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 a potential progenitor channel for certain merger-kind, but long-duration $γ$-ray bursts (GRBs), e.g., GRBs 230307A and 211211A. The relatively large tidal disruption radius of the WD can result in non-negligible residual orbital eccentricity ($0 \lesssim e \lesssim 0.2$), causing episodic mass transfer, i.e., repeated tidal 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 ensuing accretion process, leading to variations of the accretion rate on the orbital period. Across all simulations, the peak accretion rates range from $4 \times10^{-4}$ to 0.2 $M_{\odot} \rm \ s^{-1}$, while the RPD duration spans from $\sim$ 10 s to an hour. More compact systems, i.e., those with a higher mass ratio (higher WD mass and lower accretor mass), tend to undergo fewer RPD cycles, resulting in shorter durations and higher accretion rates. If such events can launch relativistic jets, three categories of non-thermal 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 that has a GRB appearing only at its later phase ; (3) an ultra-long X-ray transient lasting for $\sim 10^{3}$ s without a GRB. A generic feature of these transients is that their prompt emission light curves are probably periodically modulated with periods of a few to tens of seconds.