Transcript Poster_silvia

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