Presentation #317.02D in the session Type Ia and Iax Supernovae.
We present optical and UV analysis of Type Ia supernova (SN Ia) SN 2021fxy, including early-time, rapid-cadence ground-based photometry from the POISE group starting within a day of discovery. In addition, we present spectroscopic follow-up with HST/STIS in the mid- and near-UV. SN 2021fxy shows unexpected similarities to SN 2017erp, including nearly identical UV spectra with flux suppression in the mid-UV. As differences between SNe Ia are expected to become more prominent in the UV, these similarities are especially interesting given the classifications of SN 2017erp and SN 2021fxy as “core-normal” and “shallow-silicon” objects, respectively, within the Branch scheme. Since the UV spectrum originates in the outermost layers of the ejecta, it serves an an important probe of variations of the density and metallicity of Type Ia progenitors. When combined with the early-time observations of the POISE group, we can better explore the connections between different Branch sub-groups and gain a deeper understanding of the nature of the progenitor systems and explosion mechanisms in SNe Ia.