High-energy neutrinos from magnetars

Magnetars can accelerate cosmic rays to high energies through the unipolar effect and are also copious soft-photon emitters. We show that young, rapidly rotating magnetars whose spin and magnetic moment point in opposite directions emit high-energy neutrinos from their polar caps through photomeson interactions. We identify a neutrino cutoff band in the magnetar period-magnetic field strength phase diagram, corresponding to the photomeson interaction threshold. Within uncertainties, we point out four possible neutrino-emission candidates among the currently known magnetars, the brightest of which may be detectable for a chance on-beam alignment. Young magnetars in the universe would also contribute to a weak diffuse neutrino background, whose detectability is marginal, depending on the typical neutrino energy.