If nothing can escape a black hole, why do they still emit x-rays?

Surrounding the center of a black hole, called the Singularity (for non rotating holes), there is a mathematically precise radius where incomming matter and light signals become trapped and can no longer escape back out to infinity. This is called the Event Horizon, and for non-rotating black holes its distance in kilometers is 2.87 times the mass of the black hole in units of solar masses. If a supernova left behind a 2 solar mass black hole collapsed core, its radius would be 2x2.87 = 5.7 kilometers. If a galaxy has a supermassive black hole at its center with a mass of 10 billion suns, its event horizon would have a radius of 28 billion kilometers...extending far outside the orbit of Pluto in our solar system!

It is true that once matter or energy passes within the Event Horizon of a black hole it can never turn around and get backout. However, in the real world, a lot can happen to matter as it approaches the Event Horizon.

Commonly, matter falls into what is called an accretion disk that orbits the black hole. If this matter happens to be gas and dust, it experiences friction, and the disk heats up as some of orbital energy is converted into heat. Parcels of this matter orbit the center of the black hole just like a planet would, following roughly 'Keplerian' orbits.But at the same time it is losing energy through friction and so it is also slowly falling into orbits that are closer and closer to the black hole.

The closer the disk material is to the black hole, the more rapidly it orbits so that the greater is the heating effect. Just before it reaches the Event Horizon, this disk matter can be heated by friction to thousands of degrees which is enough to produce X-rays. Even higher temperatures approaching a million degrees can occur, which can produce gamma rays. The outer portion of the accretion disk glows with infrared light because it is only a few hundred degrees hot, but the inner regions may glow as bright as the surface of a star!

This disk radiation, being outside the black hole, is what we detect as we look at black holes.Of course as it struggles to get out of the vicinity of the Event Horizon, the radiation loses some of its energy, which means what may have started out as x-rays are received back on Earth as visible light radiation at a lower energy. Also, if material near the horizon has an orbital period of a few minutes, the time dilation effect due to the warped spacetime and intense gravity will make these periodic signals appear at longer intervals back on earth. Finally, the intensity of the light energy will appear to get dimmer the closer it is to the event horizon.

Aside from these radiation effects, there are also optical distortions that occur. Here, for instance, is what the accretion disk looks like from the vantage point of an observer orbiting close-by.If you watched the movie Interstellar, the optical 'ray tracing' that wass performed to render the black hole was generated by Physicist Kip Thorne and is as real as we think we can get it.

What you see here is the a view from the equatorial plane of the accretion disk and its swirling gases. The back-side of the disk behind the black hole has been optically bent into what is called an 'Einstein ring' around the black hole, which merges with the direct images of the foreground disk matter to create this compound effect. Closer in to the event horizon, you see a ring of light from the photons that are trapped in nearly closed 'photon orbits' just outside the horizon.

Return to Dr. Odenwald's FAQ page at the Astronomy Cafe Blog.