Non-thermal emission from mildly relativistic dynamical ejecta of neutron star mergers: spectrum and sky image

Abstract

Binary neutron star mergers are expected to produce fast dynamical ejecta, with mildly relativistic velocities extending to $\beta =v/c>0.6$. In a preceding paper, we derived an analytic description of the time-dependent radio to X-ray synchrotron flux produced by collisionless shocks driven by such fast ejecta into the interstellar medium, for spherical ejecta with broken power-law mass (or energy) distributions, $M(>\gamma \beta )\propto (\gamma \beta {)}^{-s}$ with $s=s_{\text{KN}}$ at $\gamma \beta <{\gamma }_0{\beta }_0$ and $s=s_{\text{ft}}$ at $\gamma \beta >{\gamma }_0{\beta }_0$ (where $\gamma$ is the Lorentz factor). Here, we extend our analysis and provide analytic expressions for the self-absorption frequency, the cooling frequency, and the observed angular size of the emitting region (which appears as a ring in the sky). For parameter values characteristic of merger calculation results -- a "shallow" mass distribution, $1<s_{\text{KN}}<3$, for the bulk of the ejecta (at $\gamma \beta \approx 0.2$), and a steep, $s_{\text{ft}}>5$, "fast tail" mass distribution -- the analytic results reproduce well (to tens of percent accuracy) the results of detailed numeric calculations, a significant improvement over earlier order-of-magnitude estimates (based on extrapolations of results valid for $\gamma \beta \ll 1$).

8 pages, 7 figures, accepted to MNRAS

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Cited by in corpus (5)

• Late-time non-thermal emission from mildly relativistic tidal ejecta of compact objects merger
• Synchrotron break frequencies of mildly-to-highly relativistic outflows observed off-axis
• Numerical Modeling of Relativistic Effects in Synchrotron-Emitting Shocks
• Inferring kilonova ejecta photospheric properties from early blackbody spectra
• The non-thermal emission following GW170817 is consistent with a conical radially-stratified outflow with initial Lorentz factor $\lesssim 10$