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Register a new userThe space density of magnetic cataclysmic variables
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Added by
Magaretha Pretorius
Publication date
2013
fields
Physics
and research's language is
English
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We use the complete, X-ray flux-limited ROSAT Bright Survey (RBS) to measure the space density of magnetic cataclysmic variables (mCVs). The survey provides complete optical identification of all sources with count rate >0.2/s over half the sky ($|b|>30^circ$), and detected 6 intermediate polars (IPs) and 24 polars. If we assume that the 30 mCVs included in the RBS are representative of the intrinsic population, the space density of mCVs is $8^{+4}_{-2} times 10^{-7},{rmpc^{-3}}$. Considering polars and IPs separately, we find $rho_{polar}=5^{+3}_{-2} times 10^{-7},{rm pc^{-3}}$ and $rho_{IP}=3^{+2}_{-1} times 10^{-7},{rm pc^{-3}}$. Allowing for a 50% high-state duty cycle for polars (and assuming that these systems are below the RBS detection limit during their low states) doubles our estimate of $rho_{polar}$ and brings the total space density of mCVs to $1.3^{+0.6}_{-0.4} times 10^{-6},{rm pc^{-3}}$. We also place upper limits on the sizes of faint (but persistent) mCV populations that might have escaped detection in the RBS. Although the large uncertainties in the $rho$ estimates prevent us from drawing strong conclusions, we discuss the implications of our results for the evolutionary relationship between IPs and polars, the fraction of CVs with strongly magnetic white dwarfs (WDs), and for the contribution of mCVs to Galactic populations of hard X-ray sources at $L_X ga 10^{31} {rm erg/s}$. Our space density estimates are consistent with the very simple model where long-period IPs evolve into polars and account for the whole short-period polar population. We find that the fraction of WDs that are strongly magnetic is not significantly higher for CV primaries than for isolated WDs. Finally, the space density of IPs is sufficiently high to explain the bright, hard X-ray source population in the Galactic Centre.
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Binary evolution theory predicts that accreting white dwarfs with sub-stellar companions dominate the Galactic population of cataclysmic variables (CVs). In order to test these predictions, it is necessary to identify these systems, which may be difficult if the signatures of accretion become too weak to be detected. The only chance to identify such dead CVs is by exploiting their close binary nature. We have therefore searched the Sloan Digital Sky Survey (SDSS) Stripe 82 area for apparently isolated white dwarfs that undergo eclipses by a dark companion. We found no such eclipses in either the SDSS or Palomar Transient Factory data sets among our sample of 2264 photometrically selected white dwarf candidates within Stripe 82. This null result allows us to set a firm upper limit on the space density, $rho_0$, of dead CVs. In order to determine this limit, we have used Monte-Carlo simulations to fold our selection criteria through a simple model of the Galactic CV distribution. Assuming a $T_{WD}=7,500$ K, the resulting 2$sigma$ limit on the space density of dead CVs is $rho_0 lesssim 2 times 10^{-5}$ pc$^{-3}$, where $T_{WD}$ is the typical effective temperature of the white dwarf in such systems.
We combine two complete, X-ray flux-limited surveys, the ROSAT Bright Survey (RBS) and the ROSAT North Ecliptic Pole (NEP) survey, to measure the space density (rho) and X-ray luminosity function (Phi) of non-magnetic CVs. The combined survey has a flux limit of F_X ga 1.1 times 10^{-12} erg cm^{-2}s^{-1} over most of its solid angle of just over 2pi, but is as deep as simeq 10^{-14} erg cm^{-2}s^{-1} over a small area. The CV sample that we construct from these two surveys contains 20 non-magnetic systems. We carefully include all sources of statistical error in calculating rho and Phi by using Monte Carlo simulations; the most important uncertainty proves to be the often large errors in distances estimates. If we assume that the 20 CVs in the combined RBS and NEP survey sample are representative of the intrinsic population, the space density of non-magnetic CVs is 4^{+6}_{-2} times 10^{-6} pc^{-3}. We discuss the difficulty in measuring Phi in some detail---in order to account for biases in the measurement, we have to adopt a functional form for Phi. Assuming that the X-ray luminosity function of non-magnetic CVs is a truncated power law, we constrain the power law index to -0.80 pm 0.05. It seems likely that the two surveys have failed to detect a large, faint population of short-period CVs, and that the true space density may well be a factor of 2 or 3 larger than what we have measured; this is possible, even if we only allow for undetected CVs to have X-ray luminosities in the narrow range 28.7< log(L_X/erg,s^{-1})<29.7. However, rho as high as 2 times 10^{-4} pc^{-3} would require that the majority of CVs has X-ray luminosities below L_X = 4 times 10^{28} erg s^{-1} in the 0.5--2.0 keV band.
