  A white dwarf (WD) approaching the Chandrasekhar mass may in several cases
undergo accretion-induced collapse (AIC) to a neutron star (NS) before a
thermonuclear explosion ensues. It has generally been assumed that AIC does not
produce a detectable supernova (SN). If, however, the progenitor WD is rapidly
rotating (as may be expected due to its prior accretion), a centrifugally
supported disk forms around the NS upon collapse. We calculate the subsequent
evolution of this accretion disk using time-dependent height-integrated
simulations with initial conditions taken from the AIC calculations of Dessart
et al. (2006). Initially, the disk is cooled by neutrinos and its composition
is driven neutron-rich (electron fraction Ye ~ 0.1) by electron captures.
However, as the disk viscously spreads, it is irradiated by neutrinos from the
central proto-NS, which dramatically alters its neutron-to-proton ratio. We
find that electron neutrino captures increase Ye to ~ 0.5 by the time that weak
interactions in the disk freeze out. Because the disk becomes radiatively
inefficient and begins forming alpha-particles soon after freeze out, powerful
winds blow away most of the disk's remaining mass. These Ye ~ 0.5 outflows
synthesize up to a few times 1e-2 Msun in 56Ni. As a result, AIC may be
accompanied by a radioactively powered SN-like transient that peaks on a
timescale of ~ 1 day. Since few intermediate mass elements are likely
synthesized, these Ni-rich explosions should be spectroscopically distinct from
other SNe. PanSTARRs and the Palomar Transient Factory should detect a few AIC
transients per year if their true rate is ~1/100 of the Type Ia rate, and LSST
should detect hundreds per year. High cadence observations (< 1 day) are
optimal for the detection and follow-up of AIC (abridged).
