A Tale of Two Jets: Double Relativistic Outflows from Close Binary GRB Progenitors

Abstract

Gravitational wave astronomy has revealed that close binaries with compact companions are widespread. Long GRBs (LGRBs) from massive star collapse face persistent challenges in achieving the rapid core rotation required by the collapsar model. Binary interaction via tidal spin-up offers a natural solution; recent population synthesis studies suggest a substantial fraction of LGRBs may originate from close binaries with a compact companion. In this scenario, supernova ejecta from the primary can be accreted by the companion, potentially launching a second relativistic jet after a delay set by the binary separation. We develop a comprehensive model for these double-jet systems, analyzing the dynamics of the second jet and its interaction with the first. The resulting observational signatures depend critically on the Lorentz factor ratio, the alignment angle, and the time delay. For aligned jets, two regimes arise: a fast second jet producing multiple gamma-ray triggers with distinct spectral/polarization evolution, and a slow second jet where its emission appears as an X-ray flare followed by an afterglow plateau from energy injection. For misaligned jets, the observed signal ranges from normal GRBs with late-time radio structures to fast X-ray transients followed by off-axis rebrightening. These features have observational parallels in existing GRB data. High-resolution radio interferometry with SKA, time-resolved polarimetry with eXTP, and multi-wavelength surveys with Einstein Probe and SVOM will test these predictions, providing constraints on the evolution of close massive binaries as progenitors of GRBs and gravitational wave sources.

Figures (2)

• Figure 1: Schematic illustration of the two-jet scenario. Phase I: the primary jet J1 produces prompt gamma-ray emission. Phase II: the second jet J2 is launched after a time delay $(\Delta t)$. For aligned jets (left), the relative speed determines whether J2 collides violently ($\Gamma_2>\Gamma_1$) or merges gradually ($\Gamma_2<\Gamma_1$). For misaligned jets (right), the line of sight determines the observed signal: a normal GRB with late-time radio lobes if aligned with J1, or a fast X-ray transient followed by off-axis rebrightening if aligned with J2.
• Figure 2: Schematic light curves for different two-jet scenarios. Different colors represent different observing bands, and dashed lines indicate possible components depending on specific parameters. Aligned fast jet $(\Gamma_2>\Gamma_1)$: three emission episodes (J1 prompt, J2 internal, jet-jet collision) may appear as multiple gamma-ray pulses. Aligned slow jet $(\Gamma_2<\Gamma_1)$: J2 internal emission appears as an X-ray flare, followed by an afterglow plateau from energy injection. Misaligned observer aligned with J2: an initial fast X-ray transient (FXT) is followed by late-time off-axis rebrightening from J1.