Swift is a first-of-its-kind multi-wavelength observatory dedicated to the study of gamma-ray burst (GRB) science. Its three instruments work together to observe GRBs and afterglows in the gamma-ray, X-ray, optical, and ultraviolet wavebands. Swift, part of NASA™s medium explorer (MIDEX) program, was developed by an international collaboration. It was launched into a low-Earth orbit on a Delta 7320 rocket on November 20th, 2004. In 8 months Swift has already observed 45 GRBs, and during its nominal 2-year mission is expected to observe more than 200 bursts, which will represent the most comprehensive study of GRB afterglows to date.

  • Determine the origin of gamma-ray bursts.
  • Classify gamma-ray bursts and search for new types.
  • Determine how the blastwave evolves and interacts with the surroundings.
  • Use gamma-ray bursts to study the early universe.
  • Perform a sensitive survey of the sky in the hard X-ray band.
Swift has a complement of three co-aligned instruments for studying gamma-ray bursts and their afterglow: the Burst Alert Telescope (BAT), the Xray Telescope (XRT), and the Ultraviolet/Optical Telescope (UVOT). The largest instrument on-board Swift is the BAT, which can view approximately a sixth of the entire sky at one time. It will detect approximately 100 or more gamma-ray bursts per year. Within seconds of detecting a burst, the spacecraft will "swiftly" and autonomously repoint itself to aim the XRT and UVOT at the burst to enable high-precision X-ray and optical positions and spectra to be determined. The positions will then be relayed to the ground for use by a network of observers at other telescopes. Swift will determine redshifts for most of the bursts that it detects (allowing scientists to know how far away they are and how absolutely bright they are), and will also provide detailed multi-wavelength light curves for the duration of the afterglow (allowing scientists to probe the physical environment in which the event took place). Key data taken by Swift will be relayed to the ground in near real-time, allowing the GRB Coordinate Network (GCN) to immediately distribute it to the world via the internet for follow-up observations and study. Swift will also use the BAT to perform an all-sky survey of low-energy gamma-rays that will be significantly more sensitive than any previous survey.


Swift, unlike most NASA satellites, it not an acronym. Instead Swift is named for a bird of the same name. This bird is special because it can change angles very quickly in mid-flight, like Swift will do. For more information on how Swift got its name visit this link to Goddard's Science Question of the Day about it.

The Mission Operations Center (MOC) at Penn State University provides real-time command and control of the spacecraft and monitors the observatory, while also taking care of science and mission planning, Targets of Opportunity (ToO) handling, and data capture and accounting. The Italian Space Agency™s ground station at Malindi, Kenya provides the primary communications. Swift burst alerts and burst characteristics are relayed almost instantaneously through the NASA TDRSS space data link to the GCN for rapid distribution to the community.

Swift data will be made available to the world via three different data centers located in the United States (the High Energy Astrophysics Science Archive Research Center, HEASARC), the UK (the UK Swift Science Data Center, UKSSDC), and Italy (the Italian Swift Archive Center, ISAC).

The Swift Science Center (SSC) assists the science community in fully utilizing the Swift data. It is also responsible for coordinating the development of the data analysis tools for Swift data. The BAT instrument team and the Italian Swift Archive Center will develop data analysis tools for the BAT and XRT data respectively. The Swift Science Center is responsible for developing the UVOT tools.

Swift mission details
  • Launch Date: November 20th, 2004
  • Prime Mission Duration: 2 years
  • Launcher: Delta II (7320)
  • Orbit: LEO 600 km circular
  • Orbital Life: 7 years
  • Inclination: 22 degrees
  • Dimentions:18.5 feet tall x 17.75 feet wide.
  • Spacecraft Partner: Spectrum Astro
  • Peak Slew Rate: 50 degrees in < 75 seconds
  • Arrival: Within 3 arcmin of target
  • Operations and Pointing: Autonomous
  • Uplink/Downlink: Dual Path, 2 kbps GRB alert downlink and uplink real-time using TDRSS MA link, 2.25 Mbps data rat for store and dump using Malindi-ASI seven orbits per day
GRB Detection Guidelines
  • 0s: GRB detection
  • 20s: Slew Begins/BAT approximate location distributed
  • ~ 50s: GRB acquired
  • 70s: XRT location distributed
  • 240s: UVOT finding chart distributed
  • 300s: XRT light curve distributed
  • 1200s: XRT spectrum distributed
  • ~ 60,000s: All automated observations complete (20,000 sec exposure)