The space density of the various classes of cataclysmic variables (CVs) could only be weakly constrained in the past. Reasons were the small number of objects in complete X-ray flux-limited samples and the difficulty to derive precise distances to CVs. The former limitation still exists. Here the impact of Gaia parallaxes and implied distances on the space density of X-ray selected complete, flux-limited samples is studied. The samples are described in the literature, those of non-magnetic CVs are based on ROSAT (RBS - ROSAT Bright Survey & NEP -- North Ecliptic Pole), that of the Intermediate Polars stems from Swift/BAT. All CVs appear to be rarer than previously thought, although the new values are all within the errors of past studies. Upper limits at 90% confidence for the space densities of non-magnetic CVs are $rho_{rm RBS} < 1.1 times 10^{-6}$ pc$^{-3}$, and $rho_{rm RBS+NEP} < 5.1 times 10^{-6}$ p$^{-3}$, for an assumed scale height of $h=260$ pc and $rho_{rm IPs} < 1.3 times 10^{-7}$ p$^{-3}$ for the long-period Intermediate Polars at a scale height of 120 pc. Most of the distances to the IPs were under-estimated in the past. The upper limits to the space densities are only valid in the case where CVs do not have lower X-ray luminosities than the lowest-luminosity member of the sample. These results need consolidation by larger sample sizes, soon to be established through sensitive X-ray all-sky surveys to be performed with eROSITA on the Spektrum-X-Gamma mission.
The NSFs Karl G. Jansky Very Large Array (VLA) is used to observe 122 magnetic cataclysmic variables (MCVs) during three observing semesters (13B, 15A, and 18A). We report radio detections of 33 stars with fluxes in the range 6--8031 uJy. Twenty-eight stars are new radio sources, increasing the number of radio detected MCVs to more that 40. A surprising result is that about three-quarters (24 of 33 stars) of the detections show highly circularly polarized radio emission of short duration, which is characteristic of electron cyclotron maser emission. We argue that this emission originates from the lower corona of the donor star, and not from a region between the two stars. Maser emission enables a more direct estimate of the mean coronal magnetic field of the donor star, which we estimate to be 1--4 kG assuming a magnetic filling factor of 50%. A two-sample Kolmogorov-Smirnov test supports the conclusion that the distribution function of radio detected MCVs with orbital periods between 1.5-5 hours is similar to that of all MCVs. This result implies that rapidly-rotating (Pspin < 10 days), fully convective stars can sustain strong magnetic dynamos. These results support the model of Taam & Spruit (1989) that the change in angular momentum loss across the fully convective boundary at Porb = ~3 hours is due to a change in the magnetic field structure of the donor star from a low-order to high-order multipolar field.
In a series of recent papers, it has been proposed that high field magnetic white dwarfs are the result of close binary interaction and merging. Population synthesis calculations have shown that the origin of isolated highly magnetic white dwarfs is consistent with the stellar merging hypothesis. In this picture, the observed fields are caused by an alpha-Omega dynamo driven by differential rotation. The strongest fields arise when the differential rotation equals the critical break-up velocity and result from the merging of two stars (one of which has a degenerate core) during common envelope evolution or from the merging of two white dwarfs. We now synthesise a population of binary systems to investigate the hypothesis that the magnetic fields in the magnetic cataclysmic variables also originate during stellar interaction in the common envelope phase. Those systems that emerge from common envelope more tightly bound form the cataclysmic variables with the strongest magnetic fields. We vary the common envelope efficiency parameter and compare the results of our population syntheses with observations of magnetic cataclysmic variables. We find that common envelope interaction can explain the observed characteristics of these magnetic systems if the envelope ejection efficiency is low.
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