Distinct neutrino signatures and onset condition of quark deconfinement in accretion-induced collapse of white dwarfs

We present the first general relativistic, neutrino-radiation hydrodynamics simulations of accretion-induced collapse (AIC) extending to seconds after core bounce, using realistic hadron–quark hybrid equations of state (EOSs). A first-order QCD phase transition (PT) triggers a second dynamical collapse and the formation of a quasistable protohybrid star (PHS) with a deconfined quark core and a distinctive second neutrino burst. We find that the thermally suppressed onset of the mixed phase allows low-mass protoneutron stars to enter the hadron-quark mixed phase during long-term evolution, even for hybrid EOSs with high onset densities. In contrast to core-collapse supernovae (CCSNe), AIC models exhibit a tightly constrained onset mass with minimal EOS dependence, owing to the absence of a massive envelope and thus the reduced postbounce accretion. This enhances the sensitivity of neutrino observables in AIC to hybrid EOS properties. We establish empirical relations between PT onset density and neutrino signatures, revealing a distinct behavior in AIC not seen in CCSNe. Our results suggest that a single Galactic AIC neutrino detection could place strong constraints on QCD PT thresholds, hybrid EOS characteristics, and the existence of PHSs. PT in AIC may also produce gravitational waves, gamma-ray bursts, and r -process elements, motivating multidimensional simulations with rotation, magnetic fields, and improved microphysics for realistic multimessenger predictions.