The diffusion coefficient in the Large Magellanic Cloud

TL;DR

We address the problem of measuring cosmic-ray diffusion in the Large Magellanic Cloud by developing an end-to-end numerical model that propagates CR electrons and computes their diffuse synchrotron emission. Using DRAGON and HERMES, and a source population traced to H II regions, the morphology of low-frequency radio emission constrains the diffusion coefficient to $D_0 = 4.22^{+0.21}_{-0.10} \times 10^{28}$ cm$^2$/s at 1 GeV. The results indicate diffusion properties in the LMC are similar to the Milky Way and demonstrate a robust, morphology-driven method that is applicable to other galaxies and to indirect dark matter searches. This framework thus provides a scalable tool for interpreting non-thermal signals in nearby galaxies and for deriving robust constraints on CR transport and potential DM signals from radiative emission.

Abstract

The Large Magellanic Cloud (LMC) is the largest satellite galaxy of the Milky Way and provides a unique laboratory for high-energy astrophysics and dark matter studies. In this work, we develop an end-to-end numerical description of cosmic-ray transport and the associated non-thermal emission in the LMC, extending the public DRAGON and HERMES codes. Within this framework, we compute the diffuse synchrotron radiation produced by cosmic-ray electrons in the LMC and compare our predictions with observed low-frequency radio maps. Because electron diffusion imprints a characteristic morphology on the radio emission, this comparison allows us to infer the effective average diffusion coefficient in the LMC. We find a diffusion coefficient D0 = (3-6) $\times 10^{28} \; \rm{cm}^2 \; \rm{s}^{-1}$ at 1 GeV, comparable to but slightly larger than values typically inferred for the Milky Way. More generally, this work provides a scalable tool for interpreting non-thermal signals in nearby galaxies and constraining their cosmic-ray transport properties.

Figures (14)

• Figure 1: MWA radio map at 88 MHz 2018MNRAS.480.2743F, expressed in brightness temperature (in K), along with the associated segmentation mask. The green dots correspond to the $H_{\alpha}$ regions from Ref. 2012ApJ...755...40P.
• Figure 2: Synchrotron emission at 88 MHz, expressed in brightness temperature (in K), for the best-fit model of the joint analysis. The white dots correspond to the $H_{\alpha}$ regions from Ref. 2012ApJ...755...40P.
• Figure 3: Posterior distributions of $D_0$ for the analyses of the individual observational maps and the combined analysis.
• Figure 4: Posterior distributions of $L$ for the analyses of the individual observational maps and the combined analysis.
• Figure 5: Contours at 68% and 95% credible level