The Burst Alert Telescope

The Burst Alert Telescope (BAT) is Swift's GRB watchdog. It keeps a constant eye on the gamma-ray sky, waiting for a sudden, bright burst of gamma-rays. Once a GRB is discovered, the BAT quickly calculates its position, so the spacecraft can slew to point the X-Ray Telescope and Ultra-Violet/Optical Telescope at the new burst.

  • Aperture: Coded Mask
  • Detecting Area: 5200 cm2
  • Detector: CdZnTe
  • Detector Operation: Photon Counting
  • Field of View: 2.0 sr (partially coded)
  • Detection Elements: 256 modules of 128 elements
  • Detector Size: 4mm x 4mm x 2mm
  • Telescope PSF: 17 arcminutes
  • Location Accuracy: 1-4 arcminutes
  • Energy Range: 15-150 keV
  • Burst Detection Rate: > 100 bursts/year

How it Works

Since it is not possible to focus gamma-rays with current technology, the BAT uses a technique called coded aperture imaging to image and localize incoming gamma-rays. A coded aperture is a mask positioned in front of the gamma-ray detectors. Swift's coded aperture mask sits 1-meter from the detector plane, and is made from about 54,000 lead tiles arranged in a random half-open/half-closed pattern. Each tile measures 5 x 5 x 1 mm. While the overall tile pattern is random, computers know the position of each tile. When gamma-rays come through the BAT, the lead tiles stop some, while the open areas let others through to the detector. In this way, the mask casts a shadow on the detector plane. Using the position of the shadow, the computers can determine the direction of the gamma-ray source.

Diagram of a coded aperture mask. See text above for description. Image of a technician inspecting the Swift coded aperture mask.

The BAT detector plane consists of 32,768 pieces of 4 x 4 x 2 mm cadmium-zinc-telluride (CZT). These CZT detectors are arranged into a four-tiered hierarchical structure. Arrays of 8 x 16 CZT detectors are arranged by twos into detector modules. Eight such modules are combined to form a detector block, and sixteen blocks make up the entire detector plane. This hierarchial structure, along with the coded-aperture imaging, allows the BAT to continue operations even with individual pixels, whole arrays, modules or even blocks missing.

Swift detector module. See decription in text above.

Swift Detector Module: on the right you can see the CZT detectors that form one 8 x 16 array, while the left half is open to show some of the electronics.


When the BAT detects a gamma-ray count rate significantly above the background rate, it triggers a burst detection. The BAT then calculates a 5 arcminute burst location within about 10 seconds of the trigger. This location, along with the burst's intensity, is transmitted to the ground and immediately distributed to all interested parties through the GRB Coordinates Network (GCN).

The BAT's secondary science includes an all-sky hard X-ray survey. While the BAT watches the sky for a GRB, it accumulates maps every 5 minutes in hard X-rays. Over Swift's lifetime, these images will result in an all-sky survey 20 times more sensitive than the last such survey by HEAO-1 A4 in the late 1970s. While performing this survey, the BAT watches for hard X-ray transient sources, which are distributed to the scientific community in the same manner as GRBs.

The X-Ray Telescope

The X-Ray Telescope (XRT) is one of Swift's two narrow-field instruments. Once the BAT detects a gamma-ray burst (GRB), the spacecraft slews to bring the new GRB into the XRT's field-of-view.

