The Signature of Single-Degenerate Accretion Induced Collapse

Abstract: The accretion induced collapse (AIC) of a white dwarf to a neutron star has long been suggested as a natural theoretical outcome in stellar evolution, but there has never been a direct detection of such an event. This is not surprising since the small amount of radioactive nickel synthesized ($\sim10^{-3}\,M_\odot$) implies a relatively dim optical transient. Here we argue that a particularly strong signature of an AIC would occur for an oxygen-neon-magnesium (ONeMg) white dwarf accreting from a star that is experiencing Roche-lobe overflow as it becomes a red giant. In such cases, the $\sim10^{50}\,{\rm erg}$ explosion from the AIC collides with and shock-heats the surface of the extended companion, creating an X-ray flash lasting $\sim1\,{\rm hr}$ followed by an optical signature that peaks at an absolute magnitude of $\sim -16$ to $-18$ and lasts for a few days to a week. These events would be especially striking in old stellar environments where hydrogen-rich supernova-like, transients would not normally be expected. Although the rate of such events is not currently known, we describe observing strategies that could be utilized with high cadence surveys that should either detect these events or place strong constraints on their rates.