Cyclotron versus free-free emission from the intermediate polar RXJ1712.6-2414

The intermediate polar RX J1712.6-2414, discovered by Buckley et al. (1995), turned out to have a phasedependent circular polarization of up to −5% in the Ic band, the largest value observed in any intermediate polar. By comparing the slope of the observed polarized flux with theoretical models, Buckley et al. (1995) obtained an estimate of 8 MG for the magnetic field strength. In this fit, only the polarized flux in the Ic band is dominated by cyclotron radiation. Toward shorter wavelengths, the polarization is caused by free-free emission. We reexamine this fit and investigate its consequences. By also taking into account themagnitude of the observed polarized flux, we conclude that this system must be at a small distance (5-50 pc), because the polarized flux from free-free emission is very low. This would make this system the closest intermediate polar yet discovered. Furthermore, the unpolarized background flux present in this system must be lower than that in RE 0751+14 by about a factor 103. As an alternative, we examine whether cyclotron radiation can cause all of the observed polarization. We find that the resulting fit is still consistent with the observations with the most likely magnetic field strength such that cyclotron radiation turns from optically thin to optically thick in the Ic band. As a consequence, themagnetic field strength in this systemmay be significantly higher than the value obtained by Buckley et al. (1995). Furthermore, the resulting distance and unpolarized background flux are consistent with those of other known intermediate polars. For a range of likely white dwarf masses (0.4 − 0.8 M ) and specific accretion rates (0.1 − 1 g cm−2 s−1), we obtain magnetic field strengths at the pole ranging from 9 − 20 MG, though it may be as high as 15− 27 MG with a different dipole inclination.