3 years ago

Can an Off-axis Gamma-Ray Burst Jet in GW170817 Explain All the Electromagnetic Counterparts?.

Kunihito Ioka, Takashi Nakamura

Gravitational waves from a merger of two neutron stars 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 the future observations of radio flares and X-ray remnants.

Publisher URL: http://arxiv.org/abs/1710.05905

DOI: arXiv:1710.05905v2

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