Simulating the Late Stages of WD–BH/NS Mergers: An Origin for Fast X-Ray Transients and GRBs with Periodic Modulations

Abstract Recent studies indicate that mergers of a white dwarf (WD) with a neutron star (NS) or a stellar-mass black hole (BH) may be progenitors of certain merger-type long-duration γ -ray bursts (GRBs), e.g., GRBs 230307A and 211211A. The relatively large tidal disruption radius of the WD can result in nonnegligible residual orbital eccentricity (0 ≲ e ≲ 0.2), causing episodic mass transfer, i.e., repeated partial disruptions (RPDs) of the WD. We perform smoothed particle hydrodynamics simulations of RPDs in sixteen WD–BH/NS systems, capturing the subsequent mass transfer and accretion. The WD undergoes RPDs near the orbital periastron, modulating the accretion and leading to accretion rate variations on the orbital timescale. Across all simulations, the peak accretion rates range from 4 × 10 −4 to 0.16, M ⊙ s −1 , while the RPD duration spans from ∼10 s to 1 hr. More compact systems (i.e., those with more symmetrical mass ratios) tend to undergo fewer RPD cycles, resulting in shorter durations and higher accretion rates. If such events can launch relativistic jets, three categories of nonthermal X/ γ -ray transients are predicted, in decreasing order of their mean accretion rates: (1) an X-ray transient with a simultaneous GRB, both lasting for 10 1−2 s; (2) a longer X-ray transient lasting up to 10 2−3 s with a GRB appearing at the later phase; (3) an ultralong X-ray transient lasting for ∼10 3 s without a GRB. A generic feature is that their prompt emission light curves are probably periodically modulated with periods of a few to tens of seconds.