The energy balance of polars revisited

In the EXOSAT and ROSAT eras, a significant number of polars were found to show a soft/hard X-ray ratio much greater than that expected from the standard accretion shock model. This was known as the 'soft X-ray excess'. We have made an snapshot survey of polars using XMM-Newton and determined their soft/hard ratios. We find that less than one in five systems show a significant soft X-ray excess, while the rest show ratios consistent with that predicted by the standard model. We have investigated the discrepancy between this and the previous investigations by re-examining all the available ROSAT PSPC-pointed observations of polars using more recent calibrations than in the original studies. We find that these data show an energy balance ratio which is broadly consistent with that of our XMM-Newton results. We conclude that the previous studies were affected by the data being less well calibrated. We discuss which physical mechanisms might give rise to a high soft X-ray excess and whether systems with high ratios show more variation in soft X-rays. Surprisingly, we find that six out of 21 systems found in a high accretion state did not show a distinct soft X-ray component. Two systems showed one pole with such a component and one which did not. Based on the ratio of the observed soft X-ray to UV flux measurements (which were obtained simultaneously using the Optical Monitor) we suggest that this is because the reprocessed component in these systems is cool enough to have moved out of the soft X-ray band and into the extreme ultraviolet (EUV) or the UV band.