The Ultraviolet Colors of Type Ia Supernovae and Their Photospheric Velocities

We compare ultraviolet (UV) and optical colors of a sample of 29 type Ia supernovae (SNe Ia) observed with the Swift satellite's UltraViolet Optical Telescope with theoretical models of an asymmetric explosion viewed from different angles from Kasen & Plewa. This includes mid-UV (1600-2700 angstrom; uvw2 and uvm2) and near-UV (2700-4000 angstrom; uvw1 and u) filters. We find the observed colors to be redder than the model predictions, and that these offsets are unlikely to be caused by dust reddening. We confirm that high-velocity SNe Ia have red UV-optical observed colors. After correcting the colors for dust reddening by assuming a constant b - v color, we find no correlation between the uvw1 - v or u - v colors and the ejecta velocities for 25 SNe Ia with published velocities and/or spectra. When assuming an optical color-velocity relation, weak correlations of 2 and 3.6 sigma are found for uvw1 - v and u - v. However, we find that weak correlations can be reproduced with shuffled velocities and colors that are corrected for reddening. The slope and significance of a correlation between the UV colors and the velocity is thus dependent on the slope of the optical color-velocity relation. Even with a correction, a significant scatter still remains in the uvw1 - v colors including a large spread at low velocities, demonstrating that the NUV-blue/red spread is not caused by the photospheric velocity. The uvm2 - uvw1 colors also show a large dispersion uncorrelated with the velocity.