2017/06/30 by Gavin P Lamb, Shiho Kobayashi · 1 citation
Physics and Astronomy · #astro-ph.HE #gr-qc
paper · pdf · doi:10.1093/mnras/stx2345
15 pages, 5 figures, accepted by MNRAS
arxiv created 2017/09/07 · arxiv updated 2017/10/18
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 θj. Short γ-ray bursts (GRB) with a homogeneous jet structure and θj>10^∘, the orphan afterglow viewed at the typical inclination for a GW detected event, 38^∘, is brighter at optical frequencies than the comparable macronova emission. Structured jets, where the energetics and Lorentz factor Γ vary with angle from the central axis, may have low-Γ 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 Γ component: for homogeneous jets, with a θj=6^∘ and typical short GRB parameters, we find \it r-band magnitude mr≤21 counterparts for ∼ 13.6% of GW detected mergers; where jet structure extends to a half-opening angle of 25^∘, two-component jets produce mr≤21 counterparts in ∼30% of GW detected mergers; power-law structured jets result in ∼37%; and Gaussian jets with our parameters ∼13%. We show the features in the lightcurves from orphan afterglows can be used to indicate the presence of extended structure.