Measuring the Intensity of the Interstellar Radiation Field with Ultra-high Energy $\gamma$ ray Spectra

TL;DR

This paper proposes using ultra-high-energy gamma-ray spectra from multiple sources to indirectly measure the spatial variations of the interstellar radiation field's intensity across the Galaxy, aiding future gamma-ray observations.

Contribution

It introduces a novel method to infer interstellar radiation field variations through gamma-ray spectral analysis, complementing direct observational data.

Findings

Gamma-ray spectra can reveal interstellar radiation field variations.

Method enables indirect mapping of Galactic radiation intensity.

Supports predictions for next-generation gamma-ray observatories.

Abstract

Our understanding of the intensity distribution of the interstellar radiation background is based on the observational data from IRAS, COBE-FIRAS and Planck. The intensity of this radiation field increases rapidly towards the Galactic plane and is the highest near the Galactic centre due to the high density of stars and dust. However, a precise determination of the variations of this radiation field with spatial and angular coordinates is not feasible observationally. We explore how future studies of gamma-ray spectra from numerous ultra-high-energy gamma-ray sources can indirectly probe variations in the interstellar radiation field's intensity across different distances from the Galactic centre and across Galactic latitudes and longitudes. This study is crucial for making self-consistent predictions of high energy gamma-ray fluxes from Galactic sources detected by observatories like LHAASO, Tibet AS$_{\gamma}$ and the next-generation gamma-ray telescopes.