The diffuse gamma-ray flux associated with sub-PeV/PeV neutrinos from starburst galaxies

TL;DR

This paper models the gamma-ray flux from starburst galaxies as a source of IceCube neutrinos, showing that synchrotron losses significantly reduce high-energy gamma-ray emission, aligning with observed backgrounds.

Contribution

It introduces a semi-analytic calculation of gamma-ray flux considering synchrotron losses, providing new insights into the gamma-ray/neutrino connection in starburst galaxies.

Findings

Synchrotron radiation reduces gamma-ray flux by about half.

Diffuse gamma-ray flux accounts for roughly 20% of observed background.

Steeper neutrino spectra lessen the impact of synchrotron losses.

Abstract

One attractive scenario for the excess of sub-PeV/PeV neutrinos recently reported by IceCube is that they are produced by cosmic rays in starburst galaxies colliding with the dense interstellar medium. These proton-proton ($pp$) collisions also produce high-energy gamma-rays, which finally contribute to the diffuse high-energy gamma-ray background. We calculate the diffuse gamma-ray flux with a semi-analytic approach and consider that the very high energy gamma-rays will be absorbed in the galaxies and converted into electron-position pairs, which then lose almost all their energy through synchrotron radiation in the strong magnetic fields in the starburst region. Since the synchrotron emission goes into energies below GeV, this synchrotron loss reduces the diffuse high-energy gamma-ray flux by a factor of about two, thus leaving more room for other sources to contribute to the gamma-ray background. For a $E_\nu^{-2}$ neutrino spectrum, we find that the diffuse gamma-ray flux contributes about 20% of the observed diffuse gamma-ray background in the 100 GeV range. However, for a steeper neutrino spectrum, this synchrotron loss effect is less important, since the energy fraction in absorbed gamma-rays becomes lower.