The space density of magnetic cataclysmic variables. (arXiv:1303.4270v1 [astro-ph.SR])

The space density of magnetic cataclysmic variables. (arXiv:1303.4270v1 [astro-ph.SR]):
We use the complete, X-ray flux-limited ROSAT Bright Survey (RBS) to measure
the space density of magnetic cataclysmic variables (mCVs). The survey provides
complete optical identification of all sources with count rate >0.2/s over half
the sky ($|b|>30^\circ$), 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 $8^{+4}_{-2} \times
10^{-7}\,{\rmpc^{-3}}$. Considering polars and IPs separately, we find
$\rho_{polar}=5^{+3}_{-2} \times 10^{-7}\,{\rm pc^{-3}}$ and
$\rho_{IP}=3^{+2}_{-1} \times 10^{-7}\,{\rm pc^{-3}}$. Allowing for a 50%
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 $1.3^{+0.6}_{-0.4}
\times 10^{-6}\,{\rm pc^{-3}}$. 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 $L_X \ga 10^{31} {\rm erg/s}$.
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 CV 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.