  • Telescope: Wolter I
  • Detector: XMM EPIC CCD
  • Effective Area: 135 cm2 at 1.5 keV
  • Detector Operation: Photon Counting, Integrated Imaging, and Rapid Timing
  • Field of View: 23.6 x 23.6 arcminutes
  • Detection Element: 600 x 600 pixels
  • Pixel Scale: 2.36 arcsec/pixel
  • Telescope PSF: 18 arcsec HPD at 1.5 keV
  • Location Accuracy: 3-5 arcseconds
  • Energy Range: 0.2-10 keV
  • Sensitivity: 2 x 10-14 ergs cm-2 s-1 in 104 sec

How it Works

While X-rays can be focused (unlike gamma-rays), it is more difficult than focusing optical light. X-rays tend to pass through most things. For example, when a doctor takes an "X-ray" of your arm, she shoots X-rays through your arm onto a film. Some parts of your arm are more likely to stop those X-rays, like the bone. Other parts of your arm, like the muscles and soft tissue, don't stop the X-rays at all.

If you try to focus X-rays by putting a mirror directly in front of them, like you might focus optical light, the X-rays will pass right through the mirror! However, if those X-rays hit the mirror at a "grazing incidence", then they will be reflected. Grazing incidence means that the X-rays hit the mirror at an angle nearly parallel to the mirror.

An image showing the difference between optical and X-ray light incident on a mirror. See text for details.

This means that to focus X-rays, an X-ray telescope must use mirrors that lie nearly parallel to the telescope's line-of-sight. These mirrors are nested, one inside another to cover as much area as possible. The XRT mirrors were originally built and tested for use on the JET-X program.

The XRT mirror housing. The mirrors are nearly parallel to the telescope's line-of-sight and nested, one inside another.

The mirrors which will be used to focus X-rays onto the XRT's CCD.

X-rays coming into the XRT are focused onto a state-of-the-art CCD, which was originally designed for the XMM-Newton EPIC telescope. The CCD registers the time and energy of incoming X-rays.


The XRT refines the BAT localization to 5 arcsecond accuracy, and measures fluxes, spectra, and light curves of GRBs and afterglows. Emission or absorption features in the X-ray spectra may reveal information about the material surrounding the GRB source. Redshift measurements from the XRT spectra provide distances to observed GRBs. This is particularly exciting for the class of short GRBs (see "About GRBs"), since to-date there have been no redshift measurements for GRBs in this class.


The Ultra-Violet/Optical Telescope

The Ultra-Violet/Optical Telescope (UVOT) is the other of Swift's two narrow-field instruments. As with the XRT, once the BAT detects a gamma-ray burst (GRB), the spacecraft slews to bring the new GRB into the UVOT's field-of-view.

  • Telescope: Modified Ritchey-Chrétien
  • Aperture: 30 cm diameter
  • F-number: 12.7
  • Detector: Intensified CCD
  • Detector Operation: Photon Counting
  • Field of View: 17 x 17 arcminutes
  • Detection Element: 2048 x 2048 pixels
  • Telescope PSF: 0.9 arcsec at 350 nm
  • Location Accuracy: 0.3 arcseconds
  • Wavelength Range: 170 nm - 650 nm
  • Colors: 6
  • Spectral Resolution (Grisms): ?/?? ~ 200 at 400 nm
  • Sensitivity: B = 24 in white light in 1000 sec
  • Pixel Scale: 0.48 arcseconds
  • Bright Limit: mv = 7 mag

How it Works

The UVOT's design is based on the Optical Monitor aboard XMM-Newton. It is a 30-cm reflecting telescope. Optical and ultraviolet light entering the UVOT is directed to one of two redundant CCD detectors, each with an 11-position filter wheel. These filters consist of a blocked position for detector safety; a white light filter; a field magnifier; two grisms; U, B, and V filters; two broadband UV filters centered on 180 and 260 nm; and a narrow UV filter centered on 220 nm. These filters allow observations of spectra in the blue or UV band (using the grisms) and of different colors.

UVOT filter wheel and detector.

These are the filter wheels that will be on the UVOT
(one in front, the other in back)


The UVOT further improves the BAT and XRT localizations, giving a position to 0.3 arcsecond accuracy. Upon acquiring the GRB, the UVOT performs a preprogrammed series of exposures and filter combinations. This program can be altered during Swift's mission to optimize the optical/UV observations. The filtered observations reveal the behavior of the burst and afterglow over time in different colors. In addition, if the burst is at a redshift greater than one, these observations may also provide redshift measurements.


