Variability in Short Gamma-ray Bursts: Gravitationally Unstable Tidal Tails

Abstract

Short gamma-ray bursts are thought to result from the mergers of two neutron stars or a neutron star and stellar mass black hole. The final stages of the merger are generally accompanied by the production of one or more tidal "tails" of ejecta, which fall back onto the remnant-disc system at late times. Using the results of a linear stability analysis, we show that if the material comprising these tails is modeled as adiabatic and the effective adiabatic index satisfies $\gamma \ge 5/3$, then the tails are gravitationally unstable and collapse to form small-scale knots. We analytically estimate the properties of these knots, including their spacing along the tidal tail and the total number produced, and their effect on the mass return rate to the merger remnant. We perform hydrodynamical simulations of the disruption of a polytropic (with the polytropic and adiabatic indices $\gamma$ equal), $\gamma =2$ neutron star by a black hole, and find agreement between the predictions of the linear stability analysis and the distribution of knots that collapse out of the instability. The return of these knots to the black hole induces variability in the fallback rate, which can manifest as variability in the lightcurve of the GRB and -- depending on how rapidly the instability operates -- the prompt emission. The late-time variability induced by the return of these knots is also consistent with the extended emission observed in some GRBs.

Small corrections, additional references included to reflect ApJL published version

References in corpus (19)

• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• Origin of the heavy elements in binary neutron-star mergers from a gravitational wave event
• Constraints on a phenomenologically parameterized neutron-star equation of state
• SPLASH: An interactive visualisation tool for Smoothed Particle Hydrodynamics simulations
• Accretion disc viscosity: how big is alpha?
• An enigmatic long-lasting gamma-ray burst not accompanied by a bright supernova
• Stellar disruption by a supermassive black hole: is the light curve really proportional to $t^{-5/3}$?
• New Hyperon Equations of State for Supernovae and Neutron Stars in Density-dependent Hadron Field Theory
• Fallback accretion in the aftermath of a compact binary merger
• A Comparison of the Afterglows of Short- and Long-Duration Gamma-Ray Bursts
• Phase transitions and He-synthesis driven winds in neutrino cooled accretion disks: prospects for late flares in short gamma-ray bursts
• A Second Relativistic Mean Field and Virial Equation of State for Astrophysical Simulations
• Tidal disruptions by rotating black holes: effects of spin and impact parameter
• Numerical simulations of neutron star-black hole binaries in the near-equal-mass regime
• Relativistic effects on tidal disruption kicks of solitary stars
• Bimodal Long-Lasting Components in Short Gamma-Ray Bursts: Promising Electromagnetic Counterparts to Neutron Star Binary Mergers
• Imprints of r-process heating on fall-back accretion: distinguishing black hole-neutron star from double neutron star mergers
• Flares in Gamma Ray Bursts: Disc Fragmentation and Evolution
• r-Process enrichment in the Galactic halo characterized by nucleosynthesis variation in the ejecta of coalescing neutron star binaries

Cited by in corpus (7)

• Partial tidal disruption events by stellar mass black holes: gravitational instability of stream and impact from remnant core
• The Eccentric Nature of Eccentric Tidal Disruption Events
• Structured, relativistic jets driven by radiation
• Stellar Revival and Repeated Flares in Deeply Plunging Tidal Disruption Events
• Self-gravitating collapsing star and black hole spin-up in long gamma ray bursts
• Short gamma-ray bursts and the decompression of neutron star matter in tidal streams
• GRB variabilities and following gravitational waves induced by gravitational instability in NDAFs