Accreting Collapsed Objects
Physics of Accreting White Dwarfs
Among the various classes of accreting compact binaries, cataclysmic variables (CVs) — semi-detached binaries composed of a low-mass secondary and an accreting white dwarf primary — are arguably the best laboratories in which to study accretion flows. Relative to neutron star binaries, white dwarf binaries have ~100 times lower velocities (v/c ~0.01). The relatively low velocities means that the lines are narrow (ΔE/E ~10-3), so that they stand out against the continuum and thus provide detailed diagnostics of the plasma temperature, density, abundances, emission measure distribution, and velocity. The relatively low luminosities mean that photoionization is not as important in CVs; whereas, the relatively low temperatures (blackbody temperatures Tbb~10 eV and virial temperatures Tvir~10 keV) mean that the plasmas in CVs produce X-ray spectra that are rich in emission lines with critical diagnostics at soft energies. Constellation-X will increase by a factor of ~100 the number of CVs for which we will be able to obtain detailed X-ray spectra. Among the numerous detailed studies possible with Constellation-X, we discuss three that are unique to CVs.
First, compared to stars, the X-ray emitting plasma in CVs is expected to be dense. In magnetic CVs, high densities are the result of the magnetic channeling of the accreting material onto small spots near the white dwarf magnetic poles, while in nonmagnetic CVs it is the result of accretion through a disk and boundary layer onto a narrow belt on the white dwarf surface. Mauche, Liedahl, and Fournier (2001, 2003) and Szkody et al. (2002) demonstrate the application of novel, high-density iron L density diagnostics to dense plasmas in EX Hya and U Gem. To apply these density diagnostics, it is necessary to resolve the Fe XVII 17.10 Å and 17.05 Å lines (Δλ = 0.05 Å ; R ~ few times 350) and the Fe XXIII 11.74 A and the Fe XXII 11.77 Å lines (Δλ = 0.03 Å ; R ~ few times 390). This is currently possible with the Chandra MEG (Δλ= 0.02 Å FWHM), but not with the XMM-Newton RGS (Δλ ~ 0.07 Å FWHM). To apply the helium-like R density diagnostic for high Z elements, it is necessary to resolve the intercombination and forbidden lines, which in sulfur, argon, and iron are separated by 16, 19, and 31 eV.
A second area of CV science in which Constellation-X can make significant improvements is the study of systems in which the white dwarf is eclipsed by the secondary. Phase and eclipse ingress/egress-resolved spectroscopy has the potential to constrain the geometric as well as the thermal and density structure of the accretion flow. Currently, such studies are limited by counting statistics; the large effective area of Constellation- X will allow the first detailed studies of this kind.
Third, the unprecedented combination of energy resolution and sensitivity will for the first time permit the study of subtle but extremely powerful line transfer effects that offer probes of the physical conditions in CV plasmas. One example involves recombination iron lines in settings where opacity effects are important, for instance in the accretion column below the standoff shock above the surface of the white dwarf in magnetic CVs. Given the observed densities and the expected path lengths, the accretion column is expected to be optically thin to photoelectric absorption, thin to thick to Compton scattering, and thick to line scattering (Matt 1999, Terada et al. 2001).
A simulated Constellation-X calorimeter observation of the line profiles of the Fe XXVI doublet for different column densities. While for low column densities, the matter is optically thin in the line center and the line shape is a Voigt profile, but for larger column densities, the matter becomes optically thick in the line centers and the lines become double-horn shaped. This spectroscopic signature provides a probe of the geometrical and physical conditions of the emitting plasma.
References
Matt, G., 1999, ASP Conf. Ser. 157: Annapolis Workshop on Magnetic Cataclysmic Variables, 299Mauche, C. W., Liedahl, D. A., and Fournier, K. B., 2001, ApJ, 560, 992
Mauche, C. W., Liedahl, D. A., and Fournier, K. B., 2003, ApJ, 588, L101
Szkody, P., et al., 2002, ApJ, 574, 942
Terada, Y., et al., 2001, MNRAD, 328, 112
