If a Black Hole Is Truly Black and the Escape

question 117

Multiple Choice

If a black hole is truly black and the escape velocity associated with it is greater than the speed of light such that no light can escape it, where do the X-rays from the black hole candidates so far identified originate?

• A) The black hole is only black to visible radiation, but X-rays travel faster than the speed of light and so can escape.
• B) The X-rays originate from the normal star accompanying the black hole; its ordinary light is blueshifted into the X-ray spectral region by the intense gravity of the black hole.
• C) The X-rays originate from stars behind the black hole, the light from which is focused and concentrated such that it becomes X-ray radiation by gravitational focusing.
• D) The X-rays originate from the matter surrounding the black hole that is highly condensed and hence very hot because of the intense gravitational field.

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