Quasi-periodic oscillation in short gamma-ray bursts from black hole-neutron star mergers

Short-duration gamma-ray bursts (sGRBs) are commonly attributed to the mergers of double neutron stars (NSs) or the mergers of a neutron star with a black hole (BH). While the former scenario was confirmed by the event GW170817, the latter remains elusive. Here, we consider the latter scenario in which, a NS is tidally disrupted by a fast spinning low-mass BH and the accretion onto the BH launches a relativistic jet and hence produces a sGRB. The merging binary's orbit is likely misaligned with the BH's spin. Hence, the Lense-Thirring precession around the BH may cause a hyper-accreting thick disk to precess in a solid-body manner. We propose that a jet, initially aligned with the BH spin, is deflected and collimated by the wind from the disk, therefore being forced to precess along with the disk. This would result in a quasi-periodic oscillation or modulation in the gamma-ray light curve of the sGRB, with a quasi-period of $\sim 0.01-0.1$ s. The appearance of the modulation may be delayed respective to the triggering of the light curve. This feature, unique to the BH-NS merger, may have already revealed itself in a few observed sGRBs (such as GRB 130310A), and it carries the spin-obit orientation information of the merging system. Identification of this feature would be a new approach to reveal spin-orbit-misaligned merging BH-NS systems, which are likely missed by the current gravitational-wave searching strategy principally targeting aligned systems.