Swift Gamma-Ray Burst Mission

The Swift Gamma-Ray Burst Mission consists of a robotic spacecraft called Swift, which was launched into orbit on 20 November 2004, 17:16:00 UTC on a Delta II 7320-10C expendable launch vehicle. Swift is managed by the NASA Goddard Space Flight Center, and was developed by an international consortium from the United States, United Kingdom, and Italy. It is part of NASA's Medium Explorer Program (MIDEX).

Overview

Swift is a multi-wavelength space-based observatory dedicated to the study of gamma-ray bursts (GRBs). Its three instruments work together to observe GRBs and their afterglows in the gamma-ray, X-ray, ultraviolet, and optical wavebands.

Based on continuous scans of the area of the sky which one of the instruments monitors, Swift uses momentum wheels to autonomously slew into the direction of possible GRBs. The name "Swift" is not a mission-related acronym, but rather a reference to the instrument's rapid slew capability, and the nimble bird of the same name.^[2] All of Swift's discoveries are transmitted to the ground and those data are available to other observatories which join Swift in observing the GRBs.

In the time between GRB events, Swift is available for other scientific investigations, and scientists from universities and other organisations can submit proposals for observations.

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 at the Broglio Space Centre near Malindi on the coast of Eastern Kenya, and is operated by the Italian Space Agency. The Swift Science Data Center (SDC) and archive are located at the Goddard Space Flight Center outside Washington D.C. The UK Swift Science Data Centre is located at the University of Leicester.

The Swift spacecraft bus was built by Spectrum Astro, which was later acquired by General Dynamics Advanced Information Systems.^[3]

Instruments

Burst Alert Telescope (BAT)

The BAT detects GRBs events and computes its coordinates in the sky. It covers a large fraction of the sky (over one steradian fully coded, three steradians partially coded; by comparison, the full sky solid angle is 4π or about 12.6 steradians). 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, and some wide-field, rapid-slew ground-based telescopes can catch the GRB with this information. The BAT uses a coded-aperture mask of 52,000 randomly placed 5 mm lead tiles, 1 metre above a detector plane of 32,768 four mm CdZnTe hard X-ray detector tiles; it is purpose-built for Swift. Energy range: 15–150 keV.^[4]

X-ray Telescope (XRT)

The XRT can take images and perform spectral analysis of the GRB afterglow. This provides more precise location of the GRB, with a typical error circle of approximately 2 arcseconds radius. The XRT is also used to perform long-term monitoring of GRB afterglow light-curves for days to weeks after the event, depending on the brightness of the afterglow. The XRT uses a Wolter Type I X-ray telescope with 12 nested mirrors, focused onto a single MOS charge-coupled device (CCD) similar to those used by the XMM-Newton EPIC MOS cameras. On-board software allows fully automated observations, with the instrument selecting an appropriate observing mode for each object, based on its measured count rate. The telescope has an energy range of 0.2 - 10 keV.^[5]

Ultraviolet/Optical Telescope (UVOT)

After Swift has slewed towards a GRB, the UVOT is used to detect an optical afterglow. The UVOT provides a sub-arcsecond position and provides optical and ultra-violet photometry through lenticular filters and low -esolution spectra (170–650 nm) through the use of its optical and UV grisms. The UVOT is also used to provide long-term follow-ups of GRB afterglow lightcurves. The UVOT is based on the XMM-Newton mission's Optical Monitor (OM) instrument, with improved optics and upgraded onboard processing computers.^[6]

On November 9, 2011, UVOT photographed the asteroid 2005 YU₅₅ as the asteroid made a close flyby of the Earth.^[7]

Mission goals

The Swift mission has four key scientific objectives:

• To 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.
• To use GRBs to expand understanding of the young universe. GRBs seem to take place at "cosmological distances" of many millions or billions of light-years, which means they can be used to probe the distant, and therefore young, cosmos.
• To conduct an all-sky survey which will be more sensitive than any previous one, and will add significantly to scientific knowledge of astronomical X-ray sources. Thus, it could also yield unexpected results.
• To serve 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.

Mission history

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 were 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.

