Swift Gamma-Ray Burst Mission

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Swift Gamma-Ray Burst Mission
Swift
The Swift Spacecraft (credit: NASA)
Organization NASA
Wavelength regime gamma-ray/X-ray/UV/optical
Orbit Height 600 km
Orbit period ~90 min
Launch date 20 November 2004
Orbit re-entry date no sooner than 2020
Mass 1500 kg
Webpage http://swift.gsfc.nasa.gov/
Physical Characteristics
Telescope Style coded mask (BAT), Wolter I (XRT), Ritchey-Chretien (UVOT)
Diameter 30 cm (UVOT)
Collecting Area 5200 cm2 (BAT)
Effective Focal Length 381 cm (UVOT)
Instruments
BAT Burst Alert (gamma-ray) Telescope
XRT X-Ray Telescope
UVOT UltraViolet/Optical telescope

The Swift Gamma-Ray Burst Mission consists of an unmanned spacecraft called Swift, which was launched into orbit on November 20, 2004, at 17:16:00 UTC (12:16 PM, EST) by a Delta II 7320-10C expendable launch vehicle. Swift is managed by the NASA Goddard Space Flight Center.

Contents

[edit] Overview

Swift is a multi-wavelength space-based observatory dedicated to the study of gamma-ray burst (GRB) science. Its three instruments work together to observe GRBs and their afterglows in the gamma-ray, X-ray, ultraviolet, and optical wavebands. This mission is part of NASA's "Medium Explorer Program" (MIDEX), and the satellite was developed by an international consortium from the United States, United Kingdom, and Italy.

During its nominal mission of two years about 200 GRBs should be observed, and as with many other NASA missions, once the two years are concluded successfully the mission will probably be extended. The Burst Alert Telescope (BAT) covers a large fraction of the sky (over one steradian fully coded, three steradians partially coded). It locates the position of each event with an accuracy of 1 to 4 arc-minutes within 15 seconds. This crude position is immediately relayed to the ground; latency is measured in seconds. Some wide-field, rapid-slew ground telescopes can catch the GRB with this much position error. Interpolation between Swift's solution, and the solutions of other spacecraft in the Interplanetary Network, may refine the position solution enough to observe with conventional telescopes.

Swift is then able to reorient itself automatically (autonomously slew) in about 20 to 75 seconds to point the fine-resolution instruments at the burst location. The rapid slew time demonstrated by this ability is the reason for the mission's name "Swift." In order to achieve this fast slew rate, the observatory is equipped with momentum wheels as part of its reaction control system. Relative to the spacecraft, these wheels are more massive than on any past mission, and are therefore more effective at reorienting the spacecraft when they spin. The rest of the spacecraft has had its mass concentrated, to reduce loads on the wheels.

If the other instruments detect the burst, its fine position is reported to the ground within one orbit, and typically much less. The new solution is fine enough to observe from most ground and space observatories without a rastering search.

In the time between GRB events, the BAT instrument conducts an all-sky survey in the hard X-ray region. After its successful completion, the survey is anticipated to yield new black hole candidates in the sky.

The Swift Mission Operation Center (MOC), where commanding of the satellite is performed, is located in State College, Pennsylvania and operated by the Pennsylvania State University and industry subcontractors. The Swift main ground station is located in Malindi on the coast of Eastern Kenya, Africa and is operated by the Italian Space Agency. The Swift Science Data Center (SDC) and archive are located at the Goddard Space Flight Center. The UK Swift Science Data Centre is located at the University of Leicester.

