  Dynamo processes are ubiquitous in astrophysical systems. In relativistic
astrophysical systems, such as accretion disks around black holes or neutron
stars, they may critically affect the launching of winds and jets that can
power electromagnetic emission. Dynamo processes are governed by several
microscopic parameters, one of them being magnetic helicity. As a conserved
quantity in nonresistive plasmas, magnetic helicity is transported across the
system. One important implication of helicity conservation is, that in the
absence of helicity fluxes some mean-field dynamos can be quenched, potentially
affecting the large-scale evolution of the magnetic field. One of the major
challenges in computing magnetic helicity is the need to fix a meaningful
electromagnetic gauge. We here present a fully covariant formulation of
magnetic helicity transport in general-relativistic plasmas based on the
concept of relative helicity by Berger & Field and Finn & Antonsen. This
formulation is separately invariant under gauge-transformation of the Maxwell
and Einstein equations. As an application of this new formalism we present the
first analysis of magnetic helicity transport in the merger of two neutron
stars. We demonstrate the presence of global helicity fluxes into the outer
layers of the stellar merger remnant, which may impact subsequent large-scale
dynamo amplification in these regions.
