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The radiation drag in photon-rich environments of cosmic explosions can seed kinetic instabilities by inducing velocity spreads between relativistically streaming plasma components. Such microturbulence is likely imprinted on the breakout signals of radiation-mediated shocks. However, large-scale, transverse magnetic fields in the deceleration region of the shock transition can suppress the dominant kinetic instabilities by preventing the development of velocity separations between electron–positron pairs and a heavy ion species. We use a 1D five-fluid radiative transfer code to generate self-consistent profiles of the radiation drag force and plasma composition in the deceleration region. For increasing magnetization, our models predict rapidly growing pair multiplicities and a substantial radiative drag developing self-similarly throughout the deceleration region. We extract the critical magnetization parameter σc, determining the limiting magnetic field strength at which a three-species plasma can develop kinetic instabilities before reaching the isotropized downstream. For a relativistic, single ion plasma drifting with γu = 10 in the upstream of a relativistic radiation-mediated shock, we find the threshold σc ≈ 10−7 for the onset of microturbulence. Suppression of plasma instabilities in the case of multi-ion composition would likely require much higher values of σc. Identifying high-energy signatures of microturbulence in shock breakout signals and combiningmore »them with the magnetization limits provided in this work will allow a deeper understanding of the magnetic environment of cosmic explosions like supernovae, gamma-ray bursts, and neutron star binary mergers.

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A core collapse supernova occurs when exothermic fusion ceases in the core of a massive star, which is typically caused by exhaustion of nuclear fuel. Theory predicts that fusion could be interrupted earlier by merging of the star with a compact binary companion. We report a luminous radio transient, VT J121001+495647, found in the Very Large Array Sky Survey. The radio emission is consistent with supernova ejecta colliding with a dense shell of material, potentially ejected by binary interaction in the centuries before explosion. We associate the supernova with an archival x-ray transient, which implies that a relativistic jet was launched during the explosion. The combination of an early relativistic jet and late-time dense interaction is consistent with expectations for a merger-driven explosion.

Relativistic radiation-mediated shocks are likely formed in prodigious cosmic explosions. The structure and emission of such shocks are regulated by copious production of electron–positron pairs inside the shock-transition layer. It has been pointed out recently that substantial abundance of positrons inside the shock leads to a velocity separation of the different plasma constituents, which is expected to induce a rapid growth of plasma instabilities. In this paper, we study the hierarchy of plasma microinstabilities growing in an electron-ion plasma loaded with pairs and subject to a radiation force. Linear stability analysis indicates that such a system is unstable to the growth of various plasma modes which ultimately become dominated by a current filamentation instability driven by the relative drift between the ions and the pairs. These results are validated by particle-in-cell simulations that further probe the non-linear regime of the instabilities, and the pair-ion coupling in the microturbulent electromagnetic field. Based on this analysis, we derive a reduced-transport equation for the particles via pitch-angle scattering in the microturbulence and demonstrate that it can couple the different species and lead to non-adiabatic compression via a Joule-like heating. The heating of the pairs and, conceivably, the formation of non-thermal distributions, arising frommore »the microturbulence, can affect the observed shock-breakout signal in ways unaccounted for by current single-fluid models.

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Abstract We present the full panchromatic afterglow light-curve data of GW170817, including new radio data as well as archival optical and X-ray data, between 0.5 and 940 days post-merger. By compiling all archival data and reprocessing a subset of it, we have evaluated the impact of differences in data processing or flux determination methods used by different groups and attempted to mitigate these differences to provide a more uniform data set. Simple power-law fits to the uniform afterglow light curve indicate a t 0.86±0.04 rise, a t −1.92±0.12 decline, and a peak occurring at 155 ± 4 days. The afterglow is optically thin throughout its evolution, consistent with a single spectral index (−0.584 ± 0.002) across all epochs. This gives a precise and updated estimate of the electron power-law index, p = 2.168 ± 0.004. By studying the diffuse X-ray emission from the host galaxy, we place a conservative upper limit on the hot ionized interstellar medium density, <0.01 cm −3 , consistent with previous afterglow studies. Using the late-time afterglow data we rule out any long-lived neutron star remnant having a magnetic field strength between 10 10.4 and 10 16 G. Our fits to the afterglow data using anmore »analytical model that includes Very Long Baseline Interferometry proper motion from Mooley et al., and a structured jet model that ignores the proper motion, indicates that the proper-motion measurement needs to be considered when seeking an accurate estimate of the viewing angle.« less