[Paper Review] Particle acceleration and multimessenger emission from starburst-driven galactic winds

Enrico Peretti, Giovanni Morlino+2 more|arXiv (Cornell University)|Apr 22, 2021

Astrophysics and Cosmic PhenomenaPhysics and Astronomy110 references33 citations

TL;DR

This paper investigates particle acceleration at the termination shock of starburst-driven galactic winds, showing that such shocks can produce cosmic rays up to ~300 PeV. It predicts detectable diffuse gamma-ray and neutrino emission from wind bubbles, with the latter dominating above 300 TeV and matching IceCube data, while the cosmic-ray flux from these sources is consistent with observations and may contribute to the extragalactic cosmic-ray flux near 10^17 eV.

ABSTRACT

The enhanced star forming activity, typical of starburst galaxies, powers strong galactic winds expanding on kiloparsec (kpc) scales and characterized by bubble structures. Here we discuss the possibility that particle acceleration may take place at the termination shock of such winds. We calculate the spectrum of accelerated particles and their maximum energy, that turns out to range up to a few hundred petaelectronvolt (PeV) for typical values of the parameters. Cosmic rays accelerated at the termination shock are advected towards the edge of the bubble excavated by the wind and eventually escape into extragalactic space. We also calculate the flux of gamma rays and neutrinos produced by hadronic interactions in the bubble as well as the diffuse flux resulting from the superposition of the contribution of starburst galaxies on cosmological scales. Finally, we compute the diffuse flux of cosmic rays from starburst bubbles and compare it with existing data.

Motivation & Objective

• To investigate the role of starburst-driven galactic wind termination shocks as sites for particle acceleration.
• To model the resulting multimessenger emission (gamma rays and neutrinos) from hadronic interactions in the wind bubble.
• To estimate the diffuse flux of cosmic rays escaping from starburst wind bubbles and compare it with observational data.
• To assess the detectability of such emission with current and upcoming observatories, particularly in the VHE gamma-ray and neutrino domains.

Proposed method

• Uses a semi-analytic model of diffusive shock acceleration (DSA) at the wind termination shock, incorporating shock parameters and plasma conditions.
• Applies a transport model that includes diffusion, advection, adiabatic energy losses, and catastrophic losses for high-energy particles in the wind bubble.
• Calculates gamma-ray and neutrino fluxes from pp and pγ interactions in the wind bubble, accounting for target density and magnetic fields.
• Estimates the diffuse flux of cosmic rays from the superposition of all starburst galaxies on cosmological scales.
• Compares theoretical predictions with Fermi-LAT, IceCube, and cosmic-ray observatory data (IceTop, KASCADE-GRANDE, Tunka).
• Evaluates detectability using upcoming facilities like CTA and ASTRI, focusing on morphological and spectral signatures in the VHE range.

Experimental results

Research questions

• RQ1 Can particle acceleration at the termination shock of starburst-driven galactic winds produce cosmic rays up to several hundred PeV?
• RQ2 What is the expected diffuse flux of gamma rays and neutrinos from the superposition of starburst galaxies, and how does it compare with observational constraints?
• RQ3 Is the cosmic-ray flux from starburst wind bubbles consistent with the observed extragalactic cosmic-ray spectrum near 10^17 eV?
• RQ4 Can upcoming VHE gamma-ray and neutrino observatories distinguish emission from wind bubbles versus central starburst nuclei?
• RQ5 What conditions (e.g., mass loss rate, shock strength) are required for starburst winds to produce a detectable neutrino flux?

Key findings

• The maximum energy of accelerated particles at the wind termination shock reaches up to ~300 PeV for typical parameters, with a spectrum close to E^-4 at lower energies.
• The diffuse gamma-ray flux from starburst galaxies is dominated by the central starburst nucleus below ~1 TeV, but the wind contribution becomes comparable at higher energies.
• Neutrino emission from the wind region extends up to ~300 TeV and dominates the diffuse flux above ~50 TeV, matching IceCube observations.
• The diffuse cosmic-ray flux from starburst wind bubbles is consistent with current observations and peaks at energies below a few hundred PeV.
• The model predicts that ultra-luminous starbursts or those with AGN activity may produce cosmic rays with energies exceeding a few hundred PeV.
• Detection of VHE gamma-ray emission from the wind bubble is feasible with CTA and ASTRI, especially via spectral features unabsorbed by infrared photons.