The Relationship Between X-ray Luminosity and Duty Cycle for Dwarf Novae and their Specific Frequency in the Inner Galaxy

We measure the duty cycles for an existing sample of well observed, nearby dwarf novae using data from AAVSO, and present a quantitative empirical relation between the duty cycle of dwarf novae outbursts and the X-ray luminosity of the system in quiescence. We have found that $log DC=0.63(pm0.21)times(log L_{X}({rm erg,s^{-1}})-31.3)-0.95(pm0.1)$, where DC stands for duty cycle. We note that there is intrinsic scatter in this relation greater than what is expected from purely statistical errors. Using the dwarf nova X-ray luminosity functions from citet{Pretorius12} and citet{Byckling10}, we compare this relation to the number of dwarf novae in the Galactic Bulge Survey which were identified through optical outbursts during an 8-day long monitoring campaign. We find a specific frequency of X-ray bright ($L_{X}>10^{31},{rm erg,s^{-1}}$) Cataclysmic Variables undergoing Dwarf Novae outbursts in the direction of the Galactic Bulge of $6.6pm4.7times10^{-5},M_{odot}^{-1}$. Such a specific frequency would give a Solar neighborhood space density of long period CVs of $rho=5.6pm3.9times10^{-6},$pc$^{-3}$. We advocate the use of specific frequency in future work, given that projects like LSST will detect DNe well outside the distance range over which $rhoapprox{textrm const}$.

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

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 4.01pm 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.