Late-time evolution of afterglows from off-axis neutron star mergers

Gavin P Lamb,Lekshmi Resmi,Ilya Mandel

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 $F_\nu \propto t^{-\alpha}$. The steepest decline for a jet afterglow is $\alpha>3p/4$ or $> (3p+1)/4$, 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 $\Gamma_0\lesssim 2$, is $\alpha\lesssim(15p-19)/10$ or $\alpha\lesssim(15p-18)/10$, in the respective spectral regimes. If the afterglow from GW170817 fades with a maximum index $\alpha > 1.5$ then we are observing the core of an initially ultra-relativistic jet viewed off the central axis, while a decline with $\alpha\lesssim 1.4$ after $\sim 5$--10 peak times indicates that a wide-angled and initially $\Gamma_0\lesssim 2$ outflow is responsible. At twice the peak time, the two outflow models fall on opposite sides of $\alpha \approx 1$. 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.

Highlights

Binary neutron star or neutron star–black hole mergers are the likely progenitor systems for gamma-ray bursts (GRBs) with a duration 2 s, the so-called short GRBs (Eichler et al 1989; Narayan, Paczynski & Piran 1992; Mochkovitch et al 1993; Bogomazov, Lipunov & Tutukov 2007; Nakar 2007)
Gravitational wave (GW) detected mergers involving a neutron star provide a new trigger for afterglow searches free of the inclination constraints of the GRB trigger, where the GRB is highly beamed from within an ultrarelativistic jet
By including the cocoon with each jet, we show that the cocoon accompanying a jet does not contribute to the afterglow at late times, where the energy in the cocoon is less than the jet energy

Summary

Introduction

Binary neutron star or neutron star–black hole mergers are the likely progenitor systems for gamma-ray bursts (GRBs) with a duration 2 s, the so-called short GRBs (Eichler et al 1989; Narayan, Paczynski & Piran 1992; Mochkovitch et al 1993; Bogomazov, Lipunov & Tutukov 2007; Nakar 2007). The GW- detected merger of a binary neutron star system, GW170817 (Abbott et al 2017a), was associated with a weak GRB (Abbott et al 2017b; Fermi-LAT Collaboration 2017; Savchenko et al 2017), an optical afterglow over the days following the merger (e.g. Arcavi et al 2017; Chornock et al 2017; Coulter et al 2017; Covino et al 2017; Cowperthwaite et al 2017; Dıaz et al 2017; Drout et al 2017; Evans et al 2017; Kilpatrick et al 2017; Lipunov et al 2017; McCully et al 2017; Nicholl et al 2017; Pian et al.2017; Shappee et al 2017; Smartt et al 2017; Soares-Santos et al 2017; Tanvir et al 2017; Valenti et al 2017; Buckley et al 2018; Tominaga et al 2018), and from ∼10 d after the merger, a broadband afterglow that appeared first at X-ray frequencies (Haggard et al 2017; Margutti et al 2017; Troja et al 2017) and later at radio frequencies (Alexander et al 2017; Hallinan et al 2017; Kim et al 2017). The later broad-band afterglow continued to rise over ∼150 d with optical observations by the Hubble Space Telescope confirming the single power law from radio to X-ray frequencies at ∼90 d (Lyman et al 2018), continued radio brightening >100 d (Resmi et al 2018), and a turnover detected at radio and X-ray frequencies (Alexander et al 2018; Dobie et al 2018; Nynka, Ruan & Haggard 2018)

Methods

Discussion

Conclusion