Nonthermal emission in the central starburst region of M82

Context. Diffuse nonthermal (NT) emission from the central starburst (CSB) region of M82 has been measured at radio, X-ray, and γ-ray energies. Far-infrared (FIR), radio, and X-ray emission maps are mutually consistent, with the radio and X-ray emissions being spectrally similar. These observational results suggest that NT X-ray emission is likely produced in Compton scattering of radio emitting electrons off the ambient FIR field. We present results of our analysis of 16.3 years of Fermi-LAT measurements, which – combined with the newly published, improved VERITAS point-source data – constitute the deepest, most extensive currently available γ-ray dataset on M82. Aims. We aim to self-consistently model the NT radio to γ-ray spectral energy distribution (SED) of the CSB as emission by relativistic electrons and protons. Key features of our models are the use – for the first time in a broadband NT spectral study of a starburst galaxy – of diffuse X-ray and radio emission from the CSB region, which allow for an overall calibration of the electron spectrum, and the identification of the ≳50 GeV emission as pionic in origin. This enables the determination of the zero-point and slope of the proton (and secondary electron) spectrum, and meaningful estimates of the energy densities of particles and magnetic fields. Methods. We consider all relevant radiative and adiabatic processes involving relativistic and thermal electrons and protons. We use detailed descriptions of the radiation fields in the CSB region, the most important of which is the local FIR field, a graybody parametrized by the dust emission index and temperature (β, T _d).Results. Our SED modeling indicates that (i) the ≳10 GeV emission is mostly pionic, (ii) the 0.1 ≲ E _γ /GeV ≲ 10 emission is a combination of pionic and Compton-scattered interstellar light (and subdominant NT bremsstrahlung), (iii) the ≲0.1 GeV γ-ray emission is leptonic, and (iv) the radio spectrum arises from primary and secondary electron synchrotron emissions at comparable levels. The primary and secondary electron populations are described by a power-law spectrum and a curved spectrum, respectively. Averaged over the set of viable FIR graybody models, the proton spectral index and energy density are q _p ≃ 2.3 and u _p ≃ 385 eV cm⁻³ (for n _H = 200 cm⁻³), the (primary) electron and proton maximum energies are ∼30 GeV and 7 TeV, respectively, and the magnetic field is B ≃ 120 μG. The derived particle and magnetic energy densities are in approximate equipartition.