  Fomalhaut b is a directly imaged object in the debris disk of the star
Fomalhaut. It has been hypothesized to be a planet, however there are issues
with the observed colours of the object that do not fit planetary models. An
alternative hypothesis is that the object is a neutron star in the near fore-
or background of Fomalhaut's disk. We test if Fomalhaut b could be a neutron
star using X-ray observations with Chandra's HRC-I instrument in the energy
range of 0.08-10 keV. We do not detect X-ray emission from either Fomalhaut b
or the star Fomalhaut itself. Our nondetection corresponds to an upper limit on
the X-ray flux of Fomalhaut b of F_X < 1.3e-14 erg/cm/s^2 in the energy range
0.08-10 keV. For the A-type central star Fomalhaut, we derive an X-ray upper
limit of L_X < 2e25 erg/s in the energy range 0.08-10 keV. Fomalhaut b's X-ray
non-detection constrains the parameter space for a possible neutron star
significantly, implying surface temperatures lower than 91000 K and distances
closer than 13.3 pc to the solar system. In addition we find that reflected
starlight from the central star fits the available optical detections of
Fomalhaut b; a smaller planet with a large ring system might explain such a
scenario.
