The effect of two-temperature post-shock accretion flow on the linear polarization pulse in magnetic cataclysmic variables

The temperatures of electrons and ions in the post-shock accretion region of a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass flow rates or for sufficiently weak magnetic fields. At lower mass flow rates or in stronger magnetic fields, efficient cyclotron cooling will cool the electrons faster than the electrons can cool the ions and a two-temperature flow will result. Here we investigate the differences in polarized radiation expected from mCV post-shock accretion columns modeled with one- and two-temperature hydrodynamics. In an mCV model with one accretion region, a magnetic field >~30 MG and a specific mass flow rate of ~0.5 g/cm/cm/s, along with a relatively generic geometric orientation of the system, we find that in the ultraviolet either a single linear polarization pulse per binary orbit or two pulses per binary orbit can be expected, depending on the accretion column hydrodynamic structure (one- or two-temperature) modeled. Under conditions where the physical flow is two-temperature, one pulse per orbit is predicted from a single accretion region where a one-temperature model predicts two pulses. The intensity light curves show similar pulse behavior but there is very little difference between the circular polarization predictions of one- and two-temperature models. Such discrepancies indicate that it is important to model some aspect of two-temperature flow in indirect imaging procedures, like Stokes imaging, especially at the edges of extended accretion regions, were the specific mass flow is low, and especially for ultraviolet data.Comment: Accepted for publication in Astrophysics & Space Scienc

New polarimetric observations and a 2 pole model for the cyclotron emission from AM Herculis

Original article can be found via: http://cdsads.u-strasbg.fr/ Copyright Royal Astronomical Society [Full text of this article is not available in the UHRA]High signal-to-noise ratio phase dependent linear polarization and intensity data on AM Herculis which show the presence of structured linear pulses in the optical and near IR regions is presented. The data have been analyzed to construct a detailed model which reproduces closely the observed pulse structure and polarization angle variations and clearly demonstrates the presence of two interfering cyclotron emission regions located close to the foot points of a closed field line in an offset dipole field distribution. Both emission regions are linearly extended on the white dwarf surface. Th main region has a high-density edge at magnetic colatitude theta = 16 deg and extends up to theta to about 8 deg at almost constant magnetic longitude. The secondary region has a similar angular extent in theta but a larger width in magnetic longitude psi (Delta psi about 20-30 deg). The linear bright phase which, in the model corresponds to the phases of visibility of the secondary region, coincides with the X-ray bright phase of the anomalous state of AM Herculis. It is shown that the positions of the emission regions and their relative contributions to the total intensity change with time and that the linear pulses in different wavebands originate from different regions of structured shocks. The coupling region in the orbital plane extends from 18 R(wd) to 7 R(wd) with the distance of closest approach being about a third of the value that is calculated from standard theory for the magnetospheric radius.Peer reviewe