TeV gamma-rays from photo-disintegration/de-excitation of nuclei in Westerlund 2

TL;DR

This paper proposes that TeV gamma-rays from Westerlund 2 originate from photo-disintegration and de-excitation of cosmic ray nuclei in a UV-rich environment, predicting a suppressed gamma-ray spectrum below 1 TeV.

Contribution

It introduces a novel mechanism involving photo-disintegration of nuclei in stellar environments to explain gamma-ray emissions, linking cosmic ray physics with gamma-ray observations.

Findings

Gamma-ray flux consistent with photo-disintegration model

Spectrum suppression below 1 TeV predicted

Upcoming observations by GLAST will test the model

Abstract

TeV gamma-rays can result from the photo-de-excitation of PeV cosmic ray nuclei after their parents have undergone photo-disintegration in an environment of ultraviolet photons. This process is proposed as a candidate explanation of the recently discovered HESS source at the edge of Westerlund 2. The UV background is provided by Lyman-alpha emission within the rich O and B stellar environment. The HESS flux results if there is efficient acceleration at the source of lower energy nuclei. The requirement that the Lorentz-boosted ultraviolet photons reach the Giant Dipole resonant energy (~ 20 MeV) implies a strong suppression of the gamma-ray spectrum compared to an E_\gamma^{-2} behavior at energies below about 1 TeV. This suppression is not apparent in the lowest-energy Westerlund 2 datum, but will be probed by the upcoming GLAST mission.