Non-thermal emission in the central starburst region of M82

Abstract: Diffuse non-thermal (NT) emission from the central starburst (CSB) of M82 has been measured at radio, X-ray and gamma-ray energies. Far-infrared (FIR), radio, and X-ray emission maps are mutually consistent, with radio and X-ray emissions spectrally similar - suggesting the latter to be Compton/FIR radiation. We present our analysis of 16.3 years of Fermi-LAT measurements, which - combined with newly-published VERITAS data - constitute the deepest, most extensive currently available gamma-ray dataset on M82. We model the NT radio to gamma-ray emission of the CSB as due to relativistic particles (CR). Key features of our models are the identification of the >50 GeV emission as pionic and the use of X-ray and radio emission to calibrate the CRe spectrum. This enables determination of the zero-point and slope of the CRp (and secondary CRe) spectrum, and meaningful estimates of the energy densities of CR and magnetic fields. We consider all relevant radiative processes involving CR, and use published detailed descriptions of the soft radiation fields in the CSB region - the most important of which is the FIR field, modeled as a graybody. Our SED modeling indicates that 1) the >10 GeV emission is mostly pionic, 2) the 0.1 < E/GeV < 10 emission is a combination of pionic and Compton/starlight (and subdominant NT bremss), 3) the <0.1 GeV gamma-ray emission is leptonic, and 4) the radio spectrum arises from primary and secondary CRe synchrotron at comparable levels: the corresponding CRe populations are described by a PL and a curved spectrum, respectively. Averaged over the FIR graybody models, the CRp spectral index and energy density are 2.3 and 385 eV/cm3 (for n(H) = 200 1/cm3), the primary-CRe and CRp maximum energies are 30 GeV and 7 TeV, and the magnetic field is B = 120 uG. The derived CR and B energy densities are in equipartition.