A test of the neutron star hypothesis for Fomalhaut b

Fomalhaut b is a directly imaged object in the debris disc 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 disc. 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 non-detection corresponds to an upper limit on the X-ray flux of Fomalhaut b of F_X < 1.3 × 10^-14 erg cm^-2 s^-1 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 < 2.0 × 10²⁵ erg s^-1 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 91 000 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.