Solar neutrino flux at keV energies

Abstract

We calculate the solar neutrino and antineutrino flux in the keV energy range. The dominant thermal source processes are photoproduction ($\gamma e\to e \nu\bar\nu$), bremsstrahlung ($e+Ze\to Ze+e+\nu\bar\nu$), plasmon decay ($\gamma\to\nu\bar\nu$), and $\nu\bar\nu$ emission in free-bound and bound-bound transitions of partially ionized elements heavier than hydrogen and helium. These latter processes dominate in the energy range of a few keV and thus carry information about the solar metallicity. To calculate their rate we use libraries of monochromatic photon radiative opacities in analogy to a previous calculation of solar axion emission. Our overall flux spectrum and many details differ significantly from previous works. While this low-energy flux is not measurable with present-day technology, it could become a significant background for future direct searches for keV-mass sterile neutrino dark matter.