Spectral and timing properties of transient anomalous x-ray pulsars

In the last years, thanks to new generation satellites (mostly Chandra and XMM-Newton), an increasing number of high resolution spectral and timing observations of X-ray emitting isolated neutron stars (INS)has become available. Several new classes of X-ray emitting INSs have been discovered, all of them radio silent or with radio properties much at variance with those of Pulsar Radio (PSRs): X-ray Dim Isolated Neutron Stars (XDINSs), Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs), Central Compact Objects (CCOs), Rotating Radio Transient (RRATs, in one case). AXPs and SGRs represent two peculiar groups of INSs, totalling 15 objects (6 SGRs and 9 AXPs plus one candidate). Although much less noumerous than PSRs, these objects have been extensively studied because of their peculiar and extreme properties. SGRs exhibit bursts and flares with intensity between $\sim 10^2$ and $\sim 10^{11}$ times their quiescent X-ray luminosity ($10^{34} - 10^{36} \ {\rm erg/s})$. In the last decade transient phenomena have been observed from few AXPs, with luminosity increases between few 10 and few 100 times the X-ray quiescent values. The many similarities in the timing and spectral properties among SGRs and AXPs led to the suggestion that they could be two different manifestations of the same phenomenon. These objects are believed to host a Magnetar, an ultra-magnetized neutron star with magnetic field $\sim 10^{14}-10^{15}$ G. In the Magnetar model emission from SGRs and AXPs comes to the expanse of the ultra strong magnetic field rather than of the rotational, gravitational or thermal energy. In Magnetars the external magnetic field can acquire a toroidal component as a consequence of the deformation of the star crust induced by the super-strong internal field. As a result the star magnetosphere twists up and the currents required to support the non-potential field may provide a large enough optical depth to resonant cyclotron scattering (RCS). As a consequence a distortion in the thermal spectra emitting by the star surface is expected, because primary photons gain energy in the repeated scattering with the magnetospheric charges. Spectra emerging from a twisted magnetosphere in presence of RCS has been investigated by several authors (Lyutikov and Gavriil, 2006, Fernandez and Thompson, 2007, Nobili Turolla and Zane, 2008) and applied to SGRs and AXPs spectra by Rea et al. 2008 and Zane et al., 2008. The NTZ model 3D montecarlo simulation is the more advanced tool to compute both lightcurves and spectra. Resulting spectra are described in terms of 5 parameters: the twist angle $\Delta \phi$, the electron bulk velocity $\beta$, the surface temperature $T$ and the two geometrical angles$\xi$ and $\chi$($\xi$ is the misalignement between spin and magnetic axis while $\chi$ is the line of sight inclination with respect to the spin axis). In this thesis the application of NTZ model to two Transient AXPs XTE J1810-197 and CXOU J164710.2-455216 is discussed. XTE J1810-197 exhibited an outburst in late 2002 - early 2003, increasing its luminosity by two orders of magnitude. After the outburst onset the source has been monitored by XMM-Newton between September 2003 and September 2007, showing a decrease in flux until the pre-outburst value has been reached. CXOU J164710.2-455216 exhibited an outburst on 21 September 2006, when it was observed at a luminosity 300 times greater than that measured just five days before. The source has been observed by XMM-Newton 6 times after the outburst onset, between September 2006 and August 2009. In this work, adapted from Albano et al., 2010 paper, a comprehensive study of the pulse profile and spectral evolution of the two TAXPs following their outbursts onsets is presented. By confronting timing data with synthetic lightcurves obtained from the twisted magnetosphere model (Nobili, Turolla and Zane, 2008), we were able to estimate how the physical parameters of the source (surface temperature and emitting area, electron energy, twist angle) evolve in time. The fits of the pulse profiles also allowed us to infer the geometry of the two systems, i.e. the angles between the magnetic and rotational axes and the line of sight. Spectral models, obtained with the parameter values derived for the timing analysis, provide acceptable fits to XMM-Newton data.