Properties of the ultraviolet flux of Type Ia supernovae: an analysis with synthetic spectra of SN 2001ep and SN 2001eh

The spectral properties of Type Ia supernovae in the ultraviolet (UV) are investigated using the early-time spectra of SN 2001ep and SN 2001eh obtained using the Hubble Space Telescope. A series of spectral models is computed with a Monte Carlo spectral synthesis code, and the dependence of the UV flux on the elemental abundances and the density gradient in the outer layers of the ejecta is tested. A large fraction of the UV flux is formed by reverse-fluorescence scattering of photons from red to blue wavelengths. This process, combined with ionization shifts due to enhanced line blocking, can lead to a stronger UV flux as the iron-group abundance in the outer layers is increased, contrary to previous claims.