Originally Posted by mhgaffney
Yes and no.
Yes brem-strahlung -- but not because of impact on the surface of the comet.
If you check you will see that the area of x-ray production is not on the surface of the cometary head -- but in space some distance out in front of the comet.
The x-rays are produced because the electrical current (or lightning bolt) from the sun slows down slightly when it encounters the comet -- the same mechanism that causes lightning to produce x-rays in the earth's atmosphere.
The planets also discharge the solar capacitor and receive electricity from the sun -- the energy source of lightning. But in the case of planets the discharge is much less because planets revolve in a more nearly circular orbit. Comets usually move at a much steeper angle with respect to the sun.
X-rays are only produced from brems-strahlung when high energy electrons impact on dense materials. Brems-strahlung is for example not created in measurable amounts when a human undergoes radiotherapy with electrons.
X-rays are not produced in lighting when the bolt hits, but before or when it initiates - if the x-ray production in the halo is the same mechanism as that of atmospheric lightning then that means lightning must initiate on the comet and fire outwards, not the other way around.