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Astron. Astrophys. 364, 587-596 (2000)

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6. Discussion and conclusions

I have estimated linear polarization levels of pure electron scattering accretion disks using the Monte Carlo method. The simulations produce linear polarization levels of order 1% for the bolometric flux. As most of the accretion disk luminosity is released in UV wavelengths, polarization levels of this order may be expected in these wavelengths. In optical light, the polarization levels might be a little lower due to flatter color temperature profiles. The effect of small-scale azimuthal asymmetries to azimuth-averaged Stokes parameters was also estimated, and found relatively small. Large-scale asymmetries may be observed as polarization variations with the orbital period. Orbital variations of polarization can be studied only in a few brightest X-ray binaries.

Single-scattering approximation should not be used when modeling the polarization of accretion disks, as it produces significantly larger polarization levels than multiple-scattering models. Decreasing polarization with multiple scattering is also seen in a plane-parallel atmosphere, when polarization is evaluated from Chandrasekhar functions (Coulson et al. 1960). The geometry of accretion disk is similar to that of Coulson et al., so qualitatively similar results are produced. In Be star disks, simulations indicate increasing polarization with multiple scattering (Wood et al. 1996). This does not contradict my results, as the source of radiation is the central star in Be star disks, and the disk itself in LMXB accretion disks.

The polarization level produced by a disk is sensitive to vertical distribution of emissivity, and no dependence on the distribution of absorbing gas or radial temperature profile is seen. This eliminates several dimensions from the parameter space, so the interpretation of observations is easier than expected from the amount of free parameters. As the radial structure of accretion disks is generally better known than vertical, it is important to note that polarization is more sensitive to vertical structure. In priciple spectropolarimetric observations could be used to estimate the inclination and vertical emissivity distribution of an accretion disk, but more detailed studies of this are required. Spectropolarimetric observations of LMXB:s may be useful in estimating the emission region of the disk.

The disk models used in simulations are probably relevant to two types of LMXB:s, SXT:s in outburst and Z sources. The disks of VY Sculptoris-type CV:s are also probably similar to the simulated models, but the radiation of the secondary star should be included to the model. Irregularities in disks may reduce the polarization.

Better polarization estimates can be obtained, if other opacity sources and deviations from axial symmetry are taken into account. This is particularly important for CV:s. Both linear and circular polarization of the disk should be measured to avoid confusion between polarization components produced by electron scattering and magnetic effects. This is especially important in neutron star LMXB:s.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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