The burst detection rate is 100 per year, with a sensitivity ~3 times fainter than the BATSE detector aboard the
Compton Gamma Ray Observatory. The Swift mission was launched with a nominal on-orbit lifetime of two years. Swift is a NASA MIDEX (medium-class Explorer) mission. It was the third to be launched, following
While originally designed for the study of gamma-ray bursts, Swift now functions as a general-purpose multi-wavelength observatory, particularly for the rapid followup and characterization of astrophysical transients of all types. As of 2020, Swift received 5.5 Target of Opportunity observing proposals per day, and observes ~70 targets per day, on average.
Based on continuous scans of the area of the sky with one of the instrument's 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
swift (bird of the same name). 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 organizations can submit proposals for observations.
The BAT detects GRB 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
arcminutes 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 (0.20 in)
lead tiles, 1 m (3 ft 3 in) above a detector plane of 32,768 4 mm (0.16 in)
Cadmium zinc telluride (CdZnTe) hard X-ray detector tiles; it is purpose-built for Swift. Energy range: 15–150
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.
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 resolution 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's Optical Monitor (OM) instrument, with improved optics and upgraded onboard processing computers.
In August 2017, UVOT imaged UV emissions from gravitational wave event
GW170817 detected by LIGO & Virgo detectors.
Burst Alert Telescope (BAT)
BAT (Burst Alert Telesope) is a gamma ray telescope, built by NASA's Goddard Space Flight Center, uses a coded aperture to locate the source. The software to locate the source is provided by the
Los Alamos National Laboratory (LANL). The CdZnTe detector of 5,200 cm2 (810 sq in) area, consisting of 32,500 units of 4 × 4 × 2 mm (0.157 × 0.157 × 0.079 in), can pin-point the location of sources within 1.4 arcminutes. The energy range is 15-150 keV.
Ultraviolet/Optical Telescope (UVOT)
UVOT (Ultraviolet/Optical Telescope) monitors the afterglow in ultraviolet and visible light, and locates the source at an accuracy of one arcsecond. Its aperture is 30 cm (12 in), with an f-number equal to 12.7, and is backed by 2048 x 2048
photon counting CCD
pixels. The source location accuracy is better than one arcsecond.
X-Ray Telescope (XRT)
XRT (X-Ray Telescope) aims at the source more accurately, and monitors the afterglow in X-rays. It was built jointly by the
Pennsylvania State University (PSU), the
Brera Astronomical Observatory, Italy, and the
University of Leicester, United Kingdom. It has a detector of area 135 cm2 (20.9 sq in) consisting of 600 x 600 pixels, and covers the energy range of 0.2-10 keV. It can locate the afterglow source at an accuracy of four arcseconds.
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
On 4 December 2004, 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 University of Leicester and Pennsylvania State University were able to determine on 8 December 2004 that the XRT would be usable even without the TEC being operational. Additional testing on 16 December 2004 did not yield any further information as to the cause of the anomaly.
On 17 December 2004 at 07:28:30 UTC, 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 17 January 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 1 February 2005, the mission team released the
first light picture of the UVOT instrument and declared Swift operational.
By May 2010, Swift had detected more than 500 GRBs.
By October 2013, Swift had detected more than 800 GRBs.
On 27 October 2015, Swift detected its 1,000th GRB, an event named GRB 151027B and located in the constellation
On 10 January 2018, NASA announced that the Swift spacecraft had been renamed the Neil Gehrels Swift Observatory in honor of mission PI
Neil Gehrels, who died in early 2017.
9 May 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.
4 September 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
18 February 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
14 June 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.
8 and 13 February 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
19 March 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.
13 September 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.
23 April 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.
29 April 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 2010: Swift tied its record by again detecting and localizing four bursts in a single day.
28 March 2011: Swift detected Swift J1644+57 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. "This is truly different from any explosive event we have seen before", said
Joshua Bloom of the
University of California, Berkeley, the lead author of the study published in the June issue of Science.
16 and 17 September 2012: BAT triggered two times on a previously unknown hard X-ray source, named
Sw J1745-26, a few degrees from the
Galactic Center. The outburst, produced by a rare X-ray nova, announced the presence of a previously unknown stellar-mass black hole undergoing a dramatic transition from the low/hard to the high/soft state.
2013: Discovery of ultra-long class of gamma-ray bursts
24 April 2013: Swift detected an X-ray flare from the Galactic Center. This proved not to be related to
Sgr A* but to a previously unsuspected
magnetar. Later observations by the
NuSTAR and the
Chandra X-ray Observatory confirmed the detection.
27 April 2013: Swift detected the "shockingly bright" Gamma-ray burst
GRB 130427A. Observed simultaneously by the
Fermi Gamma-ray Space Telescope, it is one of the five closest GRBs detected and one of the brightest seen by either space telescope.
3 June 2013: Evidence for kilonova emission in short GRB
23 April 2014: Swift detected the strongest, hottest, and longest-lasting sequence of stellar flares ever seen from a
nearby red dwarf star. The initial blast from this record-setting series of explosions was as much as 10,000 times more powerful than the largest solar flare ever recorded.
3 May 2014: Detection of a UV Pulse from an iPTF discovered young Type Ia SN
Launches are separated by dots ( • ), payloads by commas ( , ), multiple names for the same satellite by slashes ( / ).
Cubesats are smaller. Crewed flights are underlined. Launch failures are marked with the † sign. Payloads deployed from other spacecraft are (enclosed in parentheses).