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Register a new userConstraints on the space density of intermediate polars from the Swift-BAT survey
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Added by
Magaretha Pretorius
Publication date
2014
fields
Physics
and research's language is
English
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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 ($F_X > 2.5 times 10^{-11},mathrm{erg,cm^{-2}s^{-1}}$ 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 $1^{+1}_{-0.5} times 10^{-7},mathrm{pc^{-3}}$. 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 $sim 10^{33} {rm erg s^{-1}}$, there is evidence of a fainter population at $L_X sim 10^{31} {rm erg s^{-1}}$. We find that a population of IPs with this luminosity may have a space density as large as $5times 10^{-6},mathrm{pc^{-3}}$. 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.
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We present a review of the results of long-term photometric monitoring of selected magnetic cataclysmic binary systems, which belong to a class named Intermediate polars. We found a spin period variability in the V2306 Cygni system. We confirm the strong negative superhump variations in the intermediate polar RX J2133.7+5107 and improved a characteristic time of white dwarf spin-up in this system. We have investigated the periodic modulation of the spin phases with the orbital phase in MU Camelopardalis. We can propose simple explanation as the influence of orbital sidebands in the periodic signal produced by intermediate polar.
The disc instability model (DIM) has been very successful in explaining the dwarf nova outbursts observed in cataclysmic variables. When, as in intermediate polars (IP), the accreting white dwarf is magnetized, the disc is truncated at the magnetospheric radius, but for mass-transfer rates corresponding to the thermal-viscous instability such systems should still exhibit dwarf-nova outbursts. Yet, the majority of intermediate polars in which the magnetic field is not large enough to completely disrupt the accretion disc, seem to be stable, and the rare observed outbursts, in particular in systems with long orbital periods, are much shorter than normal dwarf-nova outbursts. We investigate the predictions of the disc instability model for intermediate polars in order to determine which of the observed properties of these systems can be explained by the DIM. We use our numerical code for the time evolution of accretion discs, modified to include the effects of the magnetic field, with constant or variable mass transfer from the secondary star. We show that intermediate polars have mass transfer low enough and magnetic fields large enough to keep the accretion disc stable on the cold equilibrium branch. We show that the infrequent and short outbursts observed in long period systems, such as e.g., TV Col, cannot be attributed to the thermal-viscous instability of the accretion disc, but instead have to be triggered by an enhanced mass-transfer from the secondary, or, more likely, by some instability coupling the white dwarf magnetic field with that generated by the magnetorotational instability operating in the accretion disc. Longer outbursts (a few days) could result from the disc instability.
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