Constraining the Geometry of the Neutron Star RX J1856.5-3754

TL;DR

This study models the magnetic atmosphere of neutron star RX J1856.5-3754, constraining its geometry through X-ray pulsation analysis, and discusses implications for its classification as a radio pulsar and future polarization observations.

Contribution

It introduces a model combining magnetic and temperature distributions to constrain the star's geometry using recent X-ray pulsation data.

Findings

Constraints on the star's magnetic and rotation axis angles.

Alignment suggests possible classification as a radio pulsar.

Potential for future polarization measurements to refine models.

Abstract

RX J1856.5-3754 is one of the brightest, nearby isolated neutron stars, and considerable observational resources have been devoted to its study. In previous work, we found that our latest models of a magnetic, hydrogen atmosphere matches well the entire spectrum, from X-rays to optical (with best-fitting neutron star radius R=14 km, gravitational redshift z_g~0.2, and magnetic field B~4x10^12 G). A remaining puzzle is the non-detection of rotational modulation of the X-ray emission, despite extensive searches. The situation changed recently with XMM-Newton observations that uncovered 7 s pulsations at the 1% level. By comparing the predictions of our model (which includes simple dipolar-like surface distributions of magnetic field and temperature) with the observed brightness variations, we are able to constrain the geometry of RX J1856.5-3754, with one angle < 6 deg and the other angle = 20-45 deg, though the solutions are not definitive given the observational and model uncertainties. These angles indicate a close alignment between the rotation and magnetic axes or between the rotation axis and the observer. We discuss our results in the context of RX J1856.5-3754 being a normal radio pulsar and a candidate for observation by future X-ray polarization missions such as Constellation-X or XEUS.