Implications of the radio and X-ray emission that followed GW170817

Abstract

The radio and X-rays that followed GW170817 increased gradually over $\sim~100$ days, resembling the radio flare predicted to arise from the interaction of a binary neutron star merger outflow with the ISM \citep{nakar2011}. Considering a blast wave moving with a Lorentz factor $\Gamma$, we show that an off-axis observer, namely an observer at $\theta_{obs}>1/\Gamma$, sees a light curve rising faster than $F_\nu\propto t^3$. Therefore, the observed rise, $F_\nu\propto~t^{0.78}$, implies that at all times we have seen an on-axis emission. Namely, the emitting matter was within $\theta_{obs}<1/\Gamma$ at the time of observations (even if it was off-axis beforehand). The observations tightly constrain the blast wave Lorentz factor: $\Gamma\sim(1.5-7)(t/10~{\rm day})^{-0.21}$.The isotropic equivalent energy in the observed region increases with time, $E_{iso}\sim~10^{50}{\rm~erg} ~(t/150~{\rm day})^{1.3}$, implying that the outflow was structured. The energy increase can arise from a slower material moving behind the blast wave or from a matter moving at larger angles that has slowed down. Such a structure could have different origins, however, the only physically motivated one, proposed so far, is the interaction of a relativistic jet with the ejecta and the resulting cocoon. The jet could have been choked or successful. In the latter case, it has produced a short GRB pointing elsewhere (this successful jet-cocoon system is sometimes called a "structured jet"). Although circumstantial evidence disfavors a successful jet, the jet fate cannot be decisively determined from current observations. The light curve alone may not be sufficient to resolve this question, since both chocked and successful jets can lead to a gradual rise to a peak, followed by a decay. Therefore, the recent turnover of the light curve does not necessitate a successful jet.