Detecting and quantifying the Kelvin-Helmholtz instability in interstellar jets by radiation observable on Earth

We present particle-in-cell simulations of the relativistic Kelvin-Helmholtz instability (KHI) at unprecedented spatial resolution and size and including, for the first time, complete far field radiation spectra. Based on these simulations we demonstrate that the emitted polarized radiation observable on Earth can be used to identify and characterize the microscopic plasma dynamics of a KHI light-years away.
The KHI is expected in shear flow regions of astrophysical plasma jets, which are significant sites for particle acceleration and radiation.
We have simulated the radiation of the KHI using the particle-in-cell code PIConGPU. Its synthetic in situ radiation diagnostic is capable of calculating the angularly and spectrally radiation of billions of electrons simulated, thereby allowing quantitative predictions for both the coherent and incoherent part of the radiation. The calculations are based on Liénard-Wiechert potentials and were performed for 481 observation directions on a half dome covering frequencies over 3 orders of magnitude. Compared to any previous KHI simulation, we increased the spatial resolution by more than a factor 4 and covered a 46 times larger volume. The simulations were conducted on 18,000 GPUs of the TITAN cluster at Oak Ridge National Laboratory reaching a peak performance of 7.2 Peta FLOPs.
The simulated spectra show that the time-dependent changes in the radiation polarization and power correlate directly with the stages of the KHI, thus allowing to identify the linear growth phase of the KHI and quantifying its characteristic growth rate. In order to support these findings we introduce an analytic kinetic model that is capable of reproducing both polarization and power characteristics. We will discuss both model and simulation in detail. We also focus on the temporal evolution of the plasma dynamic and radiation signature to show why the polarization is a clear signature for the presence of the KHI.
The findings presented are a vital step towards a better understanding of astro-physical jets since observed radiation growth rates can now be linked to kinetic KHI models allowing to constrain jet properties such as jet-to-ambient density contrast and velocity gradients.