Detection of a light echo from SN 1998bu

About 500 days after the explosion, the light curve of the Type Ia supernova SN 1998bu suddenly flattened, and at the same rime the spectrum changed from the typical nebular emission to a blue continuum with broad absorption and emission features reminiscent of the SN spectrum at early phases. We show that in analogy to SN 1991T, this can be explained by the emergence of a light echo from a foreground dust cloud. Using a simple model, we argue that the amount of dust required can consistently explain the extinction that has been estimated by completely independent methods. Because of the similar echo luminosity but much higher optical depth of the dust in SN 1998bu compared with SN 1991T, we expect that the echo ring size of SN 1998bu grows faster than in SN 1991T. Hubble Space Telescope observations have indeed confirmed this prediction.