Thermal X‐Ray Emission from Young Type Ia Supernova Remnants

In the years since the launch of Chandra and XMM-Newton, the increase in the quantity and quality of X-ray observations of supernova remnants (SNRs) has been spectacular. In many of the brightest SNRs in our Galaxy and the Magellanic Clouds, the X-ray spectrum is dominated by emission lines from the elements in the shocked supernova ejecta, including O, Ne, Mg, Si, S, Ar, Ca, and Fe. Unfortunately, the lack of adequate tools for the spectral analysis of X-ray emission from shocked SN ejecta has prevented the systematic use of these excellent observations to quantitatively constrain the physics of supernova explosions. Even the most sophisticated nonequilibrium ionization (NEI) models available in software packages like XSPEC make simplistic assumptions, like a homogeneous composition for the shocked plasma, that cannot be applied to the ejecta in young SNRs. As a result, the most fundamental parameters of SNe (total kinetic energy, mass of each element synthesized in the explosion) cannot be determined from Xray observations of young SNRs in a reliable way by an analysis that is based on these spectral models alone. In view of the state-of-the-art in supernova explosion models, however, these quantitative constrains would be extremely useful. In the case of Type Ia (thermonuclear) supernovae, this is especially relevant, because despite the continued efforts of the last decades, fundamental details of the explosion mechanism still remain obscure (see Hillebrandt & Niemeyer 2000, ARAA Travaglio et al. 2004, AA Garcia-Senz & Bravo 2005, A&A, 430, 585). The advances have been spectacular, but the zoo of possible Type Ia SN models remains very large, and the optical and infrared observations of supernovae have only been partially successful in reducing it so far. The goal of this Ph.D. thesis was to build tools that allow observers to use the X-ray spectra from young, ejecta-dominated Type Ia SNRs to constrain the physics of Type Ia SN explosions in an effective way. In order to do this, we have coupled onedimensional hydrodynamic calculations of the interaction between a grid of Type Ia SN explosion models and a uniform ambient medium (AM) to a code that follows the NEI ionization in the shocked plasma and calculates its X-ray emission. In this way, an extensive grid of synthetic spectra for the ejecta emission has been produced that can be compared with observations of real objects. These spectra only depend on four parameters: the Type Ia SN explosion model (chosen from our grid of 23 models, which includes examples of all the paradigms under discussion), the age t of the SNR, the density of the AM, , and the amount rAM of collisionless electron heating at the reverse shock, b (see Laming 2000, ApJS, 127, 409, for a discussion on collisionless heating in SNR shocks). The grid of synthetic spectra has been presented in two papers, Badenes et al. (2003, ApJ, 593, 358) and (2005, ApJ, 624, 198), in which the impact of each of the four parameters mentioned above on the final X-ray spectrum from the shocked ejecta is discussed in detail. The main conclusion from our study is that the physical processes acting at the time of the Type Ia SN explosion have a profound impact on the X-ray emission from the SNR, hundreds and even thousands of years after the explosion itself. Thus, synthetic spectra calculated assuming different Type Ia SN explosion models have very different properties, in terms of the emission measures, ionization timescales, and electron temperatures associated with the elements present in the spectrum. The application of this grid of synthetic spectra to the X-ray spectrum of the Tycho SNR has produced very promising results (Badenes et al. 2005, in preparation). The delayed detonation (DDT) paradigm of models is very successful in reproducing most of the fundamental characteristics of the line emission in the Tycho SNR, while the performance of all the other paradigms (deflagrations, pulsating delayed detonations, and sub-Chandrasekhar explosions) is clearly inferior. The well-mixed three-dimensional Type Ia models also fail, and we find strong evidence that Type Ia ejecta retain at least part of the elemental stratification characteristic of one-dimensional models. Within the DDT models, those with moderate kinetic energies (∼1051 ergs) are preferred over the highly energetic or subenergetic ones.