X-ray Spectra of Accreting Magnetic White Dwarfs

New Opportunities

The launch of the next generation of X-ray observatories, and particularly XMM, will inaugurate a new era in our understanding of the accretion flows in Magnetic White Dwarf systems (mCVs). We will be able to obtain phase resolved X-ray spectra of high S/N ratios over a wide range of energies (0.1-20 keV) and with high enough spectral resolution to begin to do line studies. These observations will for the first time be sufficient to understand the physical conditions in the accretion stream and in the impact region on the white dwarf.

Why are accretion flows in mCVs important?

Accretion is an important source of energy release in astrophysics, powering the emissions from AGN, from black hole, neutron star and white dwarf interacting binaries and from Young Stellar Objects. Recent work has highlighted the common physical processes occurring over the whole range of astrophysical systems. MCVs have attracted considerable interest because of the simplicity of the quasi-one dimensional nature of their accretion flows and because of the additional information available on the accretion region at optical wavelengths from the cyclotron radiation.

X-ray emission spectra

It is important for understanding the accretion physics to clarify the relationship between the different emission, absorption and reflection components in these systems, and their dependence on the bolometric luminosity. At MSSL we have therefore begun the process of modelling the X-ray emission from mCVs in much more detail. This has entailed the construction of optically thin bremsstrahlung and line spectra from the multi-temperature cooling flow of material as it settles onto the surface of the white dwarf. The temperature structure takes into account the additional cooling at each point resulting from the cyclotron emission from electrons spiralling in the intense magnetic field. In addition, the contribution of the Compton reflection from the surface of the white dwarf is included. The two spectra in Figure 1 indicate the difference between the emitted flux for a typical 40 MegaGauss magnetic field and one with zero field.

Other factors

The accreting material also causes heating of the white dwarf surface, resulting in a strong soft X-ray component. The infalling material is partially ionised by the X-rays from the accretion region, so that its absorption is strongly wavelength dependent. We have assembled all of these components to predict the spectrum of the prototype AM Her system which will be obtained by XMM in a 100 ksec observation in Figure 2.

Fundamental parameters of the system

We have used these model spectra to extract information on the conditions governing the accretion flow using the lower quality data available from existing satellites. One of the main determining factors is the mass of the white dwarf: more massive white dwarfs liberate more accretion energy, all the more so because they have smaller radii. The mass of the white dwarf is a fundamental parameter which enters into almost all aspects of these systems, and is also important for their evolution. We have therefore determined the white dwarf masses of a sample of ~15 mCVs. One of these, XY Ari, is an eclipsing system, and for this system we have cross-checked the masses from the X-ray spectral fits using several different satellites to those determined from the eclipse studies to explore the reliability if the method. The results are shown in Figure 3.

Figure 1.
The optically thin emission spectra from the heated material in the accretion region on the surface of the white dwarf. The emitting plasma is stratified in temperature and density, and includes the effects of cyclotron cooling in the case of the lower curve. Note that the spectrum is softer when cyclotron cooling is taken into account.

Figure 2.
The expected X-ray spectrum for a 100 ksec exposure of AM Her at the throughput and spectral resolution of the XMM EPIC cameras. We have used an absorbed blackbody (30eV) together with a stratified thermal bremsstrahlung model plus warm absorber. The two emission components are plotted together with their sum.

Figure 3.
We plot the mass of the white dwarf in XY Ari derived from fitting X-ray data obtained using RXTE, Ginga and ASCA using our stratified thermal bremsstrahlung model. We also show the effect of including an Iron fluorescence line at 6.4 keV. For comparison we show the results of Hellier who obtained a mass from timing the duration of the X-ray eclipse. There are small systematic differences between the masses derived using different satellites. Further, the mean mass from X-ray spectral fitting is higher, and not consistent, with that of the mass found by Hellier.

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This page written by Mark Cropper (msc@mssl.ucl.ac.uk).
Last modified 8th January 1998

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