Title: Probing the possibility of hotspots on the central neutron star in HESS J1731-347,
Abstract: The X-ray spectra of the neutron stars located in the centers of supernova
remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models.
These fits yield plausible neutron star sizes for the known or estimated
distances to these supernova remnants. The evidence in favor of the presence of
a pure carbon envelope at the neutron star surface is rather indirect and is
based on the assumption that the emission is generated uniformly by the entire
stellar surface. Although this assumption is supported by the absence of
pulsations, the observational upper limit on the pulsed fraction is not very
stringent. In an attempt to quantify this evidence, we investigate the
possibility that the observed spectrum of the neutron star in HESS J1731-347 is
a combination of the spectra produced in a hydrogen atmosphere of the hotspots
and of the cooler remaining part of the neutron star surface. The lack of
pulsations in this case has to be explained either by a sufficiently small
angle between the neutron star spin axis and the line of sight, or by a
sufficiently small angular distance between the hotspots and the neutron star
rotation poles. As the observed flux from a non-uniformly emitting neutron star
depends on the angular distribution of the radiation emerging from the
atmosphere, we have computed two new grids of pure carbon and pure hydrogen
atmosphere model spectra accounting for Compton scattering. Using new hydrogen
models, we have evaluated the probability of a geometry that leads to a pulsed
fraction below the observed upper limit to be about 8.2 %. Such a geometry thus
seems to be rather improbable but cannot be excluded at this stage.