SN 2021fxy: Mid-Ultraviolet Flux Suppression is a Common Feature of Type Ia Supernovae

Abstract We present ultraviolet (UV) to near-infrared (NIR) observations and analysis of the nearby Type Ia supernova SN 2021fxy. Our include UV photometry from Swift/UVOT, spectroscopy HST/STIS, high-cadence optical with Swope 1-m telescope capturing intra-night rises during early light curve. Early B − V colours show 2021fxy is first ‘shallow-silicon’ (SS) follow a red-to-blue evolution, compared other SS objects which blue earliest observations. Comparisons spectroscopically normal SNe HST spectra reveal one several flux suppression in mid-UV. These also blue-shifted mid-UV spectral features strong high-velocity Ca ii features. One possible origin this increased effective opacity due line blanketing high velocity material, but differences explosion mechanism cannot be ruled out. Among suppression, 2017erp substantial similarities their properties despite belonging different Branch subgroups, same order as those found between 2011fe 2011by. Differential comparisons multiple sets synthetic difference likely originates luminosity 2017erp, not differing progenitor metallicities suggested for 2011by 2011fe. illustrate complicated nature formation, need more determine physical source diversity.