Detection of a Light Echo from Sn 1998

About 500d after explosion the light curve of the Type Ia SN 1998bu suddenly flattened and at the same time 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 (Schmidt et al. 1994), this can be explained by the emergence of a light echo from a foreground dust cloud. Based on a simple model we argue that the amount of dust required can consistently explain the extinction which 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. HST observations have indeed confirmed this prediction. a nearby Sab galaxy. It had very good photometric and spectroscopic coverage by several groups (Jha et al. 1999; Hernandez et al. 2000; Suntzeff et al. 1999) showing that the luminosity decline was slower than the average (∆m 15 (B), the magnitude decline in the early 15 days after maximum, was 1.01 ± 0.05 mag). This is almost as slow as the slowest SN Ia on record, SN 1991T, although, unlike SN 1991T, SN 1998bu was spectroscopi-cally normal before and around maximum. The distance to the host galaxy was measured using Cepheids variables (Tanvir, Ferguson & Shanks 1999) as µ = 30.25 ± 0.19. However, a calibration of the absolute luminosity of the SN depends also on the estimated reddening which in the case of SN 1998bu is significant. Hernandez et al. (2000) estimated E(B − V) = 0.86 ± 0.15 based on the comparison of the spectral energy distribution of SN 1998bu with that of the template SN Ia 1981B. SN 1998bu was included in our program of monitoring of the nebular phase of SN Ia. During this monitoring we realized that SN 1998bu was not declining with the expected rate; this, combined with the change in the spectral appearance gave the first evidence of the emergence of a light echo in SN 1998bu (Cappellaro et al. 2000). These observations are presented here for the first time along with a simple modeling. 2. evidence for a light echo in sn 1998bu When the SN Ia ejecta become optically thin, about 100 days after the explosion, …