Unveiling the thermal and magnetic map of neutron star surfaces though their X‐ray emission: method and light‐curve analysis

Recent Chandra and XMM–Newton observations of a number of X‐ray ‘dim’ pulsating neutron stars have revealed quite unexpected features in the emission from these sources. Their soft thermal spectrum, believed to originate directly from the star surface, shows evidence for a phase‐varying absorption line at some hundred eVs. The pulse modulation is relatively large (pulsed fractions in the range ∼12–35 per cent), the pulse shape is often non‐sinusoidal, and the hard X‐ray colour appears to be anticorrelated in phase with the total emission. Moreover, the prototype of this class, RX J0720.4−3125, has been found to undergo rather sensible changes in both its spectral and timing properties over a time‐scale of a few years. All these new findings seem difficult to reconcile with the standard picture of a cooling neutron star endowed with a purely dipolar magnetic field, at least if surface emission is produced in an atmosphere on top of the crust. In this paper we explore how a dipolar+quadrupolar star‐centred field influences the properties of the observed light curves. The phase‐resolved spectrum has been evaluated accounting for both radiative transfer in a magnetized atmosphere and general relativistic ray‐bending. We computed over 78 000 light curves, varying the quadrupolar components and the viewing geometry. A comparison of the data with our model indicates that higher‐order multipoles are required to reproduce the observations.