Constrain the cosmic-ray proton-to-electron ratio

Constrain the spatially resolved ratio of cosmic-ray proton to electron number densities, K_0, in NGC 2442 using information beyond radio-continuum observations, in order to improve equipartition magnetic-field estimates, particularly in the synchrotron island region.

Background

The equipartition magnetic-field calculation depends on K_0, defined as the ratio of cosmic-ray proton and electron number densities per particle-energy interval. The authors adopt the conventional value K_0 = 100 because radio-continuum data alone do not provide an independent constraint. Spatial variations may be especially important in the island region, where cosmic-ray electrons have likely propagated away from their injection sites and may have suffered stronger energy losses than cosmic-ray protons.

A better constraint on K_0 would reduce a systematic uncertainty in the inferred magnetic-field strength and could improve interpretation of the island’s magnetic structure and cosmic-ray electron transport.

References

A spatially resolved value of $K_0$ would allow a more accurate estimate of $B_{\rm eq}$. However, $K_0$ cannot be constrained independently from the RC data alone, because this would require additional information about the CR proton population.

— Cosmic-ray electron propagation in the peculiar barred spiral galaxy NGC 2442  (2609.10270 - Wang et al., 9 Sep 2026) in Section 3.4, “Magnetic field strength”