[edit] Instruments

There are three observing instruments on board:

  • Burst Alert Telescope (BAT), 15 - 150 keV energy range. It initially detects a new GRB event and computes its coordinates in the sky. BAT uses a coded-aperture mask of 5 mm lead tiles, above a detector plane of 4 mm CdZnTe tiles; it is purpose-built for Swift.
  • X-Ray Telescope (XRT), 0.2 - 10 keV energy range. It can take images and perform spectral analyses of the GRB afterglow. This data will provide a more precise location of the GRB, with an error circle of approximately 3.5 arcseconds radius. The XRT is also used to perform long term monitoring of GRB afterglow light-curves for days to weeks, depending on the brightness of the afterglow. The XRT is primarily composed of spare parts from the JET-X mission.
  • UV/Optical Teslescope (UVOT), 170–650 nm wavelength range. If an optical afterglow is detected, UVOT provides a sub-arcsecond resolution position for the burst. UVOT also provides photometry through lenticular filters in optical and ultra-violet and spectra through the use of its optical and UV grisms. UVOT is also used to provide long time follow-ups of afterglow lightcurves. UVOT is a spare from the XMM-Newton mission's Optical Monitor (OM) instrument.

[edit] Science goals

This mission has multiple science goals:

  • Determine the origin of GRBs. There seem to be at least two types of GRBs, only one of which can be explained with a hypernova, creating a gamma-ray beam. More data is needed to explore other explanations.
  • GRBs seem to take place at "cosmological distances," which means they can be used to probe the distant, and therefore young, universe.
  • The all-sky survey will be more sensitive than any previous one, and will add significantly to our knowledge of astronomical X-ray sources. Thus, it could also yield unexpected results.
  • Swift is also utilized as a general purpose gamma-ray/X-ray/optical observatory platform, performing rapid "Target of Opportunity" observations of many transient astrophysical phenomena, such as supernovae.

[edit] Mission progress

UVOT "first light" picture (Credit: NASA/GFSC)
Enlarge
UVOT "first light" picture (Credit: NASA/GFSC)
  • Swift was launched on November 20, 2004, and reached a near-perfect orbit of 586x601 km altitude with an inclination of 20°.
  • On December 4 an anomaly occurred during instrument activation when the Thermo-Electric Cooler (TEC) Power Supply for the X-Ray Telescope did not turn on as expected. The XRT Team at Leicester and Penn State University was able to determine on December 8 that the XRT would be usable even without the TEC being operational. Additional testing on December 16 did not yield any further information as to the cause of the anomaly.
  • December 17 at 7:28:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located on-board an apparent gamma-ray burst during launch and early operations. The spacecraft did not autonomously slew to the burst since normal operation had not yet begun, and autonomous slewing was not yet enabled.
  • Swift had its first GRB trigger during a period when the autonomous slewing was enabled on January 17, 2005, at about 12:55 UT. It pointed the XRT telescope to the on-board computed coordinates and observed a bright source in the field of view.
  • On February 1, 2005 the mission team released the "first light" picture of the UVOT instrument and declared Swift operational.
  • As of December 1, 2005, Swift has detected more than 90 GRBs and X-ray afterglows for about 70 of them, and optical afterglows for about 20 of them (40 including ground-based optical observations).

[edit] Important events and results

  • On May 9, 2005, Swift detected the flash of two neutron stars crashing together, a burst of gamma rays that lasted one-twentieth of a second. The craft slewed to look towards the direction of the flash and about 53 seconds after the initial burst, the X-ray telescope caught the rapidly fading afterglow of the event. The detection marks the first time that the location of a short-duration gamma-ray burst has been identified. (Cowen, 2005)
  • On September 4, 2005, Swift detected GRB 050904 with a redshift value of 6.29 and a duration of 200 seconds (most of the detected bursts last about 10 seconds). It was also found to be the most distant at approximately 12.6 billion light years.
  • On February 18, 2006, Swift detected GRB 060218, an unusually long (about 2000 seconds) and nearby (about 440 million light years) burst which was unusually dim despite its close distance, and may be an indication of an imminent supernova.

[edit] References

Ron Cowen (2005). "Fleeting flash. Pinpointing a short gamma-ray burst". Science News 167 (20): 308.

[edit] External links

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