The space density of magnetic cataclysmic variables

We use the complete, X-ray flux-limited ROSAT Bright Survey (RBS) to measure the space density (rho) of magnetic cataclysmic variables (mCVs). The survey provides complete optical identification of all sources with count rate >0.2 s(-1) (corresponding to F-X greater than or similar to 2 x 10(-12) erg cm(-2) s(-1)) over half the sky (vertical bar b vertical bar > 30 degrees), and detected 6 intermediate polars (IPs) and 24 polars. If we assume that the 30 mCVs included in the RBS are representative of the intrinsic population, the space density of mCVs is 8(-2)(+4) x 10(-7) pc(-3). Considering polars and IPs separately, we find rho(polar) = 5(-2)(+3) x 10(-7) pc(-3) and rho(IP) = 3(-1)(+2) x 10(-7) pc(-3). Allowing for a 50 per cent high-state duty cycle for polars (and assuming that these systems are below the RBS detection limit during their low states) doubles our estimate of rho(polar) and brings the total space density of mCVs to 1.3(-0.4)(+0.6) x 10(-6) pc(-3). We also place upper limits on the sizes of faint (but persistent) mCV populations that might have escaped detection in the RBS. Although the large uncertainties in the rho estimates prevent us from drawing strong conclusions, we discuss the implications of our results for the evolutionary relationship between IPs and polars, the fraction of CVs with strongly magnetic white dwarfs (WDs) and for the contribution of mCVs to Galactic populations of hard X-ray sources at L-X greater than or similar to 10(31) erg s(-1). Our space density estimates are consistent with the very simple model where long-period IPs evolve into polars and account for the whole short-period polar population. We find that the fraction of WDs that are strongly magnetic is not significantly higher for cataclysmic variable primaries than for isolated WDs. Finally, the space density of IPs is sufficiently high to explain the bright, hard X-ray source population in the Galactic Centre.