The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole Survey

TL;DR

This study uses the ROSAT NEP survey to estimate the space density of cataclysmic variables, discovering new systems and constraining the population of faint CVs, with implications for theoretical models.

Contribution

First X-ray flux-limited sample of CVs from ROSAT NEP survey, including new discoveries and orbital period measurements, providing constraints on CV space density.

Findings

Estimated CV space density as (1.1 +2.3/-0.7) x 10^-5 pc^-3.

Identified two new CVs with specific orbital periods.

Placed upper limits on faint CV populations, challenging some theoretical predictions.

Abstract

We use the ROSAT North Ecliptic Pole (NEP) survey to construct a small, but purely X-ray flux-limited sample of cataclysmic variable stars (CVs). The sample includes only 4 systems, 2 of which (RX J1715.6+6856 and RX J1831.7+6511) are new discoveries. We present time-resolved spectroscopy of the new CVs and measure orbital periods of 1.64 \pm 0.02 h and 4.01\pm 0.03 h for RX 1715.6+6856 and RX J1831.7+6511, respectively. We also estimate distances for all the CVs in our sample, based mainly on their apparent brightness in the infrared. The space density of the CV population represented by our small sample is (1.1 +2.3/-0.7) 10^-5 pc^-3. We can also place upper limits on the space density of any sub-population of CVs too faint to be included in the NEP survey. In particular, we show that if the overall space density of CVs is as high as 2 10^-4 pc^-3 (as has been predicted theoretically), the vast majority of CVs must be fainter than L_X \simeq 2 10^29 erg/s.