Multi-epoch spectroscopy, polarimetry and photometry of the polar UW Pic

Abstract

We present photometric, polarimetric and spectroscopic observations of the ROSAT source RX J0531.5‐4624 (UW Pic), previously identified as a polar. Our observations, spanning a decade, show that UW Pic undergoes very pronounced changes in accretion geometry. This is particularly evident from the polarimetric observations. At times, UW Pic shows low levels of both positive and negative circular polarization, suggesting simultaneous accretion on to two magnetic poles. At other times, UW Pic shows high levels of negative circular polarization only over the whole orbit, suggesting accretion on to one dominant pole. Our more extensive collection of photometric measurements also shows morphological variations on monthly time-scales. The emission lines show variability typical of polars, with Doppler tomography revealing emission at the expected location of the secondary star and the accretion stream. The He II (λ4686) emission line varies in strength between epochs, being comparable to Hβ at one epoch and almost non-existent at others. Similarly, Ca II (λ3933) can either be weak or absent. We put forward a scenario that explains our observations in terms of changing accretion rates of UW Pic coupled with a particular orientation of the system to the observer. In addition, we have modelled the polarimetric observations according to this scenario. During epochs of lower rates, accretion occurs preferentially on to the magnetic pole that is most pointing towards the observer and the secondary star (the upper pole), resulting in negative circular polarization over the whole orbit and more Ca II (λ3933) emission. During epochs of higher rates, accretion occurs preferentially on to the opposite (lower) magnetic pole, though some accretion still occurs on to the upper pole. This results in both signs of polarization being observed. The higher accretion rate produces more He II (λ4686), a lower percentage of polarization and, due to geometrical considerations, a lower amount of Ca II (λ3933) emission. Alternatively, we note that these morphological variations are also suggestive of an asynchronous polar. Although we cannot rule out this scenario at present, no strong evidence of asynchronicity has been observed in our longest consecutive data sets, implying a beat period (if present) of the order of months.