The spectral shapes of Galactic gamma-ray source

TL;DR

This paper analyzes the spectral shapes of Galactic gamma-ray sources observed by ground-based telescopes, revealing curved spectra with energy-dependent slopes and a gradually softening cumulative spectrum, which is key for understanding high-energy astrophysical processes.

Contribution

It provides a detailed characterization of the spectral shapes of Galactic gamma-ray sources and their cumulative spectrum across a broad energy range, highlighting the complexity of their spectral behavior.

Findings

Spectra of Galactic gamma-ray sources are curved with different slopes below and above ~30 TeV.

The cumulative spectrum softens gradually from a slope of ~2.2 at 1 TeV to ~3.4 at 100 TeV.

The sum of diverse source spectra results in a smooth, energy-dependent spectral shape.

Abstract

Recent observations by ground-based gamma-ray telescopes have led to the publication of catalogs listing sources observed in the TeV and PeV energy ranges. Photons of such high energy are strongly absorbed during propagation over extragalactic distances, and the catalogs are dominated by Galactic sources. Of particular interest are the observations of the LHAASO telescope, which cover a very broad energy range (from 1 to 10$^3$ TeV) and show that the spectra of all Galactic gamma-ray sources are curved, with significantly different slopes below and above $E \sim 30$ TeV. The cumulative spectrum obtained by summing the contributions of Galactic individual sources has a spectral shape that gradually softens with energy, with a slope that increases from a value of order 2.2 at $E \simeq 1$ TeV, to 2.5 at 30 TeV, and $\simeq 3.4$ at 100 TeV. It is remarkable that the smooth variation in the shape of the cumulative spectrum is obtained from the sum of contributions that have a wide range of shapes. Understanding the origin of the spectral shapes of the Galactic gamma-ray sources is a crucial challenge for high energy astrophysics.