de H.E.S.S. II est détaillée. L'ajustement de la distance entre la caméra et les miroir du cinquième télescope permet d'améliorer l'image dans le plan focal. L'amélioration de la reconstruction et le gain en sensibilité qui en découle est présenté.L'étude du rayonnement gamma de quatre candidats d'associations de vestiges de supernova et de nuages moléculaires est présentée : G349.7+0.2, W51, la nébuleuse de la Tornade, et HESS J1745-303. Une interprétation de ce rayonnement utilisant également les données de Fermi-LAT permet d'estimer l'origine dominante des mécanismes d'émission et d'appréhender l'efficacité d'accélération de particules par ces systèmes.Les travaux présentés dans cette thèse ont fait l'objet de deux notes internes à la collaboration H.E.S.S., de multiples présentations dans des conférences internationales et de publications scientifiques. / The H.E.S.S. experiment is an array of Cherenkov telescopes, located in Namibia, observing the very high energy gamma rays from astrophysical sources. H.E.S.S. greatly increased our understanding of the very high energy non thermal universe. Since 2012, a fifth telescope was installed at the center of the array. This improvement increase the energy range and the sensitivity of the detector.The optimization of a multivariate analysis method, within the H.E.S.S. framework, is presented in this thesis. The Xeff analysis is improved by taking into account the observation conditions and by increasing the number of discriminating variables. The sensitivity of this analysis compared to standard analyses is demonstrated. Xeff is then used to analyze several sources of gamma rays.The effects of the focus impacting the H.E.S.S. II data are described. They are taken into account in the analysis and an optimization of the distance between the mirrors and the camera is presented.The study of the gamma emission from four candidates of molecular clouds and supernova remnant associations is presented. The H.E.S.S. data from G349.7+0.2, W51, the Tornado nebula and HESS J1745-303 is performed. The interpretation of their emission, using also Fermi-LAT data, leads to the estimation of the particles acceleration efficiency in these objects.The work described in this thesis led to the production of two internal notes in the H.E.S.S. collaboration, to several presentations in international conferences, and scientific publications.

In Yoshii et al., we described a new method for measuring extragalactic distances based on dust reverberation in active galactic nuclei (AGNs), and we validated our new method with Cepheid variable stars. In this Letter, we validate our new method with Type Ia supernovae (SNe Ia) that occurred in two of the AGN host galaxies during our AGN monitoring program: SN 2004bd in NGC 3786 and SN 2008ec in NGC 7469. Their multicolor light curves were observed and analyzed using two widely accepted methods for measuring SN distances, and the distance moduli derived are m= 33.47 +/- 0.15 for SN 2004bd and 33.83 +/- 0.07 for SN 2008ec. These results are used to obtain independently the distance measurement calibration factor, g. The g value obtained from the SN Ia discussed in this Letter is gSN= 10.61 +/- 0.50, which matches, within the range of 1s uncertainty, gDUST = 10.60, previously calculated ab initio in Yoshii et al. Having validated our new method for measuring extragalactic distances, we use our new method to calibrate reverberation distances derived from variations of Ha emission in the AGN broad-line region, extending the Hubble diagram to z approximate to 0.3 where distinguishing between cosmologies is becoming possible.

This thesis deals with the radiative emission arising out of the interaction of several core collapse supernovae (SNe) with their dense circumstellar medium (CSM) and uses the radiative properties as diagnostics of the ionized plasma in and around the interaction region. These supernovae include: SN 1993J, SN 1995N, SN 2002ap & SN 2003bg. In a SN explosion, the outer layers of the star are set in motion with high velocities and the collision of the ejecta with the CSM leads to a less dense and hot blast wave forward shock with velocities ~ 20,000 km/s and T ~ 109K. When the external layers of the expanding ejecta decelerate upon interaction with the CSM, a reverse shock develops that starts propagating into the stellar envelope, with velocity few times 1000 km/s relative to the expanding stellar ejecta, heating it to T ~ 107K. Forward shock velocities are typically 1000 times the speed of the wind that was being lost from the progenitor prior to the SN explosion. Consequently, evolution of the shock and the radiative properties of the SN few years after the explosion probes the history of the environment of the progenitor star thousands of years before the explosion. Interaction of the shocked ejecta with the CSM gives rise to emission in radio and X-ray bands. The emission in various wavebands arising due to this interaction usually has a slower decay rate than the initial photospheric emission arising from ionic recombination and radioactive decay. Hence, one is usually able to track the supernovae for longer time. Radio emission is generated from the forward shocked shell due to the synchrotron emission by relativistic electrons in the presence of the strong magnetic fields. The strong magnetic field in the shocked shell is believed to be generated