Reddened, Redshifted, or Intrinsically Red? Understanding Near-ultraviolet Colors of Type Ia Supernovae

Abstract

The intrinsic colors of Type Ia supernovae (SNe Ia) are important to understanding their use as cosmological standard candles. Understanding the effects of reddening and redshift on the observed colors are complicated and dependent on the intrinsic spectrum, the filter curves, and the wavelength dependence of reddening. We present ultraviolet and optical data of a growing sample of SNe Ia observed with the Ultraviolet/Optical Telescope on the Swift spacecraft and use this sample to re-examine the near-UV (NUV) colors of SNe Ia. We find that a small amount of reddening (E(B - V) = 0.2 mag) could account for the difference between groups designated as NUVblue and NUV-red, and a moderate amount of reddening (E(B - V) = 0.5 mag) could account for the whole NUVoptical differences. The reddening scenario, however, is inconsistent with the mid-UV colors and color evolution. The effect of redshift alone only accounts for part of the variation. Using a spectral template of SN2011fe, we can forward model the effects of redshift and reddening and directly compare those with the observed colors. We find that some SNe are consistent with reddened versions of SN2011fe, but most SNe Ia are much redder in the uvw1 - v color than SN2011fe reddened to the same b - v color. The absolute magnitudes show that two out of five NUV-blue SNe Ia are blue because their near-UV luminosity is high, and the other three are optically fainter. We also show that SN. 2011fe is not a "normal" SN Ia in the UV, but has colors placing it at the blue extreme of our sample.