Delayed Jet Breakouts from Binary Neutron Star Mergers

Short gamma-ray bursts (sGRBs) are thought to be produced by binary neutron star mergers. While an sGRB requires a relativistic jet to break out of ejecta, the jet may be choked and fails to produce a successful sGRB. We propose a "delayed breakout" scenario where a late-time jet launched by a long-term engine activity can penetrate ejecta even if a prompt jet is choked. Observationally, such a late-time jet is supported by the long-lasting high-energy emissions in sGRBs. Solving the jet propagation in ejecta, we show that a typical late-time activity easily achieves the delayed breakout. This event shows not prompt γ-rays but long-time X-ray emissions for ∼102–3 s or even ∼104–5 s. Some delayed events may be already detected as soft-long GRBs without supernova signatures. In an optimistic case, a few events coincident with gravitational-waves (GWs) are detected by the second-generation GW detectors every year. X-ray follow-ups of merger events without γ-rays will be a probe of long-lasting engine activities in binary mergers.