Interaction of SN Ib 2004dk with a Previously Expelled Envelope

The interaction between the expanding supernova (SN) ejecta with the circumstellar material (CSM) that was expelled from the progenitor prior to explosion is a long-sought phenomenon, yet observational evidence is scarce. Here we confirm a new example: SN 2004dk, originally a hydrogen-poor, helium-rich Type Ib SN that reappeared as a strong Ha-emitting point source on narrowband H alpha images. We present follow-up optical spectroscopy that reveals the presence of a broad H alpha component with full width at half maximum of similar to 290 km s(-1) in addition to the narrow H alpha+[N II] emission features from the host galaxy. Such a broad component is a clear sign of an ejecta-CSM interaction. We also present observations with the XMM-Newton Observatory, the Swift satellite, and the Chandra X-ray Observatory that span 10 days to 15 years after discovery. The detection of strong radio, X-ray, and H alpha emission years after explosion allows various constraints to be put on pre-SN mass-loss processes. We present a wind-bubble model in which the CSM is pre-prepared by a fast wind interacting with a slow wind. Much of the outer density profile into which the SN explodes corresponds to no steady-state mass-loss process. We estimate that the shell of compressed slow wind material was ejected similar to 1400 yr prior to explosion, perhaps during carbon burning, and that the SN shock had swept up about 0.04 M-circle dot of material. The region emitting the H alpha has a density of order 10(-20) g cm(-3).