The dominant radioactive energy source powering Type Ia supernova light curves is expected to switch from the decay of 56 Co to 57 Co at very late epochs. We use archival HST images of SN1992A obtained more than 900 days after explosion to constrain its cobalt isotopic abundance ratio and compare it to the well-studied event SN2011fe. We confirm the 57 Co/ 56 Co ratio for SN2011fe of 0.026±0.004 found by Shappee et al. (2017), consistent with a "double degenerate" progenitor scenario. For SN1992A, we find a ratio of 0.034±0.010, but the large uncertainty does not allow us to differentiate between progenitor models.

We conduct a review of the basic definitions and the principle results in the study of wavelike spacetimes, that is spacetimes whose metric models massless radiation moving at the speed of light. In particular, we motivate and connect their various definitions, outline their coordinate descriptions and present some classical results in their study in a language more accessible to modern readers, including the existence of "null coordinates" and the construction of Penrose limits. We also present a thorough summary of recent work on causality in pp-waves, and progress in addressing an open question in the field -the Ehlers-Kundt conjecture.

We conduct a review of the basic definitions and the principal results in the study of wavelike spacetimes, that is spacetimes whose metric models massless radiation moving at the speed of light, focusing in particular on those geometries with parallel rays. In particular, we motivate and connect their various definitions, outline their coordinate descriptions and present some classical results in their study in a language more accessible to modern readers, including the existence of “null coordinates” and the construction of Penrose limits. We also present a thorough summary of recent work on causality in pp-waves, and describe progress in addressing an open question in the field—the Ehlers–Kundt conjecture.

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