Swift Mission Background and Overview For over 30 years astrophysicists have puzzled over the origin of gamma-ray bursts (GRBs) -- brief but brilliant flashes of gamma-rays occurring about once per day at random locations in the sky.

Some progress was made in 1997 when BeppoSAX, an Italian X-ray satellite, discovered lingering X-ray emission from a GRB called afterglow. Afterglow has since been observed in optical and radio, as well. Since this discovery was made, telescopes observing all different wavelengths world-wide have scrambled to detect GRB afterglow as soon after a burst detection as possible.

In one remarkable case on January 23, 1999, a robotic optical telescope, ROTSE, was able to catch a GRB during the burst of gamma-ray emission. It saw an optical transient source, which would have been visible to the human eye (if you'd been looking at the right spot at exactly the right time).

Sequence of 6 images from the ROTSE observations of the fading optical counterpart of GRB 990123.

Another extraordinary observation took place on December 11, 2002, when a burst was imaged in optical a mere 65 seconds after the gamma-ray burst trigger by

RAPTOR, a system designed to observe optical transients. These are the only two bursts seen in optical so close in time to the burst's in gamma-ray emission. The left exposure in the image below shows the afterglow 65 seconds after the burst, while the right exposure was taken 9 minutes after the burst. The optical afterglow has noticeably faded. Also shown is one of the RAPTOR units.

RAPTOR Image of GRB 021211. See text above for details. Credit: P. Wozniak, W.T. Vestrand, et al., RAPTOR Project, LANL Swift is an innovative mission designed specifically for GRB science.

Swift's three instruments work together to observe GRBs and afterglows in the gamma-ray, X-ray and optical wavebands. The Burst Alert Telescope (BAT) monitors the entire sky to catch a GRB and calculate an initial position. Armed with the position, the Swift spacecraft autonomously slews to bring the GRB into the Swift's X-Ray Telescope (XRT) and Ultra-Violet/Optical Telescope (UVOT) fields-of-view within 90 seconds. All three telescopes watch the gamma-ray burst and afterglow unfold. Over the next several days Swift returns to the GRB to observe the afterglow's behavior over time. Swift is part of NASA's medium explorer (MIDEX) program. It was launched into a low-Earth orbit on a Delta 7320 rocket on November 20, 2004. During its nominal 2-year mission, Swift is expected to observe more than 200 bursts and afterglows, which will represent the most comprehensive study of GRB afterglow to date.


Choice of Orbit

Why Study Gamma Ray Bursts (GRBs)?

Gamma-ray bursts (GRBs) are the most powerful explosions the Universe has seen since the Big Bang. They are seen approximately once per day and are brief, but intense, fl ashes of gamma radiation. They come from all different directions of the sky and last from a few milliseconds to a few hundred seconds. So far scientists do not know what causes them. Do they signal the birth of a black hole in a massive stellar explosion? Are they the product of the collision of two neutron stars? Or is it some other exotic phenomenon that causes these bursts?

As the brightest sources that scientists observe, GRBs can be used to study the distant Universe. There is great hope that GRBs can show when the first stars were formed and what the gas and dust content of the Universe was at early times.

With Swift, scientists will now have a tool dedicated to answering these questions and solving the gamma-ray burst mystery. Its three instruments will give scientists the ability to scrutinize gamma-ray bursts like never before. Within seconds of detecting a burst, Swift will relay a burst’s location to ground stations, allowing both ground-based and space-based telescopes around the world the opportunity to observe the burst’s afterglow.

Detailed Scientific Objectives

In its identification and analysis of over 100 gamma-ray bursts a year, Swift has several key scientific objectives. These include obtaining a deeper understanding of the GRB explosion and its interaction with the surrounding medium and using the intense radiation from the blast to probe the early universe. The BATSE instrument aboard NASA’s Compton Gamma-Ray Observatory in the 1990s opened wide the field of GRB investigation. BATSE, short for the Burst and Transient Source Experiment, made it clear that GRBs come to us from all directions and hinted that they come from cosmological distances (an ap - preciable fraction of the size of the universe). All told, BATSE detected over 2,700 GRBs.

