HST*]{}/ACS images would help constrain the evolution of the LE. Using equation (1), we can predict the evolution of the echo ring with time. If the dust formed as a result of past mass loss from the central source, the echo will be more symmetric and the expansion will slow down after the initially rapid phase; with time, its size will eventually shrink to zero. On the other hand, if the dust is of interstellar origin, the echo should expand continuously with slowly decreasing brightness as more-distant regions are illuminated. Assuming that the inner component of the echo within 2 pixels is caused by a CSM dust shell $\sim$1 pc from the SN, then the emission within 2 pixels will finally decrease to zero at $t \approx 6.5$ yr. In contrast, the local echo from distant ISM dust should remain nearly constant for a longer time.

It is worth pointing out that the recent nearby SNe Ia, SN 2007gi (CBET 1017, CBET 1021), SN 2007le (CBET 1100, CBET 1101), and probably SN 2007sr (CBET 1172,1174, ATEL 1343), may exhibit high-velocity features in their spectra similar to those of SN 2006X. If the SN 2006X-like events preferentially occur in environments with abundant ISM dust or CSM dust (Wang et al. 2008, in prep.), then we might expect to detect late-time echo emission in the above three SNe Ia. Thus, it would be interesting to obtain future high-resolution [*HST*]{}/ACS images of these SNe.

Conclusions
===========

The emergence of a LE in SN 2006X has been confirmed with PSF-subtracted [*HST*]{} ACS images which show a ring-like, but rather extended, echo 2–5 pixels ($0.05''$–$0.13''$) from the SN site at $t = 308$ d past maximum brightness. A Keck nebular spectrum of the SN taken at a similar phase provides additional evidence for the LE scenario; it can be decomposed into a nebular spectrum of a normal SN Ia and a reflection spectrum consisting of the SN light emitted at early phases.

From the resolved echo image, we derive that the intervening dust is $\sim$27–170 pc from the supernova. Based on the q