Satellite Showcase
Swift
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At Cape Canaveral Air Force Station, the Swift spacecraft is revealed.
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The Mission
The Swift Gamma-ray Burst Explorer is an new NASA mission
dedicated to investigating the mysterious phenomena of
Gamma-ray bursts (GRBs): the most powerful events in the
universe. Swift is a solar powered satellite containing three
different telescopes designed to view GRBs and their
aftermaths in gamma-ray, X-ray, ultra-violet, and optical
light. It gets its name from its ability to detect a GRB and
then 'swiftly' turn to focus its telescopes on the afterglow.
In fact, Swift moves quickly enough to allow scientists to
view some GRB's in UV/optical light while they are occurring.
An GRB in progress has only been observed in optical light
once before: with GRB 990123 in 1999.
Swift is a part of the NASA medium explorer program designed with help from American universities and NASA's international partners. It is expected to launch in October 2004.
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Artist's concept of Swift as it observes a Gamma-ray burst.
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Instrumentation
Swift has three main instruments onboard: the Burst Alert Telescope (BAT), the X-ray telescope (XRT), and the UV/Optical telescope (UVOT). The main purposes of the instruments are to measure the location of the GRB and determine if there is also an "afterglow signal": a glowing ember of the burst in the longer wavelengths of X-rays, UV and optical. When an afterglow is present, the XRT and/or UVOT will be used to refine the position of the object so high precision ground based and space based observations may be performed.
The primary differences between the instruments are that they each look at a different part of the Electromagnetic Spectrum, providing us with three different eyeballs to view the universe.
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An artists concept of Swift that shows the BAT, XRT, and UVOT.
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The Burst Alert Telescope is one of the most important parts because it detects and locates the gamma-ray bursts with better than 4 arc minute accuracy. After detecting and locating a GRB, the BAT shares this information with the on-board computer of the Swift spacecraft, which then autonomously moves to position the XRT and UVOT to view the afterglow of the burst. It also immediately sends the GRB data back to Earth where anyone can access it through the Gamma-ray Burst Coordinate Network.
The BAT is also important because when it's not viewing a GRB,
it collects data to produce the first map of the sky at higher
X-ray energies.
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A drawing of the BAT showing the aperture mask and detector plate.
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These amazing feats are accomplished with the BAT's coded aperture mask and its detector array. The aperture mask is a big metal sheet with random holes in it that casts a shadow on the detector plates. Since scientists know what the aperture mask looks like, they can use the shadow cast by a GRB on the detector to pinpoint the GRB's location. The BAT detector array is responsible for recording gamma-ray and X-ray data before it gets sent down to Earth. It is a thick semiconductor plate that measures the energy change caused by the gamma-rays that strike it.
The Ultra-Violet/Optical Telescope is a type of telescope called a Ritchey-Chrètien telescope. It has a long tube to collect light and several mirrors at the end to focus it into a detector. This detector takes images and spectra of the GRB afterglows. The images are used like maps of the stars to tell astronomers where the burst happened. In fact, they are able to obtain 0.3 - 2.5 arc-second position localizations which can be use by ground based astronomers to view the location of the GRB themselves. The spectra help to determine red-shifts of the GRBs which are important because they give us information about how far away the burst is and when it happened.
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Schematic of the UVOT
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The X-ray telescope uses special conically shaped mirrors left over from the JET-X program (Joint European X-ray Telescope). These mirrors help to focus the X-rays because X-rays pass through normal mirrors like those used in the UVOT. The cone shape of the mirrors funnels the X-rays to a detector by grazing them off of the mirrors at wide angles. After detecting the X-rays, the XRT confirms the location of the GRB and determines it's intensity. The XRT burst data is also used to provide spectra and light curves of the burst in the X-ray spectrum.
Data Analysis and Expected Scientific Results
The Swift mission hopes to determine the origin of GRBs,
investigate how the GRB blast wave interacts with the rest of
the universe, and explore the origin of the universe. But
before this exciting science is done, the data must be brought
down from the satellite as telemetry and processed here on
Earth. There are actually several places the data goes once
it reaches Earth, but perhaps the most important is the Swift
Data Center (SDC) at NASA's Goddard Space Flight Center.
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A map of the path Swift's data takes before the public gets it here on Earth.
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The SDC effectively acts just like an oil processing pipeline. First, data travels from the satellite to the center here on Earth. Then it gets stopped and refined at the data center which turns the information from the three telescopes into calibrated images, cleaned event lists, light curves, and spectra. The refined data then continues its journey on the pipeline where it is brought to scientists and the public through the SDC Quick Look Facility, which is a sort of gallery for the pictures, charts and data sheets that come out of the refinery. After one week, the pipeline data is finally moved for long term storage to data archives in the US (HEASARC), UK, and Italy.
The Gamma-ray burst Coordinate Network (GCN) is another path data can take from the satellite to Earth. Upon detecting a GRB, Swift immediately sends a position message to the GCN which places it on the internet. This allows scientists on Earth to turn their telescopes toward the GRB to observe it themselves. However, the GCN doesn't refine the data for the scientists like the SDC does. In this way, it is more like the severe weather warnings on TV than an oil pipeline. It tells you where and when the burst happens, but it doesn't provide lots of details.
Once all is said and done and the data is prepared, scientists will begin to work with it. Some of them are hired by NASA to work toward specific scientific goals (like classifying GRBs). Others are just freelance astronomers and academic researchers who retrieve the data from the archives to analyze it. This could be you! Once the data is processed at the SDC, all of it is made available to the public. There is no proprietary data that is made available only to scientists, a fact that makes Swift an even more unusual and special mission.
So, what do scientists hope to find out from the data? Click the links to find out what Swift may tell us about GRBs, the Early Universe, and the hard X-ray background. See what Swift could tell us about
Additional Links
 Swift Home Page (http://swift.gsfc.nasa.gov)
Swift Education Pages (http://swift.sonoma.edu)
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