X-ray spectra from magnetar candidates – Monte Carlo simulations Nicola Parkins, Silvia Zane, Roberto Turolla and Daniele Viganò

University of Liverpool, MSSL/UCL, University of Padova, Universidad de Alicante

The most striking effect of the geometry is seen when looking at the spectra emitted at different magnetic colatitudes ( fig.4). In model 1, a globally twisted magnetic field with a self-similar twist and an evenly distributed magnetic field and current, the spectrum didn't significantly change when viewing the star from different directions. When looking at the spectra emitted from to south to the north pole the main effect is not due to the anisotropy of the current distribution, but to a Doppler shift effect . Charges particles flowing along the B=field lines have a preferential direction of motion (towards the positive pole in this case), so an observer sitting at the south pole sees a spectrum which is less comptonized (e- are flowing away from him) with respect to an observer sitting toward the north pole (when e- are flowing toward him). A similar effect is seen in model 3, which has currents concentrated toward the entire polar axis, but a more even distribution with the magnetic field (and thus current) in the region in which scattering actually take s place. In model 2, the effect is well visible, and spectra vary substantially when seen at different colatitudes, both in the thermal bump and in the high energy tail, which emission can vary by > 1 order of magnitude. The most recent observations of pulse phase spectroscopy of magnetars have shown different spectral components appearing at different phases. A more detailed investigation about the cause of this effect, in the scenario of our simulations, is currently undertaken.

Fig. 1. Topology of the magnetosphere and current distribution for model 2. The star is represented as a point source at the centre. The poloidal (top left) and toroidal (top right ) components of the magnetic field, clearly shows a concentration along the polar axis in the southern hemisphere only. Bottom panel: the correspondent current distribution. The black lines show the location , in the magnetosphere , of the surface of scattering for photons of different energies and magnetospheric electrons with velocity = 0.5.

Fig 3. Phase averaged spectra for different values of . The first 9 curves are for between 0.1 and 0.9, step 0.1; the last one is =0.998.

Fig. 2: Same as in Fig. 1 for Model 3.

Fig 4. Spectra emitted at different magnetic latitudes for models 1,2,3 (left to right). The black line is the spectrum as seen from the south pole; green and blue lines are from just below/above the equator; red line is from the north pole. The seed blackbody is shown for comparison.

This preliminary investigation has been carried out by Nicola Parkins (University of Liverpool) during a 1 month voluntary work experience at MSSL/UCL . XXXX simulations have been carried out, with 16000000 photons each, for a total running time of xxx hours on a 32 nodes XXX machine. More detailed simulations are currently under way at MSSL/UCL.