Optical polarimetry and infrared photometry of two AM Her binaries: 1RXS J161008.0+035222 and 1RXS J231603.9-052713

Abstract

We present the first optical circular and linear polarization measurements of two polar candidates from ROSAT: 1RXS J161008.0+035222 and 1RXS J231603.9-052713. We also present H-band near-infrared photometry of the last object. The presence of strong circular polarization confirms them as AM Her systems. 1RXS J231603.9-052713 was observed in two different brightness states. The orbital phase dependence of the flux and polarization of 1RXS J161008.0+035222 is reasonably fitted with a simple model in which the binary is observed at a small inclination and the magnetic field axis is almost parallel to the white dwarf rotation axis resulting in the accretion column axis being seen from top during the whole orbital revolution. An alternative geometry with intermediate inclination can fit the observed flux and circular polarization. However, in this case, the model produces a linear polarization peak which is not corroborated by the data. The estimated magnetic field is in the 10-20 MG range. The circular polarization of 1RXS J231603.9-052713 is complex and highly variable. The light curves of that object have been fitted using a model which includes the white dwarf, a heated secondary and a point-like accretion region. The secondary emission contributes significantly even in optical wavelengths. This model also reproduces the main features of the optical polarization of 1RXS J231603.9-052713. We estimate the main parameters of the binary, of the accretion region and the distance to the system. An improved description of this system should include an extended and inhomogeneous accreting region as well as non-radial accretion. © 2006 RAS.