Spectral features in isolated neutron stars induced by inhomogeneous surface temperatures

TL;DR

This study investigates how inhomogeneous surface temperatures on neutron stars can cause spectral features in X-ray observations, potentially explaining observed deviations without invoking complex atmospheric models.

Contribution

It quantitatively demonstrates that surface temperature anisotropies can produce spectral features similar to observed lines, highlighting a simpler explanation for some neutron star spectra.

Findings

Inhomogeneous temperature distributions can mimic spectral lines in neutron star X-ray spectra.

Some observed spectral features can be explained by temperature anisotropies alone.

Surface temperature inhomogeneities may reduce the need for complex atmospheric models.

Abstract

The thermal X-ray spectra of several isolated neutron stars display deviations from a pure blackbody. The accurate physical interpretation of these spectral features bears profound implications for our understanding of the atmospheric composition, magnetic field strength and topology, and equation of state of dense matter. With specific details varying from source to source, common explanations for the features have ranged from atomic transitions in the magnetized atmospheres or condensed surface, to cyclotron lines generated in a hot ionized layer near the surface. Here we quantitatively evaluate the X-ray spectral distortions induced by inhomogeneous temperature distributions of the neutron star surface. To this aim, we explore several surface temperature distributions, we simulate their corresponding general relativistic X-ray spectra (assuming an isotropic, blackbody emission), and fit the latter with a single blackbody model. We find that, in some cases, the presence of a spurious 'spectral line' is required at a high significance level in order to obtain statistically acceptable fits, with central energy and equivalent width similar to the values typically observed. We also perform a fit to a specific object, RX J0806.4-4123, finding several surface temperature distributions able to model the observed spectrum. The explored effect is unlikely to work in all sources with detected lines, but in some cases it can indeed be responsible for the appearance of such lines. Our results enforce the idea that surface temperature anisotropy can be an important factor that should be considered and explored also in combination with more sophisticated emission models like atmospheres.