Science With X-ray Microcalorimeters

Why do astronomers look at X-rays?
What sorts of objects emit X-rays?
How are the X-rays created?
What does the spectrum tell us?
What's so great about microcalorimeters?

Why do astronomers look at X-rays?

By looking at X-rays, we can learn things which we can't learn any other way. Stars and galaxies are much too far away to go and measure directly, so we can only study them by way of the electromagnetic radiation they emit. To learn the most, we use the entire spectrum: radio waves, infrared, visible, and ultraviolet light, X-rays, and gamma-rays. Each type of astronomy is a window into different aspects of the objects we study.

Imagine the Universe has more information about X-ray astronomy as well as astronomy at other wavelengths, but here are some key points about X-ray astronomy:

It's relatively new: The first cosmic X-ray source (Scorpius X-1) was detected in 1962 with a rocket flight.
It has to be done above the atmosphere: The atmosphere blocks X-rays quite effectively, so we have to get above it to see them. This means all X-ray astronomy is done with balloons, sounding rockets, or satellites.
It tells us about hot objects: X-rays in general are emitted by hot objects (typically gas at millions of degrees or above, or very fast electrons). These include supernova remnants, active galaxies, neutron stars and black holes.
The spectrum is the thing: Traditionally, nearly all of what we have learned from X-ray astronomy has come from the shape of the spectrum. The Rossi X-ray Timing Explorer has added timing to our arsenal, and the recent launch of Chandra (originally called AXAF but renamed) will improve our imaging capabilities tremendously. However, spectral analysis remains a vitally important tool.

What sorts of objects emit X-rays?

Active Galaxies: These are galaxies that emit much more energy than can be accounted for by their stars. They include Seyfert galaxies, quasars, blazars, and probably some that don't have names yet.
Compact stars: White dwarfs, neutron stars, and black holes can all emit X-rays if material is falling onto them. In fact, X-rays are the main way of learning about these objects.
Interstellar (or intergalactic) gas: The clouds of gas within our galaxy (or even between galaxies) are so hot that they glow in X-rays.
Gamma-ray Bursts: We don't know for sure what these objects are, but studying them in X-rays may help us to figure them out.
Stellar Coronae: Like our sun, other stars are surrounded by an extremely hot corona, which emits X-rays. We can learn about how stars work by studying both our own sun and other stars.
Supernovae and their Remnants: Supernovas, being highly energetic, naturally produce lots of X-rays. But even long after they explode, the shock waves from their expanding shells of gas continue to produce X-rays.

How are the X-rays created?

There are several ways for X-rays to be made in celestial sources. These include:

Bremsstrahlung: That's a beautiful German word meaning "braking radiation", and refers to the radiation produced when an electron suddenly slows down. For example, when electrons shot away from a neutron star crash into the shell of material surrounding the star, they slow down, releasing bremsstrahlung.
Synchrotron radiation: This comes from the electrons spiraling around a magnetic field. Most neutron stars have a very large magnetic field.
Compton scattering: When a photon collides with a more energetic electron, it can absorb some of the electron's energy to become more energetic itself.
Atomic transitions: Electrons within atoms (neutral or partially ionized) emit photons when they jump between energy levels. When the energy jumps are large, the photons emitted are X-rays. Energies this large are typically associated with material at very high temperatures (millions of degrees).

Our Imagine the Universe site has a more detailed discussion of X-ray generation in space (though it doesn't include atomic transitions).

What does the spectrum tell us?

Here are some of the things we can learn by looking at an X-ray spectrum. This list is by no means complete.

Temperature: If the spectrum has the shape of a "blackbody", we know the X-rays are being produced by a region of opaque gas, and the peak of the spectrum tells us the temperature.
Anything hot emits radiation with a characteristic spectrum. This "blackbody spectrum" has a fixed shape, with the location of its peak determined by the temperature. At around 800°C. the peak is at the energy of red light, which is why the heater element in your oven glows red. At higher temperatures, the peak moves through the visible to blue, ultraviolet, and finally to X-rays.
Density of energetic electrons: If the spectrum falls off like a power law (flux proportional to Energy^M), we know it is being created by Compton scattering from energetic electrons.
Elemental abundances: As mentioned above, atoms in a celestial X-ray source emit radiation when they change energy levels. There are only certain allowed energy jumps, so there are only X-rays of certain energies emitted. These show up as spikes of high amplitude in a spectrum, and are called "emission lines". The relative strengths of emission lines in the spectrum tell us what elements are present in the object, and in what quantity.
Bulk motion: The redshifts of the emission lines tell us the speed at which the object is moving relative to the earth. Their widths tell us the range of speeds in the object.

What's so great about microcalorimeters?

Microcalorimeters can provide a large improvement in spectral resolving power plus high throughput (which means they detect almost all the X-ray photons that hit them). This will be particularly helpful when studying elemental abundances and bulk motion.

Elemental abundances: To measure the relative strengths of emission lines, we first need to see the lines distinctly! In many objects of interest, there are so many emission lines that previous instruments see only a wide blob, rather than many individual lines. Microcalorimeters will see these lines separately.
Bulk motion: When radiation is emitted from moving atoms, its frequency (and hence energy) is Doppler-shifted. From this shift we can determine the speed of the stuff that emitted the radiation (or at least the speed along our line of sight). Of course different parts of an object are moving differently, so we should see the spectrum smeared out in a specific way, depending on what's really happening. Unfortunately, previous instruments have not had the spectral resolution to see this smearing clearly.

This page written and maintained by Kevin R. Boyce (email: Kevin.R.Boyce@gsfc.nasa.gov)
This page was last modified on Wednesday, 22-Nov-2000 10:15:28 EST