Late-time non-thermal emission from mildly relativistic tidal ejecta of compact objects merger

Gilad Sadeh

doi:10.1093/mnras/stae2561

paper

Late-time non-thermal emission from mildly relativistic tidal ejecta of compact objects merger

arXiv:2406.01338 · doi:10.1093/mnras/stae2561

Abstract

Mergers of compact objects (binary neutron stars, BNS, or neutron star-black hole, NSBH) with a substantial mass ratio ( $q > 1.5$ ) are expected to produce a mildly relativistic ejecta within $\sim 2 0^{\circ}$ from the equatorial plane. We present a semi-analytic approach to calculate the expected synchrotron emission observed from various viewing angles, along with the corresponding radio maps, that are produced by a collisionless shock driven by such ejecta into the interstellar medium. This method reproduces well (up to $\sim 30%$ deviations) the observed emission produced by 2D numerical calculations of the full relativistic hydrodynamics. We consider a toroidal ejecta with an opening angle of $1 5^{\circ} \leq θ_{open} \leq 3 0^{\circ}$ and broken power-law mass distribution, $M (> γ β) \propto (γ β)^{- s}$ with $s = s_{KN}$ at $γ β < γ_{0} β_{0}$ and $s = s_{ft}$ at $γ β > γ_{0} β_{0}$ (where $γ$ is the Lorentz factor). The parameter values are chosen to characterize merger calculation results -- a "shallow" mass distribution, $1 < s_{KN} < 3$ , for the bulk of the ejecta (at $γ β \approx 0.2$ ), and a steep, $s_{ft} > 5$ , "fast tail" mass distribution. While the peak flux is dimmer by a factor of $\sim$ 2-3, and the peak time remains roughly the same (within $20%$ ), for various viewing angles compared to isotropic equivalent ejecta ( $θ_{open} = 9 0^{\circ}$ ) considered in preceding papers, the radio maps are significantly different from the spherical case. The semi-analytic method can provide information on the ejecta geometry and viewing angle from future radio map observations and, consequently, constrain the ejection mechanism. For NSBH mergers with a significant mass ejection ( $\sim 0.1 M_{⊙}$ ), this late non-thermal signal can be observed to distances of $≲ 200$ Mpc for typical parameter values.

arXiv comment11 pages, 9 figures. Minor revisions following referee's report, accepted to MNRAS

Late-time non-thermal emission from mildly relativistic tidal ejecta of compact objects merger

Abstract

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