Competitive X-Ray and Optical Cooling in the Collisionless Shocks of WR 140

The long-period, highly eccentric Wolf-Rayet star binary system WR 140 has exceptionally well-determined orbital and stellar parameters. Bright, variable X-ray emission is generated in shocks produced by the collision of the winds of the WC7pd+O5.5fc component stars. We discuss the variations in the context of the colliding-wind model using broadband spectrometry from the RXTE, Swift, and NICER observatories obtained over 20 yr and nearly 1000 observations through three consecutive 7.94 yr orbits, including three periastron passages. The X-ray luminosity varies as expected with the inverse of the stellar separation over most of the orbit; departures near periastron are produced when cooling shifts to excess optical emission in C iii λ5696 in particular. We use X-ray absorption to estimate mass-loss rates for both stars and to constrain the system morphology. The absorption maximum coincides closely with the inferior conjunction of the WC star and provides evidence of the ion-reflection mechanism that underlies the formation of collisionless shocks governed by magnetic fields probably generated by the Weibel instability. Comparisons with K-band emission and He i λ10830 absorption show that both are correlated after periastron with the asymmetric X-ray absorption. Dust appears within a few days of periastron, suggesting formation within shocked gas near the stagnation point. The X-ray flares seen in η Car have not occurred in WR 140, suggesting the absence of large-scale wind inhomogeneities. Relatively constant soft emission revealed during the X-ray minimum is probably not from recombining plasma entrained in outflowing shocked gas.

Electron Energization Dynamics in Interaction of Self-Generated Magnetic Vortices in Upstream of Collisionless Electron/Ion Shocks

Relativistic collisionless shocks are considered responsible for particle energization mechanisms leading to particle acceleration. While electron energization in shock front region of electron/ion collisionless shocks are the most commonly studied, the mechanism of electron energization in interaction with self-generated magnetic vortices (MVs) in upstream region is still unclear. We investigate electron energization mechanism in upstream region of electron/ion relativistic collisionless shocks, using two dimensional particle-in-cell (PIC) simulations. We discuss mechanism of electron energization which takes place in upstream region of the shock, where the counter stream particles interact with incoming flow. The energy gain of electrons happens during their interaction with evolving fields of self-generated magnetic vortices in this region. Three Fermi-like electron energization scenarios are discussed. Stochastic acceleration of electrons in interaction with fields of MV leads to anisotropic heating of fast electrons due to diffusion in the momentum space of electrons and, finally, synergetic effect of evolving fields of MVs leads to the formation of a power-law tail of supra-thermal particles.

Downstream Super-magnetosonic Plasma Jet Generation as a Direct Consequence of Shock Reformation

Earth's bow shock, resulting from the interaction of the super-magnetosonic solar wind and Earth's magnetic field, has been studied for over 50 years and serves as an ideal astrophysical laboratory to study collisionless shocks. The Earth's bow shock offers a unique opportunity to study it through in-situ measurements. Shocks are one of nature's most powerful particle accelerators and have been connected to relativistic electron acceleration and cosmic rays. Upstream shock observations include wave generation, wave-particle interactions and SLAMS, while at the shock and downstream, particle acceleration, magnetic reconnection and plasma jets can be observed. Here, using Magnetospheric Multiscale (MMS) we show the first in-situ evidence of super-magnetosonic downstream flows (jets) generated at the Earth’s bow shock as a direct consequence of shock reformation. Jets are observed downstream due to a combined effect of upstream plasma wave evolution and an ongoing reformation cycle of the bow shock. This generation process can also be applicable to planetary and astrophysical plasmas where collisionless shocks are commonly found.

Phase space transport in the interaction between shocks and plasma turbulence

The interaction of collisionless shocks with fully developed plasma turbulence is numerically investigated. Hybrid kinetic simulations, where a turbulent jet is slammed against an oblique shock, are employed to address the role of upstream turbulence on plasma transport. A technique, using coarse graining of the Vlasov equation, is proposed, showing that the particle transport strongly depends on upstream turbulence properties, such as strength and coherency. These results might be relevant for the understanding of acceleration and heating processes in space plasmas.