Dr James Matthews

Abstract:

In this chapter, we review some features of particle acceleration in astrophysical jets. We begin by describing four observational results relating to the topic, with particular emphasis on jets in active galactic nuclei and parallels between different sources. We then discuss the ways in which particles can be accelerated to high energies in magnetised plasmas, focusing mainly on shock acceleration, second-order Fermi and magnetic reconnection; in the process, we attempt to shed some light on the basic conditions that must be met by any mechanism for the various observational constraints to be satisfied. We describe the limiting factors for the maximum particle energy and briefly discuss multimessenger signals from neutrinos and ultrahigh energy cosmic rays, before describing the journey of jet plasma from jet launch to cocoon with reference to the different acceleration mechanisms. We conclude with some general comments on the future outlook.

Abstract:

Determining accurate orbits of binary stars with powerful winds is challenging. The dense outflows increase the effective photospheric radius, precluding direct observation of the Keplerian motion; instead the observables are broad lines emitted over large radii in the stellar wind. Our analysis reveals strong, systematic discrepancies between the radial velocities extracted from different spectral lines: the more extended a line's emission region, the greater the departure from the true orbital motion. To overcome these challenges, we formulate a novel semi-analytical model which encapsulates both the star's orbital motion and the propagation of the wind. The model encodes the integrated velocity field of the out-flowing gas in terms of a convolution of past motion due to the finite flow speed of the wind. We test this model on two binary systems. (1), for the extreme case $\eta$ Carinae, in which the effects are most prominent, we are able to fit the model to 10 Balmer lines from H-alpha to H-kappa concurrently with a single set of orbital parameters: time of periastron $T_{0}=2454848$ (JD), eccentricity $e=0.91$, semi-amplitude $k=69$ km/s and longitude of periastron $\omega=241^\circ$. (2) for a more typical case, the Wolf-Rayet star in RMC 140, we demonstrate that for commonly used lines, such as He II and N III/IV/V, we expect deviations between the Keplerian orbit and the predicted radial velocities. Our study indicates that corrective modelling, such as presented here, is necessary in order to identify a consistent set of orbital parameters, independent of the emission line used, especially for future high accuracy work.