Cosmic Ray Acceleration by a Versatile Family of Galactic Wind Termination Shocks

Abstract

There are two distinct breaks in the cosmic ray (CR) spectrum: the so-called "knee" around 3 × 10 15 eV and the so-called "ankle" around 10 18 eV. Diffusive shock acceleration (DSA) at supernova remnant (SNR) shock fronts is thought to accelerate galactic CRs to energies below the knee, while an extragalactic origin is presumed for CRs with energies beyond the ankle. CRs with energies between 3 × 10 15 and 10 18 eV, which we dub the "shin," have an unknown origin. It has been proposed that DSA at galactic wind termination shocks, rather than at SNR shocks, may accelerate CRs to these energies. This paper uses the galactic wind model of Bustard et al. (2016) to analyze whether galactic wind termination shocks may accelerate CRs to shin energies within a reasonable acceleration time and whether such CRs can subsequently diffuse back to the galaxy. We argue for acceleration times on the order of 100 Myrs rather than a few billion years, as assumed in some previous works, and we discuss prospects for magnetic field amplification at the shock front. Ultimately, we generously assume that the magnetic field is amplified to equipartition. This formalism allows us to obtain analytic formulae, applicable to any wind model, for CR acceleration. Even with generous assumptions, we find that very high wind velocities are required to set up the necessary conditions for acceleration beyond 10 17 eV. We also estimate the luminosities of CRs accelerated by outflow termination shocks, including estimates for the Milky Way wind.

Cooler and smoother – the impact of cosmic rays on the phase structure of galactic outflows

“…The CR pressure gradients that develop over scales of kiloparsecs create an additional driving mechanism for galactic outflows. Detailed one-dimensional models find steady outflow solutions with wind velocities exceeding the escape velocity of the galaxy (Breitschwerdt et al 1991;Dorfi & Breitschwerdt 2012;Bustard et al 2016Bustard et al , 2017Mao & Ostriker 2018) and are in agreement with observational constraints (Everett et al 2010). Also three-dimensional numerical models support the picture of CR-driven outflows on galactic scales (Uhlig et al 2012;Hanasz et al 2013;Booth et al 2013;Salem & Bryan 2014;Pakmor et al 2016;Ruszkowski et al 2017;Jacob et al 2018) as well as on ISM scales (Girichidis et al 2016b;Simpson et al 2016;Farber et al 2018).…”

Cosmic rays from the nearby starburst galaxy NGC 253: the effect of a low-luminosity active galactic nucleus

“…However, Romero et al (2018) have investigated the case of NGC 253 using the new available information on the wind and its mass load obtained with the help of ALMA observatory (Bolatto et al 2013) along with radio and X-ray observations and concluded that even under the most optimistic assumptions the superwind of this starburst cannot accelerate cosmic rays beyond energies of ∼ 10 16 eV for protons and ∼ 4 × 10 17 eV for iron nuclei. These results are in accordance with independent estimates by Bustard et al (2017). Acceleration up to 100 EeV would require completely unrealistic magnetic fields outside this galaxy.…”

The Propagation of Cosmic Rays from the Galactic Wind Termination Shock: Back to the Galaxy?

“…In this work we examine whether a Galactic wind which eventually forms a termination shock far outside the Galactic plane can contribute as a possible source to the observed flux in the region of interest. Previous work by Bustard et al (2017) estimated that particles can be accelerated up to energies above the 'knee' up to R max = 10 16 eV for parameters drawn from a model of a Milky Way wind . A remaining question is whether the accelerated cosmic rays can propagate back into the Galaxy.…”