Late-time Evolution of Afterglows from Off-Axis Neutron-Star Mergers

Gavin P Lamb; Ilya Mandel; Lekshmi Resmi

doi:10.1093/mnras/sty2196

paper

Late-time Evolution of Afterglows from Off-Axis Neutron-Star Mergers

arXiv:1806.03843 · doi:10.1093/mnras/sty2196

Abstract

Gravitational-wave detected neutron star mergers provide an opportunity to investigate short gamma-ray burst (GRB) jet afterglows without the GRB trigger. Here we show that the post-peak afterglow decline can distinguish between an initially ultra-relativistic jet viewed off-axis and a mildly relativistic wide-angle outflow. Post-peak the afterglow flux will decline as $F_{ν} \propto t^{- α}$ . The steepest decline for a jet afterglow is $α > 3 p /4$ or $> (3 p + 1) /4$ , for an observation frequency below and above the cooling frequency, respectively, where $p$ is the power-law index of the electron energy distribution. The steepest decline for a mildly relativistic outflow, with initial Lorentz factor $Γ_{0} ≲ 2$ , is $α ≲ (15 p - 19) /10$ or $α ≲ (15 p - 18) /10$ , in the respective spectral regimes. If the afterglow from GW170817 fades with a maximum index $α > 1.5$ then we are observing the core of an initially ultra-relativistic jet viewed off the central axis, while a decline with $α ≲ 1.4$ after $\sim 5$ --10 peak times indicates that a wide-angled and initially $Γ_{0} ≲ 2$ outflow is responsible. At twice the peak time, the two outflow models fall on opposite sides of $α \approx 1$ . So far, two post-peak X-ray data points at 160 and 260 days suggest a decline consistent with an off-axis jet afterglow. Follow-up observations over the next 1--2 years will test this model.

arXiv comment10 pages, 5 figures, version accepted by MNRAS - discussion reduced

Late-time Evolution of Afterglows from Off-Axis Neutron-Star Mergers

Abstract

References in corpus (115)

Cited by in corpus (56)