The angular structure of the GW170817 jet from prompt emission alone

Abstract

We determine the angular structure of the GW170817 jet by the prompt emission alone, without afterglow fitting, circumburst density or microphysical parameters. We assume that GRB 090510 and GW170817 have outflows of the same kind, observed respectively on-axis and at the interferometric viewing angle of $20^\circ$. We support this assumption with independent gravitational-wave data showing compatible binary masses and radiated energies. We show that for an observer whose beaming cone is filled with outflow, the point-source Doppler scalings do not apply: $E_{\rm iso}=4\pi\epsilon(\theta_v)$, with $\epsilon$ the energy radiated per unit solid angle along the line of sight, while the peak energy follows $E_{\rm p,i}\propto\Gamma(\theta_v)$. We obtain $n={\rm d}\ln\epsilon/{\rm d}\ln\Gamma = 3.76\pm0.29$ from the ratio of the two bursts, with no free parameter and no assumed angle. This excludes four structures in common use at $4.7\sigma$ to $13\sigma$; three remain above $4\sigma$ across the full reported range of the peak energy of the GW170817 jet. Two prompt spectra fix no angular scale; supplying it with the core Lorentz factor of GRB 090510 and the viewing angle gives $\epsilon\propto\theta^{-7.4}$ outside a core of $2^\circ$--$5^\circ$, in agreement with the width inferred from $367$ short bursts, the outflow remaining relativistic at $\Gamma=33$ on the line of sight. The exponent exceeds what the Lorentz boost of a uniform comoving flow can produce, so the structure is intrinsic to the outflow and not a consequence of the boost. The same structure fixes the emission radius, which contributes $0.41$~s of the $1.74$~s delay between the gravitational-wave signal and the gamma-rays, the remainder being the launch and breakout of the jet, with no free parameters. We conclude that the faintness lies in the structure of the GW170817 jet, not in the de-beaming of a bright core.