Observable Signatures of the Accretion-Induced Collapse of White Dwarfs

Despite its probable occurrence in Nature, the accretion-induced collapse (AIC) of a white dwarf (WD) has not yet been directly observed (or, at least, it has not been unambiguously identified as such). In this contribution we summarize the observational signatures of AIC, emphasizing its possible role as both an optical and high-energy transient. Due to the low quantity of ejected mass and radioactive Ni, the isolated collapse of a slowly-rotating WD is difficult to detect much beyond our own Galaxy. If, however, the progenitor WD is rapidly-rotating (a justifiable assumption), a significant portion of its mass must be shed during collapse into an accretion disk around the newly-formed proto-neutron star (NS). As a result, the observational manifestations of AIC with rotation expand considerably, possibly to include: (1) a day-long optical transient with a peak brightness of up to MV ∼ −15 and a broad-lined, Fe-group-rich spectrum (termed “naked” AIC); (2) a ∼week-long sub-luminous Type I SN, perhaps similar to SNe 2005e and 2008ha (termed “enshrouded” AIC); (3) a strong gravitational wave source; and, possibly, (4) a short-duration gamma-ray burst (or some analogous high-energy transient lasting ∼ 1 second). AIC with rotation also produces a rapidly-spinning NS, which has long been considered a promising channel for producing millisecond pulsars. If this rapidly-spinning NS forms strongly magnetized (a “magnetar”), its spindown luminosity may power a ∼ 100 second “flash” of high-energy radiation via internal shocks in the NS’s neutrino-heated wind. Isolated magnetar birth through AIC provides a natural explanation for the X-ray tails observed following some short GRBs and, potentially viewable over a larger solid angle than the gamma-ray precursor, may also be detectable with proposed all-sky X-ray survey missions such as EXIST .