On December 17 at 07:28:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located on-board an apparent gamma-ray burst during launch and early operations.^[8] 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 UTC. It pointed the XRT telescope to the on-board computed coordinates and observed a bright X-ray 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 May 2010, Swift has detected more than 500 GRBs, X-ray afterglows for more than 90% of them, and optical afterglows for more than 50% of them.^[9]

Notable detections

• May 9, 2005: Swift detected GRB 050509B, a burst of gamma rays that lasted one-twentieth of a second. The detection marked the first time that the accurate location of a short-duration gamma-ray burst had been identified and the first detection of X-ray afterglow in an individual short burst.^[10]

• 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 yet detected, at approximately 12.6 billion light-years.

• 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.

• June 14, 2006: Swift detected GRB 060614, a burst of gamma rays that lasted 102 seconds in a distant galaxy (about 1,6 Billion light years). No supernova was seen following this event (and GRB 060505 to deep limits) leading some to speculate that it represented a new class of progenitors. Others suggested that these events could have been massive star deaths, but ones which produced too little radioactive ⁵⁶Ni to power a supernova explosion.

• January 9, 2008: Swift was observing a supernova in NGC 2770 when it witnessed an X-ray burst coming from the same galaxy. The source of this burst was found to be the beginning of another supernova, later called SN 2008D. Never before had a supernova been seen at such an early stage in its evolution. Following this stroke of luck (position, time, most appropriate instruments), astronomers were able to study in detail this Type Ibc supernova with the Hubble Space Telescope, the Chandra X-ray Observatory, the Very Large Array in New Mexico, the Gemini North telescope in Hawaii, Gemini South in Chile, the Keck I telescope in Hawaii, the 1.3m PAIRITEL telescope at Mt Hopkins, the 200-inch and 60-inch telescopes at the Palomar Observatory in California, and the 3.5-meter telescope at the Apache Point Observatory in New Mexico. The significance of this supernova was likened by discovery team leader Dr. Alicia Soderberg to that of the Rosetta Stone for egyptology.^[11]

• February 8 and February 13, 2008: Swift provided critical information about the nature of Hanny's Voorwerp, mainly the absence of an ionizing source within the Voorwerp or in the neighboring IC 2497.

• March 19, 2008: Swift detected GRB 080319B, a burst of gamma rays amongst the brightest celestial objects ever witnessed. At 7.5 billion light-years, Swift established a new record for the farthest object (briefly) visible to the naked eye. It was also said to be 2.5 million times intrinsically brighter than the previous brightest accepted supernova (SN 2005ap). Swift observed a record four GRBs that day, which also coincided with the death of noted science-fiction writer Arthur C. Clarke.^[12]

• September 13, 2008: Swift detected GRB 080913, at the time the most distant GRB observed (12.8 billion light-years) until the observation of GRB 090423 a few months later.^[13][14]

• April 23, 2009: Swift detected GRB 090423, the most distant cosmic explosion ever seen at that time, at 13.035 billion light-years. In other words, the universe was only 630 million years old when this burst occurred.^[15]

• April 29, 2009: Swift detected GRB 090429B, which was found by later analysis published in 2011 to be 13.14 billion light-years distant (approximately equivalent to 520 million years after the Big Bang), even farther than GRB 090423.^[16]

• March 16, 2010, Swift tied its record by again detecting and localizing four bursts in a single day.

• April 13, 2010: Swift detected its 500th GRB.^[17]

• March 28, 2011: Swift detected GRB 110328A, which subsequent analysis showed to possibly be the signature of a star being disrupted by a black hole or the ignition of an active galactic nucleus.^[18]"This is truly different from any explosive event we have seen before," said Joshua Bloom of the University of California at Berkeley, the lead author of the study published in the June issue of Science.^[19]

References

External links

• NASA Swift site at GSFC
• UK Swift site
• Swift Mission Director's Status Report Log
• Sonoma State University Swift website
• Penn State Swift Mission Operations Center site
• Gamma-ray Burst Real-time Sky Map based on Swift data
• Swift Short-Burst Discovery
• GCN entry for first GRB observed with auto-slew
• APOD page with movie of separation from rocket
• New Scientist: Swift measures distance to gamma-ray bursts