Then in 1997 a small Italian satellite called BeppoSAX discovered that GRBs glow in X-ray , radio and optical light for hours and days after the initial flash, a phenomenon called the afterglow. Scientists could determine distances to the bursts by studying the lingering after glow, and this revealed that GRBs are indeed cosmological in origin.

In 2000, NASA launched a satellite built by the Massachusetts Institute of Technology called HETE-2, the High-Energy Transient Explorer, dedicated to the GRB mystery. HETE-2 is still active today, and one of its landmark findings has been that at least some GRBs have their origin in massive star explosions that may ultimately form a black hole. Thus, there is the intriguing hint that some GRBs simultaneously signal both star death and black hole birth.

NASA’s Swift mission is designed to respond to GRBs faster than any satellite that has come before. Swift has three instruments: one to detect a GRB and to provide a good approximation of its location within seconds; and two instruments to pinpoint the location and to study the burst after glow in X-ray, ultraviolet and optical light. This unprecedented capability will allow astronomers to pursue a number of important scientific objectives:

Swift will determine what causes GRBs and whether there are different kinds of GRBs.

We now know that some GRBs arise from the explosion of massive stars. But each GRB is a little differ - ent, leading scientists to speculate that there may be several dif ferent types of progenitors. For example, some bursts last only for a few milliseconds while others last over a minute. The bulk of the GRBs seem to last between two and ten seconds, yet characteristics in their after glow light differ. Aside from star explosions, some GRBs, in theory, may erupt from spectacular mergers, such as collisions between two neutron stars or two black holes. Some theorists speculate that a burst of Hawking radiation, signaling the complete evaporation of a black hole, could resemble a GRB. In these scenarios, scientists expect the burst to last less than two seconds. Interestingly, scientists have only detected afterglows from long bursts, longer than two seconds; and it is from this analysis that we can say that some longer bursts origi - nate from massive star explosions. Detecting the afterglow of a short burst, if it exists, may reveal their nature.

To address these questions, Swift will:

• Observe hundreds of GRBs and afterglows. • Detect bursts shorter, longer and up to three times fainter than those detected by BATSE. • Identify the galaxies where the GRBs occur (called the host galaxies) and, by obtaining a precise arcsecond position, locate GRBs relative to the host galaxy. Swift will determine if the bursts al - ways occur in star forming (and star dying) regions. Measure the redshift distribution (a distance measurement) of detected bursts and see how features such as energy and luminosity relate to distance. • Analyze the local environment of GRBs by determining how dusty (or clean) the region is. Dust is the hallmark of star formation.

Determine how the explosion blast wave evolves and interacts with its surr oundings.

Gamma-ray bursts are the most powerful explosions known in the Universe, second only to the Big Bang. The energies involved are clearly far greater than anything that can be reproduced in a laboratory. Thus, each GRB is a “cosmic laboratory” that can reveal new insights into nuclear and fundamental physics. The powerful blast wave plows the interstellar medium, with some of its debris accelerated to near light speed. This blast wave heats the surrounding gas to ultrahigh temperatures and, perhaps, triggers new star formation. It is this blast wave’s interaction with itself and the surrounding medium, in fact, that creates the gamma rays that we ultimately see. Yet what is the extent of this blast wave, and how wide is the beam of gamma rays?

To address these questions, Swift will:

• Perform detailed multiwavelength observations starting immediately after the burst. • Monitor the afterglow frequently for days after the event at various multiwavelengths to reconstruct the evolution of the blast wave. • Search for X-ray line and edge features that are indicative of the elemental composition and struc - ture of the blast wave.

Some theorists suggest that a small portion of GRBs may originate the fi rst generation of stars in the Uni - verse. These stars are thought to be upwards of 100 to 1,000 times more massive than our Sun, far more massive than stars today. These stars would contain only hydrogen and helium from the Big Bang and not heavier elements created in subsequent generations of stars. If such stars existed, they surely would have died with a resounding explosion like a GRB. Swift, through its study of these ancient bursts, could theoretically map out early star formation. No current telescope is powerful enough to see these fi rst stars. Swift will see a number of distant GRBs, and each one will have traveled for billions of light years, il - luminating its path along the way.

