The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole survey |
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| Authors: | M. L. Pretorius C. Knigge D. O'Donoghue J. P. Henry I. M. Gioia C. R. Mullis |
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| Affiliation: | School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ;South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa;Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA;INAF –Istituto di Radioastronomia del CNR, Via Gobetti 101, I-40129 Bologna, Italy;Department of Astronomy, University of Michigan, 918 Dennison, 500 Church Street, Ann Arbor, MI 48109-1042, USA |
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| 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 four systems, two 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 ± 0.02 and 4.01 ± 0.03 h for RX J1715.6+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 subpopulation 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≃ 2 × 1029 erg s−1 . |
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| Keywords: | binaries: close stars: dwarf novae novae, cataclysmic variables |
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