# A Test of the Neutron Star Hypothesis for Fomalhaut b

## TL;DR

This study tests if Fomalhaut b could be a neutron star by analyzing X-ray observations, finding no X-ray emission and thus constraining its properties, while suggesting an alternative planetary explanation with a ring system.

## Contribution

The paper provides the first X-ray observational constraints on Fomalhaut b, challenging the neutron star hypothesis and proposing a planetary model with a ring system.

## Key findings

- No X-ray emission detected from Fomalhaut b or Fomalhaut star.
- Constraints on neutron star surface temperature and distance.
- Reflected starlight explains optical detections, supporting a planetary model.

## Abstract

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.