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The Type IIb supernova (SN) 1993J is one of only a few stripped-envelopesupernovae with a progenitor star identified in pre-explosion images. SN IIbmodels typically invoke H envelope stripping by mass transfer in a binarysystem. For the case of SN 1993J, the models suggest that the companion grew to22 M_solar and became a source of ultraviolet (UV) excess. Located in M81, at adistance of only 3.6 Mpc, SN 1993J offers one of the best opportunities todetect the putative companion and test the progenitor model. Previouslypublished near-UV spectra in 2004 showed evidence for absorption linesconsistent with a hot (B2 Ia) star, but the field was crowded and dominated byflux from the SN. Here we present Hubble Space Telescope (HST) Cosmic OriginsSpectrograph (COS) and Wide-Field Camera 3 (WFC3) observations of SN 1993J from2012, at which point the flux from the SN had faded sufficiently to potentiallymeasure the UV continuum properties from the putative companion. The resultingUV spectrum is consistent with contributions from both a hot B star and the SN,although we cannot rule out line-of-sight coincidences.

Uncovering the Putative B-Star Binary Companion of the SN 1993J Progenitor