[Paper Review] Can an Off-axis Gamma-Ray Burst Jet in GW170817 Explain All the Electromagnetic Counterparts?

Kunihito Ioka, Takashi Nakamura|arXiv (Cornell University)|Oct 16, 2017

Gamma-ray bursts and supernovae23 references16 citations

TL;DR

This paper investigates whether a single off-axis gamma-ray burst (GRB) jet in GW170817 can explain all observed electromagnetic counterparts—sGRB 170817A, the blue macronova, and X-ray/radio afterglows. Using relativistic jet modeling with isotropic energy ~10⁵¹–10⁵² erg, opening angle ~20°, and viewing angle ~30°, the authors show consistent emission across all bands, with jet-driven cocoon accelerating ejecta to 0.3–0.4c and low ambient density (~10⁻³–10⁻⁶ cm⁻³) inferred from afterglow decay.

ABSTRACT

Gravitational waves from a merger of two neutron stars (NSs) were discovered for the first time in GW170817, together with diverse electromagnetic (EM) counterparts. To make constraints on a relativistic jet from the NS merger, we calculate the EM signals in (1) the short gamma-ray burst sGRB 170817A from an off-axis jet, (2) the optical-infrared macronova (or kilonova), especially the blue macronova, from a jet-powered cocoon, and (3) the X-ray and radio afterglows from the interaction between the jet and interstellar medium. We find that a typical sGRB jet is consistent with these observations, and there is a parameter space to explain all the observations in a unified fashion with an isotropic energy $\sim 10^{51}$-$10^{52}$ erg, opening angle $\sim 20^{\circ}$, and viewing angle $\sim 30^{\circ}$. The off-axis emission is less de-beamed than the point-source case because the viewing angle is comparable to the opening angle. We also analytically show that the jet energy accelerates a fair fraction of the merger ejecta to a sub-relativistic velocity $\sim 0.3$-$0.4$c as a cocoon in a wide parameter range. The ambient density might be low $\sim 10^{-3}$-$10^{-6}$ cm$^{-3}$, which can be tested by future observations of radio flares and X-ray remnants.

Motivation & Objective

• To determine if a single off-axis GRB jet can explain all electromagnetic counterparts observed in GW170817.
• To constrain the jet parameters—such as isotropic energy, opening angle, and viewing angle—that reproduce the observed sGRB, macronova, and afterglow light curves.
• To assess the role of jet-powered cocoon in accelerating sub-relativistic ejecta and producing the blue macronova emission.
• To derive constraints on the interstellar medium density from X-ray and radio afterglow decay behavior.

Proposed method

• Modeling the off-axis GRB emission using a relativistic jet with a broken power-law spectrum (Band-like) in the comoving frame, with spectral indices α_B = -1, β_B = -2.2, and s = 1.
• Calculating the isotropic energy at the observer's viewing angle using time-integrated flux, accounting for relativistic beaming and Doppler boosting via the Lorentz factor Γ and viewing angle θ_v.
• Deriving the time-integration limits T_start and T_end based on light-travel time from the jet's emission region, considering the jet's angular width Δθ and observer's position.
• Using the Doppler factor δ(θ_v) = 1/[Γ(1 - β cos θ_v)] and modified version ˜δ(θ_v) for different viewing regimes to compute flux scaling across θ_v.
• Applying analytical scaling laws to show E_iso(θ_v) ∝ const for θ_v < Δθ, ∝ ˜δ(θ_v)² for Δθ < θ_v ≲ 2Δθ, and ∝ δ(θ_v)³ for θ_v ≳ 2Δθ.
• Estimating the cocoon's kinetic energy and velocity by modeling jet-cocoon interaction, showing sub-relativistic ejecta at 0.3–0.4c over a wide parameter range.

Experimental results

Research questions

• RQ1 Can a single off-axis GRB jet explain the observed short gamma-ray burst sGRB 170817A, the blue macronova, and the X-ray and radio afterglows in GW170817?
• RQ2 What range of jet parameters—specifically isotropic energy, opening angle, and viewing angle—can simultaneously reproduce all electromagnetic counterparts?
• RQ3 How does the jet drive the acceleration of ejecta to sub-relativistic velocities (0.3–0.4c) via a cocoon, and is this mechanism consistent across a broad range of jet and ambient conditions?
• RQ4 What constraints can be placed on the interstellar medium density from the observed X-ray and radio afterglow light curves?
• RQ5 How does the viewing angle affect the observed isotropic energy, especially when it is comparable to the jet opening angle?

Key findings

• A typical off-axis GRB jet with isotropic energy ~10⁵¹–10⁵² erg, opening angle ~20°, and viewing angle ~30° can simultaneously explain sGRB 170817A, the blue macronova, and the X-ray and radio afterglows.
• The off-axis emission is less de-beamed than in the point-source approximation because the viewing angle (~30°) is comparable to the jet opening angle (~20°), enhancing detectability.
• The jet efficiently accelerates a significant fraction of the merger ejecta to sub-relativistic speeds of 0.3–0.4c via a jet-powered cocoon across a wide range of parameters.
• The ambient interstellar medium density is constrained to be low, ~10⁻³–10⁻⁶ cm⁻³, based on the afterglow decay behavior, which can be tested with future radio and X-ray observations.
• The analytical scaling of isotropic energy E_iso(θ_v) shows distinct regimes: constant for θ_v < Δθ, ∝ ˜δ(θ_v)² for Δθ < θ_v ≲ 2Δθ, and ∝ δ(θ_v)³ for θ_v ≳ 2Δθ, with the latter being most relevant for off-axis observers.
• The model predicts that the macronova luminosity arises from jet-driven cocoon heating, not solely from r-process decay, suggesting alternative energy sources may contribute to kilonova emission.