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 Å; uvw 2 and uvm 2) and near-UV (2700–4000 Å; uvw 1 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 uvw 1 − 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 σ are found for uvw 1 − 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 uvw 1 − v colors including a large spread at low velocities, demonstrating that the NUV-blue/red spread is not caused by the photospheric velocity. The uvm 2 − uvw 1 colors also show a large dispersion uncorrelated with the velocity.