In this regard, Swift will:

• Use GRBs as high-redshift (long-distance) beacons by studying their optical, ultraviolet and X-ray afterglow and providing precise positions for follow-up observations by powerful ground-based telescopes. • Use X-ray absorption to probe the intergalactic and cluster medium. That is, scientists can ascertain the contents of so-called empty (and optically dark) space by studying the X-ray light from a GRB afterglow that doesn’t reach us, blocked by invisible gas and dust. • Measure the Universe’s star formation rate out to greater distances than what is currently known. • Measure the “Lyman-alpha forest.” This is the sum of absorption lines seen in the optical spectra of quasars and other distant galaxies, essentially revealing the Universe’ s web-like structure of chains of galaxies and galaxy clusters.

When Swift isn’t detecting and analyzing bursts it will conduct a survey of the X-ray sky. The X-ray waveband comprises a wide chunk of the electromagnetic spectrum, far wider , for example, than the region we call optical light. Swift will concentrate on the “hard” or high-ener gy X-rays (which some sci - entists call soft gamma rays). This all-sky survey will be 20 times more sensitive than previous measure - ments. Scientists expect that Swift’s enhanced sensitivity relative to earlier surveys will uncover over 400 new supermassive black holes that are obscured at softer X-ray ener gies.



Orbital designed and manufactured the fully-redundant Swift spacecraft bus for NASA, and served in a leadership role at the Goddard Space Flight Center (GSFC) during instrument integration, environmental testing, launch, early orbit check-out, and initial mission operations. Orbital continues to provide sustaining engineering support to the mission.

Specifications Spacecraft

Launch Mass: 1,467 kg (3,234 lb.)
Solar Arrays: Two gimbaled, three panel, triple-junction GaAs/Ge cells, 2132 W EOL
Orbit: 600 x 600 km @ 20.6 ° inclination
Stabilization: 3-axis, zero momentum bias Pointing knowledge: 2.2 arcsec P/Y (3 s )
Data Storage: 32.0 Gbits Data Downlink: STDN/TDRSS, to 2.25 Mbps
Propulsion: None Mission Life: 2 year mission; 3 year design; 5 year goal
Current Status: Operational Launch Launch Vehicle: Delta II 7320-10
Launch Site: Cape Canaveral Air Force Station, Florida Date: November 20, 2004

Other News
Other News

Gas Molecules Identified in the Host Galaxy of a Gamma-ray Burst (GRB).

January 6, 2009

Astronomers combining data from NASA's Swift satellite, the W. M. Keck Observatory in Hawaii, and other facilities have, for the first time, identified gas molecules in the host galaxy of a gamma-ray burst (GRB).

The explosion, designated GRB 080607, occurred in June 2008. "This burst gave us the opportunity to 'taste' the star-forming gas in a young galaxy more than 11 billion light-years away," said University of California, Santa Cruz, professor Xavier Prochaska. The finding provides insight into star formation when the universe was about one-sixth its present age.

Gamma-ray bursts — the universe's most luminous explosions — create bright afterglows. Their light encodes information about the gas and dust it encounters on its way to Earth.

"We clearly see absorption from two molecular gases: hydrogen and carbon monoxide," Prochaska said. "Those are gases we associate with star-forming regions in our own galaxy." The team believes that the burst exploded behind a thick molecular cloud similar to those that spawn stars in our galaxy today.

Gamma rays from GRB 080607 triggered Swift's Burst Alert Telescope shortly after 2:07 a.m. EDT June 7, 2008. Swift calculated the burst's position, beamed the location to a network of observatories, and turned to study the afterglow.

That night, University of California, Berkeley, professor Joshua Bloom and graduate students Daniel Perley and Adam Miller were using the Low Resolution Imaging Spectrometer on the 10-meter Keck I Telescope in Hawaii. "Because afterglows fade rapidly, we really had to scramble when we received the alert," Perley said. "But in less than 15 minutes, we were on target and collecting data."

