When a black hole interacts with surrounding gas and dust, it can create jets of material that fly away from the black hole at nearly the speed of light. When those jets interact with the cosmic microwave background, something brilliant can occur. 

The cosmic microwave background (CMB) is an afterglow of the big bang. We see it as a faint glow of microwaves in all directions, and that’s because the entire universe is filled with the remnant light of the big bang. Photons from the CMB are streaming through the universe in all directions. When electrons in a black hole jet collide with these photons, they can give the photons an energy boost. Since the electrons collide at nearly the speed of light, the photons are boosted into x-rays.

Recently the Chandra x-ray observatory found an example of such a CMB enhanced jet. The x-rays from this jet began their journey more than 11 billion years ago. At this time the CMB was stronger, and so the x-rays emitted by this jet are about 150 times brighter than a jet forming in the present universe would be. It’s a great example of how the background light of the universe can be rekindled by black holes.

Interestingly, this active black hole isn’t bright at radio frequencies. Often the electrons of a black hole jet interact with magnetic fields to create strong radio emissions, and usually such black hole jets are detected by radio waves first. If more black holes can be x-ray loud but radio quiet, it could mean that there are many more distant x-ray jets that have simply been overlooked.

Paper: A. Simionescu, et al. Serendipitous discovery of an extended X-ray jet without a radio counterpart in a high-redshift quasar.  arXiv:1509.04822 [astro-ph.HE]

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Yesterday I wrote about the cosmic x-ray background, and I noted that x-ray astronomy, particularly with high-energy x-rays is difficult. But what is it about x-rays that makes x-ray astronomy so challenging?

X-rays are electromagnetic waves (light) with very short wavelengths. Visible light has wavelengths between about 400 to 700 nanometers, while x-rays have wavelengths of 0.01 to 10 nanometers. Their wavelengths are roughly the size of an atom. Because of their very short wavelength and high energy, x-ray photons act almost like hard particles. At longer wavelengths they are known as soft x-rays, while at shorter wavelengths they are known as hard x-rays. Hard x-rays in particular are very good at penetrating material. This is what makes them useful used as medical x-rays or CT scans.

This ability to penetrate is problematic for astronomy. A telescope works by focusing light from a larger area to a smaller one. An optical telescope uses lenses to refract light or curved mirrors to reflect it. Radio waves bounce easily off conductive materials such metal, so large metal dishes can be used as radio mirrors to focus radio waves to the detectors. But because x-rays tend to penetrate material rather than reflect off them, you can’t focus them as you would visible light or radio waves.

But x-rays can scatter off certain materials if they strike the surface at a very low angle. Basically an x-ray photon can make a glancing blow off the surface of the material, which changes the direction of the photon slightly. So to make an x-ray telescope, you need to make a column of reflective surfaces nested within each other, as seen in the image above. Each layer scatters x-ray photons slightly, but the amount of deflection by each layer is different so that all x-rays entering the telescope are focused toward the detector.

If there x-ray photons were only scattered once, then they would only focus after a very far distance. So you add another column of reflective surfaces to scatter the x-ray photons again. With two scattering the length of your telescope can be reduced to a more manageable length of about 10 meters.

Of course there are other challenges. Because the x-rays come in at such a shallow angle, any defect in your reflecting surface can scatter them in the wrong direction, which reduces the sensitivity of your telescope. So the reflectors need to be extremely precise. Metals such as platinum can be used for lower energy x-ray reflectors, but high-energy x-ray reflectors require multiple layers of different materials as a scattering surface. This means that after you make the reflecting surfaces, you then have to vacuum deposit a few-atoms thick layer of a low density material such as silicon or carbon, then a layer of a high density material such as platinum, and keep doing that about a hundred times. You have to do this for each mirror. In the case of the NuStar x-ray telescope, for example, there are 130 concentric focusing mirrors.

One last thing. X-rays don’t penetrate through the Earth’s atmosphere much, so this x-ray telescope, which is about the length of a bus, has to be launched into space.

This is why x-ray astronomy is such a challenge.

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You are likely familiar with the cosmic microwave background. This background is a thermal remnant of the big bang. Because of the expansion of the universe, this remnant energy has a temperature of about 2.7 K, which means it exists primarily in the microwave wavelengths. We see this cosmic background as a diffuse, low-energy glow of microwave radiation.

But there is another background that exists, known as the cosmic x-ray background. Just as the cosmic microwave background is a diffuse microwave glow, the cosmic x-ray background is a diffuse x-ray glow. You can see an image of this x-ray background in the image above. It is a false-color image, where red, green and blue represent low, medium and high x-ray energies.

Unlike the microwave background, the x-ray background is not a remnant of the big bang. Instead it is generated through several processes. Most of the background is produced by localized sources such as active galactic nuclei, but other sources are the the local bubble of interstellar media that surrounds the Sun and other stars in our local spiral arm of Orion. But there is a small portion of the background that remains unexplained.

One of the difficulties in understanding the x-ray background is the sheer challenge of observing x-rays at high resolution. X-rays tend to penetrate materials, so you can’t simply make a mirror to focus x-rays the way we do visible light or radio waves. X-ray telescopes must have special materials to reflect x-rays, and they need to have a very long focal length.

But I’ll talk about that next time.

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In observational astronomy, it’s always good to gather more data.  Sometimes this is achieved by making more observations, but at other times it is achieved by making observations at different wavelengths of light.  You’re probably most familiar with observations made in the visible spectrum.  After all, it is how we observe things with the naked eye, which is how astronomy was done for thousands of years.

In the 1930s, we started building radio telescopes, which opened a whole new realm of observational possibilities.  Expanding observations to even more wavelengths was a bit of a challenge because our atmosphere tends to absorb light in the infrared, at large radio wavelengths, and at wavelengths shorter than visible light.  This is generally a good thing, because things like ultraviolet light and x-rays tend to be harmful to us.  It means, however, that to observe these wavelengths we have to get above the atmosphere.  For this reason, satellites capable of ultraviolet and x-ray astronomy didn’t appear until the 1970s.

X-ray astronomy presents an additional challenge because you not only have to put your x-ray telescope in space, you also have to build your telescope very differently.  A visible or radio telescope typically uses a curved surface that acts as a mirror to reflect light back to your camera or detector.  But the wavelengths of x-rays are so short they act almost like tiny bullets.  If you tried to use a curved surface to reflect x-rays back to your detector, the x-rays would simply pierce through your “mirror.”

To focus x-rays, then, you have to use a series of slightly curved surfaces aligned so that the x-rays glance off the surface at a shallow angle, as you can see in the figure below.

This means x-ray telescopes have to be fairly large.  The Chandra telescope, for example, is more than 40 meters long, compared to the Hubble telescope, which is about 10 meters long.  It also makes it more difficult to make high resolution images.  Chandra’s resolution is about 1/2 an arcsecond, which is roughly the apparent size of the minor planet Ceres as seen from Earth.

The big advantage, of course, it that x-ray astronomy lets us observe many of the phenomena that occur at high energies, such as black holes and supernova.

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