by Rayleigh Taylor instability, which enhances any seed magnetic field present initially. Relativistic electrons are produced, most likely, by shock mediated acceleration processes. The early rapid rise in the radio flux density results from the shock overtaking progressively further into the progenitor’s stellar wind and therefore in the regions of decreasing optical depth. Since optical depth is larger at lower frequencies lower frequencies turn on later in time. The emission from the shocked region decreases slowly with time as the shock expands, so even when the radio absorption has become negligible, the radio light curve would show this decline. Radio emission is absorbed initially by different mechanisms depending upon the mass loss rate in the progenitor wind, shock velocity, electron temperature etc. If the emission is absorbed by an electron moving in the field of an ion (free-free absorption), then one can infer the mass loss of the progenitor. If on the other hand, the radio emission is absorbed by synchrotron self absorption in which the photon interacts with an electron in a magnetic field, then it gives information about the size of the emitting region. In contrast, X-ray emission initially comes from the forward shock and is non-thermal in nature. The X-rays could be either due to synchrotron emission or due to inverse Compton scattering in which the photospheric optical photons can be boosted to X-ray energies due to multiple scattering with the electrons. Late time X-rays, which are thermal in nature, arise from the reverse shock and probe the CSM interaction of the SN ejecta and provide information of the plasma and surroundings. However, in an alternate model due to Chugai (1993, Astron. Rep., 41, 672), X-rays can also emerge from the radiative cooling of the shocked, dense clumps (clouds) embedded in the circumstellar wind overtaken by the blast-wave shock and crushed by the pressure of the strongly shocked wind. Line-widths, elemental yields and luminosity curves are the observational signatures to distinguish between the two models. X-ray spectra of SNe can be used to determine what elements are there in the shock heated ejecta, the supernova’s nucleosynthetic yield and thence the (helium) core mass of the progenitor at the explosion stage. Nucleosynthetic studies of SNe and constraints on their progenitor masses are of vital interest to the origin and distribution of elements in the galaxy and its chemical evolution. Chapter 1 gives a general overview of supernova types, explosion scenarios and essentials of shock dynamics in the CSM. Chapter 2 gives an overview of the radiative processes relevant to radio and X-ray emission. I describe the radio and X-ray data analysis procedures in Chapter 3. I discuss synchrotron aging in young supernovae in Chapter 4. Synchrotron aging has been seen in many old sources, such as radio galaxies, Compact Steep Spectrum sources etc., where the age of the source is not known. Synchrotron aging was used to determine the age of such sources using magnetic field under equipartition (between magnetic energy density and relativistic energy density) as an input parameter. However in young supernovae (whose ages are known), the magnetic fields are generated due to the instabilities (Rayleigh Taylor) created in the plasma and hence it is difficult to estimate the field correctly. Here synchrotron aging can be used to derive the magnetic field independent of any assumption of equipartition. I discuss the synchrotron aging in detail and derive a significant conclusion about the plasma energetics from the combined GMRT and Very Large Array (VLA) spectrum of a 10 year old type IIb SN 1993J around day 3200 after explosion. I found a steepening of its spectrum caused by synchrotron aging. After taking into account the adiabatic losses and Fermi acceleration of electrons, I estimate from the synchrotron break, the magnetic field in the plasma and derive that the magnetic energy density is 10,000 times larger than the relativistic energy density. In Chapter 4, I also underscore the importance of wide band radio spectrum in dealing with issues of the physics of shocked plasma. In Chapter 5, I describe further studies of SN 1993J with the GMRT at frequencies 1420, 610, 325 and 235 MHz, from 7.5 years to 10 years since explosion. SN 1993J is a unique supernova for which magnetic field and sizes are determined from model independent measurements; the former from the synchrotron cooling break and the latter from VLBI measurements. Using GMRT spectra and earlier published spectra of SN 1993J, I compare the VLBI sizes of the SN 1993J with that of obtained from the peak of the spectra using synchrotron self absorption (SSA) model. I find that the SSA sizes are roughly equal to the VLBI sizes of the SN. This suggests that the synchrotron self absorption is responsible for the turn over in the spectra of SN 1993J at all the epochs. The size evolution shows that the ejecta expands freely initially and then show a small deceleration in the later epochs. I also plot the magnetic field evolution, which goes as Spectral index initially lies between 0.8 - 1.0 and later seems to flatten with time and lies within the range of 0.5 - 0.7. The mass loss rate roughly remains constant ( ~ 5 x 10−5M yr−1) in two years of GMRT observations, i.e., 8000-10,000 years before explosion. Light curves based on high frequency