  Magnetars are neutron stars (NSs) with extreme magnetic fields of strength $5
\times 10^{13}$ - $10^{15}$ G. They exhibit transient, highly energetic events,
such as short X-ray flashes, bursts and giant flares, all of which are powered
by their enormous magnetic energy. Quiescent magnetars have X-ray luminosities
between $10^{29}$ and $10^{35}$ erg/s, and are further classified as either
persistent or transient magnetars. Their X-ray emission is modulated with the
rotational period of the NS, with a typical relative amplitude (so-called
pulsed fraction) between 10-58 per cent, implying that the surface temperature
is significantly non-uniform despite the high thermal conductivity of the
star's crust. Here, we present the first 3D magneto-thermal MHD simulations of
magnetars with strong toroidal magnetic fields. We show that these models,
combined with ray propagation in curved space-time, accurately describe the
light-curves of most transient magnetars in quiescence and allow us to further
constrain their rotational orientation. We find that the presence of a strong
toroidal magnetic field explains the observed asymmetry in the surface
temperature, and is the main cause of the strong modulation of thermal X-ray
emission in quiescence.
