Delayed jet breakouts from binary neutron star mergers

Abstract

Short gamma-ray bursts (sGRBs) are thought to be produced by binary NS mergers. While a 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 $\gamma$-rays but long-time X-ray emissions for $\sim10^{2-3}\,\rm{s}$ or even $\sim10^{4-5}\,\rm{s}$. 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 followups of merger events without $\gamma$-rays will be a probe of long-lasting engine activities in binary mergers.