A pair of robotic observatories also responded quickly. The NASA-supported Peters Automated Infrared Imaging Telescope (PAIRITEL) on Mount Hopkins, Arizona, and the Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory on Mount Hamilton, California, observed the burst's afterglow within 3 minutes of Swift's alert.

The spectrum from Keck established that the explosion took place 11.5 billion light-years away. GRB 080607 blew up when the universe was just 2.2 billion years old.

The molecular cloud in the burst's host galaxy was so dense, less than 1 percent of the afterglow's light was able to penetrate it. "Intrinsically, this afterglow is the second brightest ever seen," Prochaska said. "That's the only reason we were able to observe it at all."

Screening from thick molecular clouds provides a natural explanation for so-called "dark bursts," which lack associated afterglows. "We suspect that previous events like GRB 080607 were just too faint to be observed," said team member Yaron Sheffer of the University of Toledo, Ohio.

Nearly half of the absorption lines found in the Keck spectrum are unidentified. The team expects that understanding them will provide new data on the simplest space molecules.

Prochaska and Sheffer presented the findings today at the 213th meeting of the American Astronomical Society in Long Beach, California. A paper describing the results will appear in a future issue of Astrophysical Journal Letters.

Most gamma-ray bursts occur when massive stars run out of nuclear fuel. As the star's core collapses into a black hole or neutron star, gas jets punch through the star and into space. Bright afterglows occur as the jets heat gas that was previously shed by the star. Because a massive star lives only a few tens of millions of years, it never drifts far from its natal cloud.

Swift Reveals Active Galaxies are Different Near and Far

An ongoing X-ray survey undertaken by NASA's Swift spacecraft is revealing differences between nearby active galaxies and those located about halfway across the universe. Understanding these differences will help clarify the relationship between a galaxy and its central black hole.

"There's a lot we don't know about the workings of supermassive black holes," said Richard Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Astronomers think the intense emission from the centers, or nuclei, of active galaxies arises near a central black hole containing more than a million times the Sun's mass. "Some of these feeding black holes are the most luminous objects in the universe. Yet we don't know why the massive black hole in our own galaxy and similar objects are so dim."

NASA's Swift spacecraft is designed to hunt gamma-ray bursts. But in the time between these almost-daily cosmic explosions, Swift's Burst Alert Telescope (BAT) scans the sky. The survey is now the largest and most sensitive census of the high-energy X-ray sky.

Mushotzky presented a progress report January 6 on the BAT Hard X-ray Survey at the American Astronomical Society meeting in Long Beach, California. "The BAT sees about half of the entire sky every day," he said. "Now we have cumulative exposures for most of the sky that exceed 10 weeks."

Galaxies that are actively forming stars have a distinctly bluish color ("blue and booming"), while those not doing so appear quite red. Nearly a decade ago, surveys with NASA's Chandra X-Ray Observatory and the European Space Agency's XMM-Newton showed that active galaxies some 7 billion light-years away were mostly massive "red and dead" galaxies in normal environments.

The BAT survey looks much closer to home, within about 600 million light-years. There, the colors of active galaxies fall midway between blue and red. Most are spiral and irregular galaxies of normal mass, and more than 30 percent are colliding. "This is roughly in line with theories that mergers shake up a galaxy and 'feed the beast' by allowing fresh gas to fall toward the black hole," Mushotzky said.

Until the BAT survey, astronomers could never be sure they were seeing most of the active galactic nuclei. An active galaxy's core is often obscured by thick clouds of dust and gas that block ultraviolet, optical, and low-energy ("soft") X-ray light. Dust near the central black hole may be visible in the infrared, but so are the galaxy's star-forming regions. And seeing the black hole's radiation through dust it has heated gives us a view that is one step removed from the central engine. "We're often looking through a lot of junk," Mushotzky said.

But "hard" X rays — those with energies between 14,000 and 195,000 electron volts — can penetrate the galactic gunk and allow a clear view. Dental X rays work in this energy range.

