Electromagnetic Counterparts to Structured Jets from Gravitational Wave Detected Mergers

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 $m_r\le 21$ 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 $m_r\le 21$ counterparts in $\sim 30\%$ of GW detected mergers; power-law structured jets result in $\sim 37\%$; and Gaussian jets with our parameters $\sim 13\%$.} We show the features in the lightcurves from orphan afterglows can be used to indicate the presence of extended structure.

15 pages, 5 figures, accepted by MNRAS

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• Observable Features of GW170817 Kilonova Afterglow
• GRB 170817A as a Refreshed Shock Afterglow viewed off-axis
• Determine the Lorentz factor and the viewing angle of GRB 170817A
• Structured, relativistic jets driven by radiation
• Orphan GRB afterglow searches with the Pan-STARRS1 COSMOS survey
• Jet Structure in the Afterglow Phase for Gamma-ray Bursts with a Precessing Jet
• Late afterglow emission statistics: a clear link between GW170817 and bright short GRBs
• Afterglows and Macronovae Associated with Nearby Low-Luminosity Short-Duration Gamma-Ray Bursts
• Binary Neutron Star (BNS) merger: What we learned from relativistic ejecta of GW/GRB~170817A