  Axion helioscopes search for solar axions by their conversion in x-rays in
the presence of high magnetic fields. The use of low background x-ray detectors
is an essential component contributing to the sensitivity of these searches. In
this work, we review the recent advances on Micromegas detectors used in the
CERN Axion Solar Telescope (CAST) and proposed for the future International
Axion Observatory (IAXO). The actual setup in CAST has achieved background
levels below 10$^{-6}$ keV$^{-1}$ cm$^{-2}$ s$^{-1}$, a factor 100 lower than
the first generation of Micromegas detectors. This reduction is based on active
and passive shielding techniques, the selection of radiopure materials, offline
discrimination techniques and the high granularity of the readout. We describe
in detail the background model of the detector, based on its operation at CAST
site and at the Canfranc Underground Laboratory (LSC), as well as on Geant4
simulations. The best levels currently achieved at LSC are low than 10$^{-7}$
keV$^{-1}$ cm$^{-2}$ s$^{-1}$ and show good prospects for the application of
this technology in IAXO. Finally, we present some ideas and results for
reducing the energy threshold of these detectors below 1 keV, using
high-transparent windows, autotrigger electronics and studying the cluster
shape at different energies. As a high flux of axion-like-particles is expected
in this energy range, a sub-keV threshold detector could enlarge the physics
case of axion helioscopes.
