Type Ia supernovae

Type Ia supernovae (SNe Ia) are important cosmological probes, but we are uncertain how they explode. There are two progenitor channels for SNe~Ia: single and double degenerate white dwarf (WD) systems. In either model, we expect the system to be detectable as a Supersoft X-ray Source (SSS) for a significant amount of time before the explosion. By studying these SSSs, we hope to improve our understanding of SNe~Ia. In Chapter 2, we analyze an interesting source (r1-25) in M31. We found that the source exhibited spectral changes to harder X-ray states. r1-25 is the first source of its kind, and we require unique physical models to fit its behavior. We find that existing WD models are inconsistent with the spectra of the source. We explore new black hole and neutron star models, and find that they can model the unusual behavior of r1-25. In Chapter 3, we study three gravitationally lensed SNe from the Cluster Lensing And Supernova survey with Hubble (CLASH). Based on photometric classification, we found that two SNe (SN CLO12Car and CLN12Did) are likely to be SNe~Ia, while the classification of the third is inconclusive. Using multi-color light-curve fits to determine a standardized SN~Ia luminosity distance, we infer that SN CLO12Car was $sim 1.0 pm 0.2$ mag brighter than field SNe~Ia at a similar redshift and ascribe this to gravitational lens magnification. Similarly, SN CLN12Did is $sim 0.2 pm 0.2$ mag brighter than field SNe~Ia. From independent CLASH strong+weak lensing maps of the clusters , we derived similar magnifications for the two SNe~Ia. The two SNe~Ia provide a new test of the cluster lens model predictions: we find that the magnifications based on the SN~Ia brightness and those predicted by the lens maps are consistent. Finally, in Chapter ef{c4} we discuss a new light curve fitter for SNe~Ia, which we call Multicolor Light Curve Shapes 3 (MLCS3). The project has not been completed, but we discuss some of the features, and the expected improvements from MLCS3 in the chapter. With nearly 1000 SNe~Ia usable for cosmology, we are currently systematics, not statistics, limited for constraining cosmology. MLCS3 will provide more accurate distance measurements for the SNe~Ia using a variety of improvements that have been suggested in the literature.