A)(i) State the key difference between cataclysmic variables and X-ray binaries

Question

A)(i) State the key difference between cataclysmic variables and X-ray binaries that makes one class of object emit in the optical/UV range and the other class of object emit in X-rays. Hence explain how the difference in their emission properties arises. (Your answer should be no more than 200 words.) (ii) Describe how we can gain evidence for orbital motion and disc accretion from emission lines in compact binaries. Explain whether cataclysmic variables or X-ray binaries are more suited for this study. (Your answer should be no more than 200 words.)

Explanation / Answer

A) (i) In Cataclysmic variables, the paired stars are results of two low mass stars dying. The thermonuclear fusion doesn't totally make the star run out of thermonuclear fuel, so the collapse of such stars result in core sustaining its gravitational pull. This modification from the hot center supported by thermal pressure to a cold center supported by degeneracy pressure preserves the mass of the star.

In such a system, the star with more mass ends first, becoming a degenerate dwarf. Its companion star remains a main-sequence for a bit longer. If the stars are close enough together, the orbit decays enough to cause the main-sequence star to overflow its Roche lobe, depositing gas onto the degenerate dwarf.

The magnetic field of the white dwarf can be powerful enough to disrupt the inner accretion disk or moreover, its formation too. Such magnetic systems can show strong and variable polarization in their optical light. The stars are so close to each other that the gravity of the white dwarf changes the structure of the secondary and accretes its matter. Strong UV emission is often seen from the accretion disc, powered by the loss of gravitational potential energy from the in-falling material.

X-ray binaries are more complex. The companion stars are bigger in size and can either end up in low mass companions or high mass companions producing a black hole or a neutron star. They can be produced by one star expanding over Roche lobe limit or feeding off the wind of the other. Mainly the loss of gravitational potential dominates the powering of the emission in such cases hence we get mainly x-rays.

(ii) The emission lines from such binary systems will be affected by Doppler shift. Hence we can learn about their orbital motion. Also, due to the Lens- Thirring/ Frame dragging effect, i.e. rotation of a massive object distorting the space-time curvature, making the orbit of a nearby test particle tilt, can help us understand the disc accretion.

In high mass X-ray binary systems, the prominent radiant sources are in close vicinity to the compact objects emitting x-rays or to the donor stars giving out optical/ UV radiation compared to the accretion disc. Mostly the mass transfer takes place in form of stellar wind and not Roche lobe overflow, so the accretion disc is comparatively less well formed and smaller than in semi-distant systems. Whereas in CVs, the accretion disc is the main source of radiation.

Also, a lot of x-ray emission is absorbed by the outer disc and the donor star re-emitted in optical/ UV part of the spectrum when it comes to x-ray binaries. Instead, CVs can help us study the accretion flow directly.

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