DIFFUSE HARD X-RAY EMISSION IN STARBURST GALAXIES AS SYNCHROTRON FROM VERY HIGH ENERGY ELECTRONS

Abstract

[Abdriged] The origin of the diffuse hard X-ray (2 - 10 keV) emission from\nstarburst galaxies is a long-standing problem. We suggest that synchrotron\nemission of 10 - 100 TeV electrons and positrons (e+/-) can contribute to this\nemission, because starbursts have strong magnetic fields. We consider three\nsources of e+/- at these energies: (1) primary electrons directly accelerated\nby supernova remnants; (2) pionic secondary e+/- created by inelastic\ncollisions between CR protons and gas nuclei in the dense ISMs of starbursts;\n(3) pair e+/- produced between the interactions between 10 - 100 TeV gamma-rays\nand the intense far-infrared (FIR) radiation fields of starbursts. We create\none-zone steady-state models of the CR population in the Galactic Center (R <=\n112 pc), NGC 253, M82, and Arp 220's nuclei, assuming a power law injection\nspectrum for electrons and protons. We compare these models to extant radio and\nGeV and TeV gamma-ray data for these starbursts, and calculate the diffuse\nsynchrotron X-ray and Inverse Compton (IC) luminosities of these starbursts. If\nthe primary electron spectrum extends to ~PeV energies and has a\nproton/electron injection ratio similar to the Galactic value, we find that\nsynchrotron contributes 2 - 20% of their unresolved, diffuse hard X-ray\nemission. Inverse Compton emission is likewise a minority of the unresolved\nX-ray emission in these starbursts, from 0.1% in the Galactic Center to 10% in\nArp 220's nuclei. We also model generic starbursts, including submillimeter\ngalaxies, in the context of the FIR--X-ray relation, finding that up to 2% in\nthe densest starbursts with our fiducial assumptions. Neutrino and TeV\ngamma-ray data can further constrain the synchrotron X-ray emission of\nstarbursts. Our models do not constrain hard synchrotron X-ray emission from\nany additional hard components of primary e+/- from sources like pulsars in\nstarbursts.\n