12 research outputs found
A UNIFIED TIMING AND SPECTRAL MODEL FOR THE ANOMALOUS X-RAY PULSARS XTE J1810-197 AND CXOU J164710.2-455216
Anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs) are two small
classes of X-ray sources strongly suspected to host a magnetar, i.e. an
ultra-magnetized neutron star with $B\approx 10^14-10^15 G. Many SGRs/AXPs are
known to be variable, and recently the existence of genuinely "transient"
magnetars was discovered. Here we present a comprehensive study of the pulse
profile and spectral evolution of the two transient AXPs (TAXPs) XTE J1810-197
and CXOU J164710.2-455216. Our analysis was carried out in the framework of the
twisted magnetosphere model for magnetar emission. Starting from 3D Monte Carlo
simulations of the emerging spectrum, we produced a large database of synthetic
pulse profiles which was fitted to observed lightcurves in different spectral
bands and at different epochs. This allowed us to derive the physical
parameters of the model and their evolution with time, together with the
geometry of the two sources, i.e. the inclination of the line-of-sight and of
the magnetic axis with respect to the rotation axis. We then fitted the
(phase-averaged) spectra of the two TAXPs at different epochs using a model
similar to that used to calculate the pulse profiles ntzang in XSPEC) freezing
all parameters to the values obtained from the timing analysis, and leaving
only the normalization free to vary. This provided acceptable fits to
XMM-Newton data in all the observations we analyzed. 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.Comment: 15 pages, 12 figures, accepted for publication in Ap