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 ($F_X>2.5\times 10^{-11}\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ at 14--195~keV, and $|b|>5^{\circ }$). We then use it to estimate the space density ($\rho$) 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 $1_{-0.5}^{+1}\times 10^{-7}\mathrm{p}{\mathrm{c}}^{-3}$. 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 BAT have 14--195~keV luminosities of $\sim 10^{33}\mathrm{erg}{\mathrm{s}}^{-1}$, there is evidence of a fainter population at $L_X\sim 10^{31}\mathrm{erg}{\mathrm{s}}^{-1}$. We find that a population of IPs with this luminosity may have a space density as large as $5\times 10^{-6}\mathrm{p}{\mathrm{c}}^{-3}$. 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.

7 pages, 4 figures, accepted for publication in MNRAS

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