Stellar winds in massive X-ray binaries

In supergiant X-ray binary systems, the strong wind of the optical star is able to drive out 10-6 -10-8 Msun/yr, with a terminal velocity of up to 2500 km/s. A compact object, like a neutron star, orbiting relatively close to its companion is therefore able to trap enough matter to emit a significant amount of X-ray radiation.

The basic mechanism that stars use to push out the wind is the absorption of photons at their spectral lines energies. When an atom in the atmosphere interacts with a photon and absorbs its energy, some momentum is also transferred. Generally, the radiation field is pointing outward from the star, and therefore the net momentum transfer also points in this direction. Coulomb interactions drag surrounding matter, enhancing the process until the attraction of gravity of the star is overcome and a wind is generated.

When an ion is accelerated and moves away from the star, it ‘sees’ the photons coming from behind as redshifted because of the Doppler effect. That is, the absorption will be produced at a higher energy than in the case where the atom is at rest. If there is any reason why an ion or molecule moves a little bit faster than the local wind, the particle will receive more Doppler shifted radiation than the surrounding particles, thereby having access to more photons in the frequencies it absorbs. This particle will be more accelerated than the ones from the local wind, and its velocity will be even faster. This type of wind is therefore highly unstable, and very prone to inhomogeneities and clumps.

Please find more information about massive stellar winds in the following links:

A Comprehensive View of Stellar Winds in Massive X-ray Binaries

Unified View of Stellar Winds in Massive X-ray Binaries

Massive stellar winds are made of tiny pieces

 

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