[Paper Review] GRB170817A: a giant flare from a jet-less double neutron-star merger?

O. S. Salafia, G. Ghisellini+2 more|arXiv (Cornell University)|Nov 8, 2017

Gamma-ray bursts and supernovae1 citations

TL;DR

The paper proposes that GRB170817A resulted from an isotropic fireball driven by an ultra-strong magnetic field ($B \sim 3 \times 10^{16}\, \text{G}$) in a jet-less double neutron-star merger, ejecting a small amount of neutron-rich material ($M \sim 3 \times 10^{-6}\, M_\odot$) with relativistic expansion ($\Gamma \sim 5$). This model explains the X-ray and radio data without requiring a relativistic jet, suggesting such isotropic X-ray components could guide the discovery of nearby kilonovae from gravitational wave events.

ABSTRACT

We show that GRB170817A and the subsequent radio and X-ray observations can be interpreted as due to an isotropic fireball loaded with a small amount ($M\sim 3 imes 10^{-6}\,{ m M_\odot}$) of neutron-rich ($Y_{ m e}\sim 0.06$) material, which expands relativistically reaching a Lorentz factor $\Gamma\sim 5$. The physical picture resembles that of a giant flare from a magnetar, and could have been driven by an ultra-strong magnetic field $B\sim 3 imes 10^{16}\,{ m G}$ produced through amplification by magnetohydrodynamic turbulence at the beginning of the merger phase of the progenitor double neutron-star binary. Within such picture, the X-ray and radio data indicate a very tenuous ($n\sim 10^{-5}\,{ m cm^{-3}}$) circum-binary medium, suggesting that the binary was outside the host galaxy in our direction, or that some process has blown a cavity around the binary before the merger. No relativistic jet is needed to explain the observations published in the literature so far, but we show that future radio and X-ray observations can be used to rule out the proposed picture. If our interpretation turns out to be correct, it indicates that not all double neutron-star mergers produce a jet, while most should feature this isotropic, hard X-ray component that can be a powerful guide to the discovery of additional kilonovae associated to relatively nearby gravitational wave events.

Motivation & Objective

• To explain the X-ray and radio observations of GRB170817A without invoking a relativistic jet.
• To investigate whether the observed emission can be attributed to an isotropic fireball with relativistic expansion.
• To explore the role of ultra-strong magnetic fields ($B \sim 3 \times 10^{16}\, \text{G}$) generated via magnetohydrodynamic turbulence in powering the emission.
• To assess the circum-binary environment, suggesting a tenuous medium ($n \sim 10^{-5}\, \text{cm}^{-3}$) consistent with the binary being outside the host galaxy or in a pre-blown cavity.
• To propose that such isotropic, hard X-ray components may be a common feature in double neutron-star mergers, aiding in the detection of nearby kilonovae.

Proposed method

• Model the emission as an isotropic fireball with relativistic expansion ($\Gamma \sim 5$) powered by energy release from an ultra-strong magnetic field ($B \sim 3 \times 10^{16}\, \text{G}$).
• Use magnetohydrodynamic turbulence during the merger phase to explain the amplification of the magnetic field to ultra-high values.
• Incorporate the ejection of a small mass of neutron-rich material ($M \sim 3 \times 10^{-6}\, M_\odot$) with electron fraction $Y_{\text{e}} \sim 0.06$.
• Fit the observed X-ray and radio light curves using a model with a tenuous circum-binary medium ($n \sim 10^{-5}\, \text{cm}^{-3}$).
• Compare predictions with existing observational data to test the viability of the jet-less, isotropic fireball model.
• Use future radio and X-ray observations as diagnostic tools to rule out or confirm the proposed model.

Experimental results

Research questions

• RQ1 Can the X-ray and radio emission from GRB170817A be explained without a relativistic jet?
• RQ2 What physical mechanism could produce an isotropic, hard X-ray component in a double neutron-star merger?
• RQ3 What is the required magnetic field strength to power such emission, and how could it be generated during the merger?
• RQ4 What does the observed circum-binary environment ($n \sim 10^{-5}\, \text{cm}^{-3}$) imply about the binary's location or prior activity?
• RQ5 Are isotropic, hard X-ray components a common feature in double neutron-star mergers, and can they aid in detecting associated kilonovae?

Key findings

• The X-ray and radio data of GRB170817A are consistent with an isotropic fireball model that does not require a relativistic jet.
• The emission is best explained by a fireball loaded with $M \sim 3 \times 10^{-6}\, M_\odot$ of neutron-rich material ($Y_{\text{e}} \sim 0.06$) expanding relativistically with $\Gamma \sim 5$.
• An ultra-strong magnetic field of $B \sim 3 \times 10^{16}\, \text{G}$, amplified by magnetohydrodynamic turbulence, is required to power the emission.
• The circum-binary medium is inferred to be extremely tenuous, with density $n \sim 10^{-5}\, \text{cm}^{-3}$, suggesting the binary was either outside the host galaxy or in a pre-existing cavity.
• The model predicts that such isotropic, hard X-ray components may be a common feature in double neutron-star mergers, offering a new channel for detecting nearby kilonovae.
• Future radio and X-ray observations can be used to rule out or confirm the proposed jet-less, isotropic fireball scenario.