  Axion helioscopes like the planned International Axion Observatory (IAXO)
search for evidence of axions and axion-like particles (ALPs) from the Sun. A
strong magnetic field is used to convert ALPs into photons via the generic
ALP-photon coupling. To observe the resulting photons, X-ray detectors with low
background and high efficiency are necessary. In addition, good energy
resolution and low energy threshold would allow for investigating the ALP
properties by studying the X-ray spectrum after its discovery. We propose to
use low temperature metallic magnetic calorimeters (MMCs). Here we present the
first detector system based on MMCs developed for IAXO and discuss the results
of the characterization. The detector consists of a two-dimensional 64-pixel
array covering an active area of 16 mm$^2$ with a fill factor of 93 %. We
achieve an average energy resolution of 6.1 eV FWHM allowing for energy
thresholds below 100 eV. This detector is the first step towards a larger 1
cm$^2$ array matching the IAXO X-ray optics. We determine the background rate
for an unshielded detector system in the energy range between 1 keV and 10 keV
to be $3.2(1) \times 10^{-4}$ keV$^{-1}$ cm$^{-2}$ s$^{-1}$ from events
acquired over 30 days. In the future, active and passive shields will
significantly reduce the background induced by cosmic muons and natural
radioactivity. Our results demonstrate that MMCs are a promising technology for
helioscopes to discover and study ALPs.
