Electromagnetic counterparts to structured jets from gravitational wave detected mergers

Abstract

We show the peak magnitude for orphan afterglows from the jets of\ngravitational wave (GW) detected black-hole/neutron star - neutron star\n(BH/NS-NS) mergers highly depends on the jet half-opening angle $\\theta_j$.\nShort $\\gamma$-ray bursts (GRB) with a homogeneous jet structure and\n$\\theta_j>10^\\circ$, the orphan afterglow viewed at the typical inclination for\na GW detected event, 38$^\\circ$, is brighter at optical frequencies than the\ncomparable macronova emission. Structured jets, where the energetics and\nLorentz factor $\\Gamma$ vary with angle from the central axis, may have\nlow-$\\Gamma$ components where the prompt emission is suppressed; GW\nelectromagnetic (EM) counterparts may reveal a population of failed-GRB orphan\nafterglows. Using a Monte Carlo method assuming a NS-NS detection limit we show\nthe fraction of GW-EM counterparts from homogeneous, two-component, power-law\nstructured, and Gaussian jets where the variable structure models include a\nwide low energy and $\\Gamma$ component: for homogeneous jets, with a\n{$\\theta_j=6^\\circ$ and typical short GRB parameters, we find {\\it r}-band\nmagnitude $m_r\\leq21$ counterparts for $\\sim 13.6\\%$ of GW detected mergers;\nwhere jet structure extends to a half-opening angle of $25^\\circ$,\ntwo-component jets produce $m_r\\leq21$ counterparts in $\\sim30\\%$ of GW\ndetected mergers; power-law structured jets result in $\\sim37\\%$; and Gaussian\njets with our parameters $\\sim13\\%$.} We show the features in the lightcurves\nfrom orphan afterglows can be used to indicate the presence of extended\nstructure.\n