We propose to test the magnetic transition moments of Majorana neutrinos by comparing the fluxes of different flavours of neutrinos coming from active galactic nuclei (AGN). We show that, with reasonable assumptions about the magnetic field of the AGN, it is possible to obtain limits on $ u_{tau} u_{e}$ and $ u_{tau} u_{mu}$ transition moments which are three to five orders of magnitude better than the laboratory limits. We also point out that with certain parameter values the ratio $ u_{tau}/ u_{e,mu}$, when measured from different sources, is expected to vary from zero to values somewhat higher than one, providing an unambigious signal of a magnetic transition within the AGN which cannot be explained by neutrino oscillations.
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