# X-ray Lightcurves from Realistic Polar Cap Models: Inclined Pulsar Magnetospheres and Multipole Fields

## TL;DR

This paper models realistic thermal X-ray lightcurves from pulsar polar caps considering complex magnetic field geometries, including quadrupole components, to better understand observed pulsar emission features.

## Contribution

It introduces advanced polar cap temperature distributions incorporating multipole magnetic fields, improving upon simple models and explaining complex lightcurve features.

## Key findings

- Realistic models show 5-10% differences in lightcurves for millisecond pulsars.
- Quadrupole magnetic components create asymmetries and complex hotspot shapes.
- Models can qualitatively reproduce features of PSR J0437-4715 lightcurve.

## Abstract

Thermal X-ray emission from rotation-powered pulsars is believed to originate from localized "hotspots" on the stellar surface occurring where large-scale currents from the magnetosphere return to heat the atmosphere. Lightcurve modeling has primarily been limited to simple models, such as circular antipodal emitting regions with constant temperature. We calculate more realistic temperature distributions within the polar caps, taking advantage of recent advances in magnetospheric theory, and we consider their effect on the predicted lightcurves. The emitting regions are non-circular even for a pure dipole magnetic field, and the inclusion of an aligned magnetic quadrupole moment introduces a north-south asymmetry. As the aligned quadrupole moment is increased, one hotspot grows in size before becoming a thin ring surrounding the star. For the pure dipole case, moving to the more realistic model changes the lightcurves by $5-10\%$ for millisecond pulsars, helping to quantify the systematic uncertainty present in current dipolar models. Including the quadrupole gives considerable freedom in generating more complex lightcurves. We explore whether these simple dipole+quadrupole models can account for the qualitative features of the lightcurve of PSR J0437$-$4715.