Spatial and Spectral Modeling of the Gamma-ray Distribution in the Large Magellanic Cloud

TL;DR

This study analyzes the gamma-ray emission of the Large Magellanic Cloud using spatial and spectral modeling, revealing insights into cosmic-ray distribution, energy loss, and magnetic field strength within the galaxy.

Contribution

It introduces a comprehensive spatial and spectral analysis of LMC gamma-ray data, incorporating multi-wavelength observations and robust statistical methods, providing new insights into cosmic-ray physics.

Findings

Gamma-ray flux of (1.38 ± 0.02) × 10^{-7} ph cm^{-2} s^{-1} from 200 MeV to 20 GeV

Cosmic-ray spectral index of 2.4 ± 0.2

Average magnetic field strength estimated at ~3 μG

Abstract

We perform spatial and spectral analyses of the Large Magellanic Cloud (LMC) gamma-ray emission collected over 66 months by the Fermi Gamma-ray Space Telescope. In our spatial analysis, we model the LMC cosmic-ray distribution and gamma-ray production using observed maps of the LMC interstellar medium, star formation history, interstellar radiation field, and synchrotron emission. We use bootstrapping of the data to quantify the robustness of spatial model performance. We model the LMC gamma-ray spectrum using fitting functions derived from the physics of $\pi^0$ decay, bremsstrahlung, and inverse Compton scattering. We find the integrated gamma-ray flux of the LMC from 200 MeV to 20 GeV to be $1.38 \pm 0.02 \times 10^{-7}$ ph cm$^{-2}$ s$^{-1}$, of which we attribute about 10% to inverse Compton scattering and 40% to bremsstrahlung. From our work, we conclude that the spectral index of the LMC cosmic-ray proton population is 2.4$\pm$0.2, and we find that cosmic-ray energy loss through gamma-ray production is concentrated within a few hundred pc of acceleration sites. Assuming cosmic-ray energy equipartition with magnetic fields, we estimate LMC cosmic rays encounter an average magnetic field strength ~3 $\mu$G.