A self-consistent model of isolated neutron stars: the case of the X-ray pulsar RX J0720.4-3125

¹ Departament de Física Aplicada, Universitat d'Alacant, Ap. Correus 99, 03080 Alacant, Spain e-mail: Jose.A.Pons@uv.es
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK

Received: 14 June 2006
Accepted: 2 August 2006

Abstract

Context.We present a unified explanation for the observed properties of the isolated neutron star RX J0720.4-3125 by obtaining a self-consistent model that accounts simultaneously for the observed X-ray spectrum and optical excess, the pulsed fraction, the observed spectral feature around 0.3 keV, and the long-term spectral evolution.

Aims.We show that all observed properties are consistent with a normal neutron star with a proper radius of about 12 km, a temperature at the magnetic pole of about 100 eV and a magnetic field strength of  G, value inferred from the observed period decay. The high magnetic field produces a strong anisotropy in the surface temperature distribution.

Methods.Fitting archival XMM–Newton observations to X-ray thermal spectra obtained in the framework of a condensed surface model composed of iron (no gaseous atmosphere) of magnetized neutron stars.

Results.The observed variability of the effective temperature, strength of the spectral feature, and pulsed fraction are in good agreement with the predictions of our model in which the star is subject to free precession, producing changes in the angle between the magnetic field and the rotation axis of tens of degrees with a periodicity of 7 years, as pointed out by other authors on the basis of phenomenological models. In addition to the evidence of strong crustal-confined toroidal components, we also find strong evidence for non-dipolar components, since all spectral properties are better reproduced with models with strong quadrupolar components.