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

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 $\sim 20^{\circ }$ 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 $15^{\circ }\le {\theta }_{\text{open}}\le 30^{\circ }$ and broken power-law mass distribution, $M(>\gamma \beta )\propto (\gamma \beta {)}^{-s}$ with $s=s_{\mathrm{KN}}$ at $\gamma \beta <{\gamma }_0{\beta }_0$ and $s=s_{\mathrm{ft}}$ at $\gamma \beta >{\gamma }_0{\beta }_0$ (where $\gamma$ is the Lorentz factor). The parameter values are chosen to characterize merger calculation results -- a "shallow" mass distribution, $1<s_{\mathrm{KN}}<3$, for the bulk of the ejecta (at $\gamma \beta \approx 0.2$), and a steep, $s_{\mathrm{ft}}>5$, "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 (${\theta }_{\text{open}}=90^{\circ }$) 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 ($\sim 0.1M_{\odot }$), this late non-thermal signal can be observed to distances of $\lesssim 200$Mpc for typical parameter values.

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

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Cited by in corpus (3)

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