Science With The XRS Instrument
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.
For an in-depth discussion of what XRS will look at, and why, check out
section of the Astro-E2 Learning Center.
Imagine the Universe also has more information about
X-ray astronomy as well as astronomy at
Or you can read the short version right here:
- 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
X-ray Timing Explorer
has added timing to our arsenal, and the recent
launch of Chandra (originally
called AXAF but
will improve our imaging capabilities tremendously.
However, spectral analysis remains a vitally important tool, and the spectral
capabilities of XRS will really rock.
- 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 objects
- White dwarfs,
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
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.
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.
- Ordinary stars
- Like our sun, other stars are surrounded by an extremely hot corona, which emits
X-rays. XRS will not be able to look at the sun (too bright!), but by studying other stars,
we can learn more about our own.
There are several ways for X-rays to be made in celestial sources. These include:
- 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
discussion of X-ray generation in space.
Here are some of the things we can learn by looking at an X-ray spectrum.
This list is by no means complete.
- 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 EnergyM),
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.
The XRS has both high sensitivity and high
spectral resolution over a broad range of energies. It achieves its best resolving power
at high energies, nicely complementing the grating-based X-ray
spectrometers on XMM-Newton
Also, because it's non-dispersive,
it can be used equally well for both point
sources and extended sources. 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. XRS, with its 6-7 eV resolving ability, 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.