Absorption of Very High Energy Gamma Rays in the Milky Way (1608.01587v2)

Abstract: Galactic Gamma ray astronomy at very high energy (E > 30 TeV) is a vital tool in the study of the non-thermal universe. The interpretation of the observations in this energy region requires the precise modeling of the attenuation of photons due to pair production interactions, where the targets are the radiation fields present in interstellar space. For gamma rays with energy E > 300 TeV the attenuation is mostly due to the photons of the Cosmic Microwave Background Radiation (CMBR). At lower energy the most important target are infrared photons with wavelengths in the range 50-500 micron emitted by dust. The evaluation of the attenuation requires a good knowledge of the density, and energy and angular distributions of the target photons for all positions in the Galaxy. In this work we discuss a simple model for the infrared radiation that depends on only few parameters associated to the space and temperature distributions of the emitting dust. The model allows to compute with good accuracy the effects of absorption for any space and energy distribution of the diffuse Galactic gamma ray emission. The absorption probability due to the Galactic infrared radiation is maximum for E around 150 TeV, and can be as large as P_abs around 0.45 for distant sources on lines of sight that pass close to the Galactic Center. The systematic uncertainties on the absorption probability are estimated as Delta P_abs < 0.08.

• The paper introduces a robust model of gamma-ray absorption where pair production leads to an absorption probability of ~0.45 at energies near 150 TeV.
• The study employs interstellar dust temperature and distribution data, validated against COBE-FIRAS and DIRBE, to compute energy-dependent absorption effects.
• The findings emphasize the need to account for gamma-ray absorption in VHE observations, guiding current and future surveys like CTA and LHAASO.

Overview of Gamma Ray Absorption in the Milky Way

The paper "Absorption of Very High Energy Gamma Rays in the Milky Way" by Vernetto and Lipari addresses the attenuation of gamma rays with energies exceeding 30 TeV as they traverse the dense and complex radiation fields within our Galaxy. The research provides a rigorous modeling of the gamma-ray absorption process, a crucial step for accurate interpretations of observational data in very high energy (VHE) astrophysics.

Key Findings and Contributions

The paper meticulously models the absorption due to pair production interactions ($\gamma \gamma \to e^+ e^-$) where cosmic gamma rays interact with radiation fields dominated by the cosmic microwave background radiation (CMBR) at energies around 300 TeV and interstellar infrared photons at lower energies. By proposing a simplified yet robust model for the infrared radiation originating from dust in the Galaxy, the authors derive an absorption probability that is most significant for gamma rays with energies near 150 TeV.

1. Numerical Results: The analysis shows that the absorption probability can reach approximately 0.45 for sources located in remote regions of the Galactic plane where sightlines approach the Galactic center. Systematic uncertainties in this probability are estimated at $\Delta P_{\rm abs} \lesssim 0.08$, providing a relatively solid foundation for interpreting gamma-ray data from Galactic sources.
2. Model Innovation: The authors present a model based on the temperature and distribution of interstellar dust, enabling efficient computation of absorption effects across various spatial and energy distributions. This model aligns well with observational data from COBE-FIRAS and DIRBE, verifying its accuracy in characterizing the infrared emission's main spectral features.
3. Practical Implications: The paper underscores the importance of considering absorption effects when interpreting VHE gamma-ray observations, particularly those above 30 TeV. This is vital for advancing our understanding of energetic processes in the nonthermal universe, including potential sources such as astrophysical neutrinos and dark matter signatures.

Comparisons and Implications

Vernetto and Lipari compare their model's absorption estimates with prior works, such as those by Moskalenko and colleagues, noting some discrepancies that affirm the need for precise modeling of the Galactic dust emission. The findings serve to complement gamma-ray observations pursued by ground-based telescopes like CTA and LHAASO, as these facilities expand the observable energy range beyond 100 TeV. As such, the research has implications for present and future high-energy cosmic surveys, helping to refine source detection and elucidate cosmic ray propagation dynamics.

Future Directions

The insights provided by this paper have significant implications for future observational programs in gamma-ray astrophysics. Continued advancements in telescope sensitivity and array configurations could refine the absorption model further, particularly through more precise mapping of the Galaxy's infrared emission. Additionally, comprehensive integration with neutrino observatories like IceCube may offer synergistic benefits, enhancing our grasp of the Milky Way's high-energy processes.

By constructing a clear framework for understanding gamma-ray absorption, this research supports ongoing efforts to decode the complex radiation interactions shaping the very high-energy universe, facilitating explorations of phenomena that push the boundaries of astrophysics.