Hadronic versus Leptonic Origin of Gamma-Ray Emission from Supernova Remnants

GeV and TeV emission from the forward shocks of supernova remnants (SNRs) indicates that they are capable particle accelerators, making them promising sources of Galactic cosmic rays (CRs). However, it remains uncertain whether this γ-ray emission arises primarily from the decay of neutral pions produced by very-high-energy hadrons, or from inverse-Compton and/or bremsstrahlung emission from relativistic leptons. By applying a semi-analytic approach to non-linear diffusive shock acceleration, and calculating the particle and photon spectra produced in different environments, we parameterize the relative strength of hadronic and leptonic emission. We show that even if CR acceleration is likely to occur in all SNRs, the observed photon spectra may primarily reflect the environment surrounding the SNR: the emission is expected to look hadronic unless the ambient density is particularly low (with proton number density ≲0.1 cm−3) or the photon background is enhanced with respect to average Galactic values (with radiation energy density u rad ≳ 10 eV cm−3). We introduce a hadronicity parameter to characterize how hadronic or leptonic we expect a source to look based on its environment, which can be used to guide the interpretation of current γ-ray observations and the detection of high-energy neutrinos from SNRs.

超新星遗迹（SNRs）前向激波产生的GeV和TeV辐射表明它们是有能力的粒子加速器，这使它们成为银河系宇宙射线（CRs）的有希望的来源。然而，这种γ射线辐射主要是由极高能强子产生的中性π介子衰变引起，还是由相对论性轻子的逆康普顿和/或轫致辐射产生，仍然不确定。通过对非线性扩散激波加速应用一种半解析方法，并计算在不同环境中产生的粒子和光子光谱，我们对强子和轻子辐射的相对强度进行了参数化。我们表明，即使宇宙射线加速可能在所有超新星遗迹中发生，观测到的光子光谱可能主要反映超新星遗迹周围的环境：除非环境密度特别低（质子数密度≲0.1 cm⁻³）或者光子背景相对于银河系平均值有所增强（辐射能密度u_rad ≳ 10 eV cm⁻³），否则辐射预计看起来是强子性的。我们引入一个强子性参数来表征根据其环境我们预期一个源看起来是强子性还是轻子性，它可用于指导对当前γ射线观测的解释以及对来自超新星遗迹的高能中微子的探测。