The imprint of the crustal magnetic field on the thermal spectra and pulse profiles of isolated neutron stars

TL;DR

This study models how different magnetic field configurations in isolated neutron stars influence their thermal spectra and pulse profiles, revealing significant effects on observed properties and implications for radius measurements.

Contribution

It introduces detailed simulations of magnetic and thermal evolution in neutron stars, highlighting the impact of internal magnetic field topology on observable X-ray spectra and pulse profiles.

Findings

Toroidal magnetic fields produce single pulse profiles with high pulsed fractions.

Blackbody fits can underestimate neutron star radii, especially with strong internal toroidal fields.

Magnetic topology significantly affects the interpretation of X-ray observations.

Abstract

Isolated neutron stars (NSs) show a bewildering variety of astrophysical manifestations, presumably shaped by the magnetic field strength and topology at birth. Here, using state-of-the art calculations of the coupled magnetic and thermal evolution of NSs, we compute the thermal spectra and pulse profiles expected for a variety of initial magnetic field configurations. In particular, we contrast models with purely poloidal magnetic fields to models dominated by a strong internal toroidal component. We find that, while the former displays double peaked profiles and very low pulsed fractions, in the latter, the anisotropy in the surface temperature produced by the toroidal field often results in a single pulse profile, with pulsed fractions that can exceed the 50-60% level even for perfectly isotropic local emission. We further use our theoretical results to generate simulated "observed" spectra, and show that blackbody (BB) fits result in inferred radii that can be significantly smaller than the actual NS radius, even as low as ~ 1-2 km for old NSs with strong internal toroidal fields and a high absorption column density along their line of sight. We compute the size of the inferred BB radius for a few representative magnetic field configurations, NS ages, and magnitudes of the column density. Our theoretical results are of direct relevance to the interpretation of X-ray observations of isolated NSs, as well as to the constraints on the equation of state of dense matter through radius measurements.