3 research outputs found
Polarisation of high-energy emission in a pulsar striped wind
Recent observations of the polarisation of the optical pulses from the Crab
pulsar motivated detailed comparative studies of the emission predicted by the
polar cap, the outer gap and the two-pole caustics models.
In this work, we study the polarisation properties of the synchrotron
emission emanating from the striped wind model. We use an explicit asymptotic
solution for the large-scale field structure related to the oblique split
monopole and valid for the case of an ultra-relativistic plasma. This is
combined with a crude model for the emissivity of the striped wind and of the
magnetic field within the dissipating stripes themselves. We calculate the
polarisation properties of the high-energy pulsed emission and compare our
results with optical observations of the Crab pulsar. The resulting radiation
is linearly polarised. In the off-pulse region, the electric vector lies in the
direction of the projection on the sky of the rotation axis of the pulsar, in
good agreement with the data. Other properties such as a reduced degree of
polarisation and a characteristic sweep of the polarisation angle within the
pulses are also reproduced.Comment: Proceedings of the 363. WE-Heraeus Seminar on: Neutron Stars and
Pulsars (Posters and contributed talks) Physikzentrum Bad Honnef, Germany,
May.14-19, 2006, eds. W.Becker, H.H.Huang, MPE Report 291, pp.108-11
Forced oscillations in relativistic accretion disks and QPOs
In this work we explore the idea that the high frequency QPOs observed in
LMXBs may be explained as a resonant coupling between the neutron star spin and
epicyclic modes of accretion disk oscillations. We propose a new model for
these QPOs based on forced oscillations induced in the accretion disk due to a
stellar asymmetric rotating gravitational or magnetic field. It is shown that
particles evolving in a rotating non-axisymmetric field are subject to three
kinds of resonances: a corotation resonance, a Lindblad resonance due to a
driving force, and a parametric resonance due to the time varying epicyclic
frequencies. These results are extends by means of 2D numerical simulations of
a simplified version of the accretion disk. The simulations are performed for
the Newtonian gravitational potential, as well as for a pseudo-general
relativistic potential, which enables us to explore the behavior of the
resonances around both rotating neutron stars and black holes. Density
perturbations are only significant in the region located close to the inner
edge of the disk near the ISCO where the gravitational or magnetic perturbation
is maximal. It is argued that the nearly periodic motion induced in the disk
will produce high quality factor QPOs.
Finally, applying this model to a typical neutron star, we found that the
strongest response occurs when the frequency difference of the two modes equals
either the spin frequency (for "slow rotators") or half of it (for "fast
rotators"). The two main excited modes may both be connected to vertical
oscillations of the disk. We emphasize that strong gravity is not needed to
excite the modes.Comment: Proceedings of the 363. WE-Heraeus Seminar on: Neutron Stars and
Pulsars (Posters and contributed talks) Physikzentrum Bad Honnef, Germany,
May.14-19, 2006, eds. W.Becker, H.H.Huang, MPE Report 291, pp.189-19