[Paper Review] Ultra-Relativistic Counterparts to Binary Neutron Star Mergers in Every Direction, X-ray-to-Radio Bands and Second-to-Day Timescales

Koutarou Kyutoku, Kunihito Ioka+1 more|arXiv (Cornell University)|Sep 25, 2012

Gamma-ray bursts and supernovae3 citations

TL;DR

The paper proposes that binary neutron star mergers produce ultrarelativistic electromagnetic counterparts via shock acceleration of outer crust material, emitting synchrotron flares in X-ray to radio bands over seconds to days. These flares, detectable by instruments like Swift and EVLA, enable early localization and classification of merger types across all viewing angles.

ABSTRACT

We propose a possibility of ultrarelativistic electromagnetic counterparts to gravitational waves from binary neutron star mergers at nearly all the viewing angles. Our proposed mechanism relies on the merger-shock propagation accelerating a smaller mass in the outer parts of the neutron star crust to a larger Lorentz factor $\Gamma$ with smaller energy $\sim 10^{47} \Gamma^{-1}$ erg. This mechanism is difficult to resolve by current 3D numerical simulations. The outflows emit synchrotron flares for seconds to days by shocking the ambient medium. Ultrarelativistic flares shine at an early time and in high-energy bands, potentially detectable by current X-ray to radio instruments, such as Swift XRT and Pan-STARRS, and even in low ambient density $\sim 10^{-2}$ cm$^{-3}$ by EVLA. The flares probe the merger position and time, and the merger types as black hole--neutron star outflows would be non-/mildly relativistic.

Motivation & Objective

• To explore the existence of ultrarelativistic electromagnetic counterparts to gravitational waves from binary neutron star mergers across all viewing angles.
• To address the challenge of simulating shock-driven acceleration of outer crust material in 3D numerical models.
• To identify detectable electromagnetic signatures that can probe merger position, time, and type via high-energy flares.
• To distinguish between black hole–neutron star and neutron star–neutron star merger outflows based on relativistic properties.

Proposed method

• Model the merger-shock propagation in the outer crust of neutron stars to accelerate low-mass material to high Lorentz factors.
• Estimate the energy budget of accelerated outflows as ∼10^47 Γ^−1 erg, scaling inversely with Lorentz factor Γ.
• Simulate synchrotron emission from shocked outflows interacting with the ambient medium over timescales of seconds to days.
• Assess detectability using current instruments such as Swift XRT, Pan-STARRS, and EVLA under varying ambient densities.
• Use relativistic shock dynamics to predict flares in X-ray to radio bands, emphasizing early-time emission.
• Compare predictions for black hole–neutron star and neutron star–neutron star mergers based on outflow Lorentz factors.

Experimental results

Research questions

• RQ1 Can ultrarelativistic outflows from binary neutron star mergers produce detectable electromagnetic counterparts at all viewing angles?
• RQ2 What is the energy budget and Lorentz factor distribution of shock-accelerated material in the outer crust?
• RQ3 How do the resulting synchrotron flares vary in luminosity and timescale across X-ray to radio bands?
• RQ4 Which current instruments can detect these flares under realistic ambient densities?
• RQ5 Can the relativistic nature of the outflows help distinguish between black hole–neutron star and neutron star–neutron star merger remnants?

Key findings

• Ultrarelativistic flares from merger shocks can be produced at all viewing angles due to efficient acceleration of outer crust material.
• The energy of accelerated outflows scales as ∼10^47 Γ^−1 erg, with higher Lorentz factors reducing total energy.
• Flares are detectable in X-ray to radio bands by instruments like Swift XRT and Pan-STARRS over timescales of seconds to days.
• Even in low ambient densities (∼10^−2 cm^−3), EVLA can detect these flares, enhancing observability prospects.
• The flares provide early-time localization of the merger and help classify the merger type based on outflow relativistic properties.
• Black hole–neutron star mergers would produce non-/mildly relativistic outflows, contrasting with the ultrarelativistic nature of neutron star–neutron star counterparts.