XMM-Newton observations of polars in low accretion states

Abstract

We have made a series of snap-shot observations of 37 polars using XMM-Newton. We found that 16 of these systems were in a low, or much reduced, accretion state. Of those, six were not detected in X-rays. This suggests that in any survey of polars, around half will be in a low accretion state. We tested if there was a bias towards certain orbital periods: this is not the case. Of the 10 systems that were detected at low but significant rates in X-rays, eight showed significant variability in their X-ray light curves. This implies that non-uniform accretion still takes place during low accretion epochs. The bolometric luminosity of these systems is ∼1030 erg s -1, two orders of magnitude less than for systems in a high accretion state. The X-ray spectra show no evidence of a distinct soft X-ray component. However, the X-ray and UV data imply that such a low-temperature component exists: its temperature is low enough for its flux distribution to move outside the bandpass of the X-ray instruments.