existing models extrapolated to low frequencies overpredict the flux densities at low frequencies. Some extra opacity is needed to incorporate the difference. This suggests that the low frequency opacity in SN 1993J is not a simple extrapolation of high frequency opacity and a hitherto unaccounted for absorption may be at work at low frequencies. I describe the Chandra X-ray observatory work on SN 1995N, which we observed on March 28, 2004, in Chapter 6. I detected the X-ray emission from the SN with most of the emission found to be below 2 keV. SN 1995N had also been observed by ROSAT and ASCA earlier on three occasions. Our reanalysis of ASCA 1998 spectra revealed certain line features which were not reported in the published work of Fox et al. (2000, MNRAS, 319, 1154). I detect a Ne X line in both ASCA and Chandra observations, and while I detect a Ne IX line in the Chandra observation this was absent in the ASCA one. At the same time I detect a 1.3 keV line in the ASCA observation, absent in the Chandra spectrum of SN 1995N. No Fe line was detected in either spectrum. The light curves of SN 1995N suggested a non-linear profile due to high ASCA flux. We re-analyzed the ASCA data in view of the high-resolution imaging data obtained by Chandra and found at least ten more sources contributing to the SN flux due to the large ASCA PSF. After taking out the contribution from the contaminating sources, the light curve appears to be consistent with a linear decline. This indicates that the X-ray emission is due to the reverse shock going through a shallow ejecta profile. I also find that the absorption column density is at least 2.5 times more than that calculated from the galactic extinction maps. This suggests that the moderate, extra absorption is likely to be due to the formation of a thin cool ejecta-shell between reverse-shock and the contact discontinuity. About 0.01 M of Ne is estimated to be present in SN 1995N from the Chandra line detection. This, most likely, arises in the partially burnt He core at velocities > 5000 km s−1 . I also observed SN 1995N with the GMRT in radio bands. I describe these results in Chapter 7. The spectrum is seen to be peaking towards lower frequencies with the time. The radio light curve suggests that the SN is already in the optically thin part of the light curve. Some but not all type Ic supernovae have shown association with Gamma Ray Burst (GRB) sources. This seems to divide the type Ic SNe in two subclasses -the ones associated with GRBs and the ones without the GRB connection. The observations of these two classes of type Ic SNe and their comparison (the ”afterglows”) are likely to be useful in determining the physical conditions inside the progenitor star which leads some of the type Ic supernovae to have GRB associations. Since these SNe are bare core SNe with no hydrogen and little or no helium envelope, their prompt emission in the radio and high energy bands provide the most promising probes of their interior at early times. In Chapter 8, I investigate the origin of prompt X-ray emission in a type Ic supernova SN 2002ap, a non-GRB supernova. An analysis of SN 2002ap, observed with XMM-Newton on Feb 3, 2002 as a Target Of Opportunity is presented and spectral model fits to the prompt X-ray emission are obtained. I model the early X-ray emission with inputs from optical photometry and light curve and find that multiple inverse Compton scattering of optical photons from the supernova photosphere by electrons in the medium can account for the observed early X-ray flux and its spectrum for modest electron temperatures and optical depths. I compare the X-ray image with the GMRT 610 MHz radio image obtained three days apart. While I find no radio counterpart of the SN at such low frequencies, several sources in the field have radio and X-ray counterparts. I compare the radio data obtained from three different supernovae in their early phases and model these using the synchrotron self absorption model. GRB associated SN 1998bw was found to be most rapidly expanding with fastest transition from optically thick to optically thin part in the spectrum. Radio studies of SN 2003bg, another type Ic supernova is discussed in Chapter 9. I observed SN 2003bg with the GMRT from day 43 till day 600 since explosion. On one occasion (day ~ 350), I combined the GMRT data with the VLA data to get a composite spectrum. From the optically thick part of the spectrum, I find that the dominant absorption mechanism in the SN is synchrotron self absorption. I deduced magnetic field and size of the supernova under the assumption of equipartition. I discuss the overall results in Chapter 10. In this thesis, I have investigated four supernovae in detail and few more have been observed with lesser sampling frequency (see Chapter 10 and P. Chandra et al 2002, BASI 30, 755). Although they all belong to the ejecta dominated free expansion phase, the core-collapse supernovae are of widely different subclasses and I have observed them at very young ages (few days) to more than 10 years of age, with multiple probes, going through a variety of emission mechanisms and absorption processes. I provide comparison between different supernovae observed by us and others. In cases, where I am able to obtain X-ray spectra, nucleosynthesis arguments lead me to constrain the mass of the progenitor star and the composition of its layers.