Modelling lightcurves and spectra of transient Anomalous X-ray Pulsars

TL;DR

This paper models the timing and spectral evolution of transient anomalous X-ray pulsars during outburst decay, using a twisted magnetosphere framework to explain surface heating, cooling, and magnetospheric untwisting.

Contribution

It provides the first joint modeling of timing and spectral data for these pulsars during outburst decay, incorporating localized surface heating and magnetospheric dynamics.

Findings

Limited surface heating near magnetic poles at outburst onset.

Evolution driven by cooling and shrinking of heated regions.

Magnetosphere gradually untwists during decay.

Abstract

We present the first detailed joint modelling of both the timing and spectral properties during the outburst decay of transient anomalous X-ray pulsars. We consider the two sources XTE J1810-197 and CXOU J164710.2-455216, and describe the source decline in the framework of a twisted magnetosphere model, using Monte Carlo simulations of magnetospheric scattering and mimicking localized heat deposition at the NS surface following the activity. Our results support a picture in which a limited portion of the star surface close to one of the magnetic poles is heated at the outburst onset. The subsequent evolution is driven both by the cooling/varying size of the heated cap and by a progressive untwisting of the magnetosphere.