Australia Telescope National Facility

Accretion Processes in Magnetic Binaries
Lilia Ferrario , Jianke Li , Curtis Saxton , Kinwah Wu, PASA, 16 (3), 234.

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Radiation properties of magnetically channelled flows

Ferrario & Wickramasinghe (1993) and Ferrario (1996) have shown that in the disced IPs, the gas in the field channelled flow (the accretion curtains) is heated by radiation from shocks at the white dwarf surface, and produce continua and emission lines which have a significant impact on the observed properties of these systems. The study of the emission properties of magnetically channeled flows have been extended to the accretion funnels in AM Hers by Ferrario & Wehrse (1999a, 1999b), who have treated the full 3-D radiation transfer problem to establish the thermal structure of magnetically confined flows.

The 3-D radiative transfer equation is given by

$\begin{displaymath} {\bf n}\cdot\nabla I = \kappa (S-I) \end{displaymath}$

(2)

here, ${\bf n}$ is a unit vector in the ray direction and S is the source function which is given by:

$\begin{displaymath} S = (1-\epsilon)J+\epsilon B \end{displaymath}$

(3)

where J is the mean intensity given by:

$\begin{displaymath} J={\int_{4\pi} I(\theta,\psi)\, d\Omega\over 4\pi}. \end{displaymath}$

(4)

Furthermore, $\kappa$ is the extinction coefficient, $\epsilon$ is the ratio of absorption to extinction, $\Omega$ is the solid angle and I and B are the radiation intensity and Planck function respectively.

Finally, the energy equation is given by

$\begin{displaymath} \kappa\epsilon(B-J)=q_{mag} \end{displaymath}$

(5)

Where q_mag is the magnetic heating term, which depends on the strength of the toroidal field formed at the base of the accretion funnel (see section 2). These equations are then solved by using the Jacobi method described in detail by Stenholm, Störzer & Wehrse (1991).

In all calculations, Ferrario & Wehrse (1999a, 1999b) have assumed LTE level occupations and the opacities have been computed by assuming that the accreting material has a solar abundance of elements.

The solution of the above radiation transfer equation gives, self-consistently, the thermal structure of the funnel. Ferrario & Wehrse (1999a, 1999b) show that while models which allow only for x-ray heating can explain the observed intensities of the Balmer lines, they cannot, at the same time, explain the intensities of the HeI and HeII lines in the optical spectra. These lines appear to be formed mainly in the magnetically heated transition region near the orbital plane. Ferrario & Wehrse (1999a, 1999b) show that with the inclusion of this region, through the magnetic heating factor q_mag in equation (5), models can be constructed which are in close agreement with the optical line and continuum emission observed in AM Herculis systems; that is, they exhibit a flat or inverted Balmer decrement, HeI lines and a strong HeII $\lambda 4686$ line (see Figures 2 and 3).

**Figure 2:** Hydrogen and helium lines phase-averaged spectra of an accretion funnel with
$\dot M=5\times 10^{16}$ g sec^-1 (top) and

$5\times \dot M=10^{15}$ g sec^-1 (bottom) heated by x-rays only.
$\begin{figure} \begin{center} \hspace{0.1in} \epsfysize=0.45\textwidth \epsfbox[15 150 590 700]{ferrario2.ps}\vspace{0.1in} \end{center} \end{figure}$

**Figure 3:** Hydrogen and helium lines phase-averaged spectra of an accretion funnel with
$\dot M=5\times 10^{16}$ g sec^-1 (top) and

$\dot M=5\times 10^{15}$ g sec^-1 (bottom). Heating is caused by x-rays from the accretion shock and by magnetic reconnection at the base of the funnel.
$\begin{figure} \begin{center} \hspace{0.1in} \epsfysize=0.45\textwidth \epsfbox[15 150 590 700]{ferrario3.ps}\vspace{0.1in} \end{center} \end{figure}$

Ferrario & Wehrse (1999a, 1999b) also showed that the emission line profiles can vary dramatically in velocity and shape over the orbital period of the white dwarf. They also showed that the continuum emission from the accretion funnel provides an important source of unpolarised background radiation which reduces the degree of polarisation of the cyclotron radiation from the accretion shocks, and produces the polarisation standstills that are a well known characteristic of these systems.

Next Section: Iron lines from AM
Title/Abstract Page: Accretion Processes in Magnetic
Previous Section: The magneto-hydrodynamics of accretion:
Contents Page: Volume 16, Number 3

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