The X-ray Sky

Finally, we will look at an x-ray survey of the sky. X-rays have very short wavelengths and high energies compared to visible light. If an astronomical object gives off x-rays, it must be a hot, bright object that gives off a great deal of energy. However, since these objects give off most of their energy in the x-ray part of the spectrum, they may look quite ordinary in the visible part of the spectrum.

The Earth’s atmosphere absorbs almost all the x-rays that come from space. That’s good for life on Earth, because high doses of x-rays are dangerous. But that’s bad for astronomers – they have to get above the atmosphere to study x-rays. Fortunately for the astronomers, satellites have allowed them to get x-ray telescopes into space.

You may have seen the launch of the Chandra X-ray observatory, a satellite that launched in 1999. Chandra has a very narrow field of view. It is very good at getting close-up views of x-ray sources, but it would not be very useful to survey the entire sky.

The Röntgen Satellite (ROSAT)

The Röntgen Satellite, or ROSAT, launched in June 1990, and has since completed an all-sky survey of x-ray sources. ROSAT found more than 60,000 x-ray sources. The temperatures of these sources ranged from about 100,000 Kelvin to 20 million Kelvin! (For comparison, the surface of the Sun is about 5,000 Kelvin.) ROSAT imaged everything from nearby asteroids and comets to distant quasars during its 8-year mission.

ROSAT data is also publicly available on the Internet. Let’s compare some ROSAT images with SDSS images.

When astronomers find an x-ray source, they know that something is giving off a great deal of energy, and they want to know what that object is. The object frequently gives off visible light in addition to x-rays. Astronomers find both the x-ray and visible images; this is called finding an “optical counterpart” to the x-ray source. In the next exercise, you will look at objects in the ROSAT survey and find their optical counterparts in the SDSS.

Exercise 3. Open the ROSAT Search Page. In the lat and lon boxes, enter the coordinates 150 and 0 (these are the RA and Dec coordinates). Click “Coordinates.” A new window will open with an image of part of the sky as seen in x-rays. Click on any of the numbered objects that lie between Dec = -1.2 and Dec = 1.2 (this is the area covered by SDSS). Another window will open telling you the RA and Dec of the object.

Now open the Navigate tool. In the boxes marked “ra” and “dec,” enter the RA and Dec of the object you selected, then click “Get Image.” A new image will come up in the main window, centered on the object you selected. The object’s data will appear in the right-hand frame.

Question 6. What type of object is the optical counterpart to the x-ray source you chose? Does it stand out when you look at it in the visible wavelengths, or does it look like a lot of the other objects?

Question 7. What do you think are the advantages of looking at objects in the x-ray region of the spectrum?

Exercise 4. Select several other x-ray sources and find their optical counterparts. What type(s) of objects tend to give off x-rays?