Constraining the coherence scale of the interstellar magnetic field using TeV gamma-ray observations of supernova remnants

TL;DR

This study uses simulations and observations of supernova remnants to constrain the magnetic coherence scale in the interstellar medium, linking gamma-ray morphology to magnetic field structure and shock acceleration processes.

Contribution

It introduces a method to constrain the magnetic coherence scale using gamma-ray observations and shock morphology, advancing understanding of magnetic field influence on cosmic ray acceleration.

Findings

Gamma-ray bright regions are associated with quasi-parallel shocks.

Estimated magnetic coherence scales are approximately 13 pc for Vela Jr. and over 200 pc for SN1006.

Both hadronic and mixed models can reproduce observed spectra and morphology.

Abstract

Galactic cosmic rays (CRs) are believed to be accelerated at supernova remnant (SNR) shocks. In the hadronic scenario the TeV gamma-ray emission from SNRs originates from decaying pions that are produced in collisions of the interstellar gas and CRs. Using CR-magnetohydrodynamic simulations, we show that magnetic obliquity-dependent shock acceleration is able to reproduce the observed TeV gamma-ray morphology of SNRs such as Vela Jr. and SN1006 solely by varying the magnetic morphology. This implies that gamma-ray bright regions result from quasi-parallel shocks (i.e., when the shock propagates at a narrow angle to the upstream magnetic field), which are known to efficiently accelerate CR protons, and that gamma-ray dark regions point to quasi-perpendicular shock configurations. Comparison of the simulated gamma-ray morphology to observations allows us to constrain the magnetic coherence scale $\lambda_B$ around Vela Jr. and SN1006 to $\lambda_B \simeq 13_{-4.3}^{+13}$ pc and $\lambda_B >200_{-10}^{+80}$ pc, respectively, where the ambient magnetic field of SN1006 is consistent with being largely homogeneous. We find consistent pure hadronic and mixed hadronic-leptonic models that both reproduce the multi-frequency spectra from the radio to TeV gamma rays and match the observed gamma-ray morphology. Finally, to capture the propagation of a SNR shock in a clumpy interstellar medium, we study the interaction of a shock with a dense cloud with numerical simulations and analytics. We construct an analytical gamma-ray model for a core collapse SNR propagating through a structured interstellar medium, and show that the gamma-ray luminosity is only biased by 30% for realistic parameters.