Constraints on the space density of intermediate polars from the Swift-BAT survey

Abstract

We construct a complete, hard X-ray flux-limited sample of intermediate polars (IPs) from the Swift-BAT 70-month survey, by imposing selection cuts in flux and Galactic latitude (FX > 2.5 × 10− 11 erg cm− 2 s− 1 at 14–195 keV, and |b| > 5°). We then use it to estimate the space density (ρ) of IPs. Assuming that this sample of 15 long-period systems is representative of the intrinsic IP population, the space density of long-period IPs is |$1^{+1}_{-0.5} \times 10^{-7}\,\rm{pc^{-3}}$|⁠. The Swift-BAT data also allow us to place upper limits on the size of a hypothetical population of faint IPs that is not included in the flux-limited sample. While most IPs detected by Swift-BAT have 14–195 keV luminosities of ∼1033 erg s−1, there is evidence of a fainter population at LX ∼ 1031 erg s− 1. We find that a population of IPs with this luminosity may have a space density as large as 5 × 10−6 pc−3. Furthermore, these low-luminosity IPs, despite appearing rare in observed samples, are probably at least as intrinsically common as the brighter systems that are better represented in the known IP sample.