Contributed by RHESSI Scientist, Dr. David Smith
Although RHESSI was designed to study the Sun, its x-ray/gamma-ray detectors are open to the whole sky, and will receive photons from objects all over the Galaxy and beyond. These x-rays and gamma-rays usually won't go through the grids, though, so there will be no way to make images from these signals. In addition, these sources are much, much fainter than solar flares: not surprising, since most of them are on the order of a billion times further away! These faint sources can be swamped by undesired "background" x-rays which come from cosmic-ray reactions with the spacecraft and the Earth's atmosphere. But there are tricks which can be used to pull out these faint signals from the background. The different tricks depend on what we're looking at.
| PULSARS |  |
OTHER SOURCES |  |
| RADIOACTIVE ISOTOPES | CRAB NEBULA |  |
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| GAMMA-RAY BURSTS |  |
DOWN TO EARTH |  |
One of the neatest tricks is used to study the energy spectrum of x-ray pulsars. A pulsar is a neutron star which shoots out a beam of x-rays from each of its magnetic poles. The neutron star spins around an axis which is tilted from the magnetic axis, just as the Earth's spin axis and magnetic axis are different. Like a lighthouse, this beam flashes by us once per spin. Because every pulsar has its own exact spin period, all we have to do is look for x-rays flashing at one of these magic periods, and we know it has to be coming from that particular neutron star, no matter how much other stuff is coming in at the same time from other sources.
There's a lot we still have to learn about these pulsars. Some of them suck in gas from a companion star: these can gradually start to spin faster or slower, and we're still trying to learn how. Some of them are alone, and they slow down very gradually just because of the radiation they emit (which carries some of the spin away). This we think we understand, but these solitary pulsars also have "glitches", small, sudden changes in spin which we don't understand. Perhaps they are due to the neutron-star equivalent of earthquakes.
We will use a completely different trick to look at another source of gamma-rays: radioactive isotopes in the central regions of our Galaxy. Near our Galaxy's center, about 30,000 light-years away, is the a large concentration of stars, many of which have undergone explosions "recently" (anywhere from a few decades to a few million years). These explosions can be supernovae, which blow up a star entirely, or novae, which are smaller explosions on the surface of a white dwarf star. These are all basically thermonuclear explosions, and they are the source of most of the elements which make up planets like Earth, people, etc. Some of the isotopes produced in these explosions are radioactive, and when they decay (over years, centuries, or millennia), they produce gamma-rays of a very particular energy, each energy telling you what element it came from. To study these gamma-rays, and learn about the origin of elements in the Galaxy, we will collect all the data when the Galaxy's center is hidden from RHESSI by the Earth, and subtract this from the data we collect when it is visible, and this should leave only those gamma-rays that came from the Galaxy's center. From these gamma-rays, we'll look for only the ones with the special energies that indicate they came from particular radioactive isotopes of aluminum, titanium, iron, etc.
Gamma-ray bursts are brilliant flashes of gamma-rays lasting from milliseconds to minutes. They come from distant galaxies, sometimes so far that we can't even see the galaxy they came from with our best optical telescopes. Even though they're so far away, they can be almost as bright as solar flares. This means that, intrinsically, they are by far the brightest explosions in the universe. No one knows what causes them, but some people guess it's a particularly massive supernova (exploding star), or perhaps an event where two neutron stars in orbit around each other merge to form a black hole.
We can decide which x-rays and gamma-rays came from a burst pretty easily, because the burst is so bright and is over so quickly. But we will count on other satellites to tell us exactly where the burst came from.
But what about x-ray/gamma-ray sources which don't have unique periods of flashing like the pulsars, or unique gamma-ray energies like the radioactive isotopes, or a very fast lifetime like the gamma-ray bursts? How do we sort them out from the background? This category includes black holes sucking gas from another star, giant black holes in the centers of galaxies, and some neutron stars which don't pulse. The task is much harder, but not impossible.
We count on RHESSI's spin: if the object is off to the side, it only shines on some of the RHESSI detectors; the others are shadowed by their companions. As we spin every four seconds, each detector goes into and out of the illumination from the source. This gives a "pulse" like the pulse of the pulsars, but with a period always equal to the spacecraft spin. We locate the object by seeing exactly WHEN each detector comes into and out of the "light". In addition, we can see when sources switch on and off entirely because they've risen or set behind this Earth; this also helps identify their position. These techniques go under the general description of occultation.
Finally, there is at least one source out in the Galaxy which RHESSI will make a real image of through its incredibly precise grids. This is the Crab Nebula, an intricate structure of plasma stretched across space, all due to the radiation and particles emitted by a solitary pulsar, one of the nearest and youngest neutron stars in the Galaxy. As the Earth moves around the Sun, the Sun appears to move around the sky, and of course RHESSI will follow it. Once a year in mid-June, the Sun comes within 1.3 degrees of the Crab Nebula, and RHESSI will then create a beautiful image of the nebula. The picture shown here gives the nebula in visible light. The best high-energy maps made to date have been with the Chandra x-ray observatory. RHESSI will make maps from x-rays and gamma-rays up to 100 times more energetic than those seen by Chandra.
After the Sun, the next brightest source of x-rays and gamma-rays that RHESSI will see is the Earth. Aside from the continuous glow of x-rays and gamma-rays that comes off the Earth's atmosphere due to the continuous bombardment of cosmic rays, there will also be a variety of extra flashes, surges, and glows due to high-energy particles generated in and around the Earth's atmosphere. Some of these particles are usually trapped in the Earth's radiation belts; they can come crashing down into the atmosphere, producing a burst of radiation. This "precipitation" of particles can be caused by spontaneous disturbances within the radiation belts themselves, but it can also be stimulated by lightning. In addition, some kinds of lightning can create a small number of extremely energetic electrons which are accelerated UPWARDS almost out of the Earth's atmosphere; on their way up, they create an extremely short (milliseconds) flash of very high-energy gamma-rays; RHESSI should see about one such event per month.