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

摘要

The temperatures of electrons and ions in the post-shock accretion region ofa magnetic cataclysmic variable (mCV) will be equal at sufficiently high massflow rates or for sufficiently weak magnetic fields. At lower mass flow ratesor in stronger magnetic fields, efficient cyclotron cooling will cool theelectrons faster than the electrons can cool the ions and a two-temperatureflow will result. Here we investigate the differences in polarized radiationexpected from mCV post-shock accretion columns modeled with one- andtwo-temperature hydrodynamics. In an mCV model with one accretion region, amagnetic field >~30 MG and a specific mass flow rate of ~0.5 g/cm/cm/s, alongwith a relatively generic geometric orientation of the system, we find that inthe ultraviolet either a single linear polarization pulse per binary orbit ortwo pulses per binary orbit can be expected, depending on the accretion columnhydrodynamic structure (one- or two-temperature) modeled. Under conditionswhere the physical flow is two-temperature, one pulse per orbit is predictedfrom a single accretion region where a one-temperature model predicts twopulses. The intensity light curves show similar pulse behavior but there isvery little difference between the circular polarization predictions of one-and two-temperature models. Such discrepancies indicate that it is important tomodel some aspect of two-temperature flow in indirect imaging procedures, likeStokes imaging, especially at the edges of extended accretion regions, were thespecific mass flow is low, and especially for ultraviolet data.