Abstract
We show the peak magnitude for orphan afterglows from the jets of gravitational wave (GW) detected black-hole/neutron star - neutron star (BH/NS-NS) mergers highly depends on the jet half-opening angle $\theta_j$. Short $\gamma$-ray bursts (GRB) with a homogeneous jet structure and $\theta_j>10^\circ$, the orphan afterglow viewed at the typical inclination for a GW detected event, 38$^\circ$, is brighter at optical frequencies than the comparable macronova emission. Structured jets, where the energetics and Lorentz factor $\Gamma$ vary with angle from the central axis, may have low-$\Gamma$ components where the prompt emission is suppressed; GW electromagnetic (EM) counterparts may reveal a population of failed-GRB orphan afterglows. Using a Monte Carlo method assuming a NS-NS detection limit we show the fraction of GW-EM counterparts from homogeneous, two-component, power-law structured, and Gaussian jets where the variable structure models include a wide low energy and $\Gamma$ component: for homogeneous jets, with a {$\theta_j=6^\circ$ and typical short GRB parameters, we find {\it r}-band magnitude $m_r\leq21$ counterparts for $\sim 13.6\%$ of GW detected mergers; where jet structure extends to a half-opening angle of $25^\circ$, two-component jets produce $m_r\leq21$ counterparts in $\sim30\%$ of GW detected mergers; power-law structured jets result in $\sim37\%$; and Gaussian jets with our parameters $\sim13\%$.} We show the features in the lightcurves from orphan afterglows can be used to indicate the presence of extended structure.
Highlights
The merger of binary neutron star (NS) systems or black hole (BH) neutron star systems is thought to be the progenitors of short gammaray bursts (GRB) (Narayan, Paczynski & Piran 1992; Mochkovitch et al 1993; Bogomazov, Lipunov & Tutukov 2007; Nakar 2007; Berger 2014)
For jets from compact-stellar-mergers with a homogeneous structure, we have shown that wide opening angles θj 10◦ result in optical orphan afterglows, when viewed at the average gravitational waves (GW) detected merger inclination of ∼38◦, that are brighter than the estimates for the equivalent peak flux from macronovae; note that this depends on the ambient density and jet energetics
We show that where jets have an extended structure to a limit of θ j = 25◦, similar to the limit predicted by numerical simulations, the fraction of EM counterparts brighter than magnitude 21 can be 2–3 times that from a narrower homogeneous jet population
Summary
The merger of binary neutron star (NS) systems or black hole (BH) neutron star systems is thought to be the progenitors of short gammaray bursts (GRB) (Narayan, Paczynski & Piran 1992; Mochkovitch et al 1993; Bogomazov, Lipunov & Tutukov 2007; Nakar 2007; Berger 2014). Rosswog et al 2000; Ruffert & Janka 2001; Yamamoto, Shibata & Taniguchi 2008; Kiuchi et al 2010; Foucart et al 2012; Deaton et al 2013; Hotokezaka et al 2013; Dietrich et al 2015; Kawaguchi et al 2016; Dietrich & Ujevic 2017; Ciolfi et al 2017) Such ejecta is more isotropic in the case of an NS–NS merger and highly anisotropic for BH–NS mergers (Kyutoku et al 2015). The jet will break out of the merger ejecta and continue to propagate into the ambient medium where the collimating pressure from the cocoon is lost This transition can result in the jet becoming structured, i.e. the energy and bulk Lorentz factor vary across the jet crosssection
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