Unlike most telescopes, the Swift's BAT contains no optics to focus incoming radiation. Instead, images are made by analyzing the shadows cast by 52,000 randomly placed lead tiles on 32,000 hard X-ray detectors.

Astronomers think that all big galaxies have a massive central black hole, but less than 10 percent of these are active today. Active galaxies are thought to be responsible for about 20 percent of all energy radiated over the life of the universe, and are thought to have had a strong influence on the way structure evolved in the cosmos.

Cosmic distance record smashed

April 28, 2009

The Swift satellite has found a gamma-ray burst from a star that died when the universe was 640 million years old, or less than 5 percent of its present age. The event, called GRB 090423, is the most distant cosmic explosion ever seen and gives astronomers an insight into the early universe. The international team, led by United Kingdom and United States astronomers, announced the discovery today.

"This is the most remote gamma-ray burst ever detected, and also the most distant object ever discovered - by some way," said Nial Tanvir of the University of Leicester.

"At its most basic level, this discovery tells us that there were massive stars at this moment in cosmic history," said Andrew Levan of the University of Warwick. "Equally important, we can use events like this to probe how the universe evolves when it is less than 5 percent of its current age."

"The burst most likely arose from the explosion of a massive star," said Derek Fox at Penn State University in University Park. "We're seeing the demise of a star - and probably the birth of a black hole - in one of the universe's earliest stellar generations."

"Swift was designed to catch these very distant bursts," said Neil Gehrels, Swift lead scientist at NASA's Goddard Space Flight Center, Greenbelt, Maryland. "We've waited 5 years, and we finally have one."

On 23rd April, Swift satellite detected a 10-second-long gamma-ray burst of modest brightness. It quickly pivoted to bring its Ultraviolet/Optical and X-Ray telescopes to bear on the burst location. Swift saw a fading afterglow in X rays but no corresponding glow in visible light.

"That alone suggested this was a very distant object," said Fox. Beyond a certain distance, the expansion of the universe shifts all optical emission into longer infrared wavelengths. While a star's ultraviolet light could be similarly shifted into the visible region, UV-absorbing hydrogen gas grows thicker at earlier times. "If you look far enough away, you can't see visible light from any object," he said.

Twenty minutes after the burst, Tanvir and his colleagues detected an infrared source at the Swift position using the Science and Technology Facilities Council's United Kingdom Infrared Telescope (UKIRT) on Mauna Kea, Hawaii. "Burst afterglows provide us with the most information about the exploded star and its environs," Tanvir said. "But we have to target afterglows quickly because they fade out so fast."

"We have worked hard to implement a rapid-response system for events just such as this," said Gary Davis, director of UKIRT. "It is rewarding to see it used so spectacularly."

Shortly after, Fox led an effort to obtain infrared images of the afterglow using the Gemini North Telescope on Mauna Kea. The source appeared in longer-wavelength images, but was absent in an image taken at the shortest wavelength (1 micron). The drop-out corresponded to a burst distance of about 13 billion light-years.

As Fox spread the word about the record distance, telescopes around the world slewed toward GRB 090423 to observe the afterglow before it faded away.

Follow-up observations made by two teams reached the same conclusion, using different observatories - the burst was a record-breaker! At the Galileo National Telescope on La Palma in the Canary Islands, a team including Guido Chincarini at the University of Milan-Bicocca, Italy, determined that the afterglow's redshift was 8.2. Tanvir's team measured the same redshift of 8.2 which equates to looking back 13 billion years in time, using the European Southern Observatory's Very Large Telescope (VLT) on Cerro Paranal in Chile.

Gamma-ray bursts are the universe's most luminous explosions. Most occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets - driven by processes not fully understood - punch through the star and blast into space. There they strike gas previously shed by the star and heat it, which generates short-lived afterglows in other wavelengths.

The previous record holder was a burst with a redshift of 6.7, which places it 180 million light-years closer than GRB 090423.

Swift Satellite Records Early Phase of Gamma-Ray Burst

March 2, 2009

United Kingdom astronomers, using a telescope aboard the NASA Swift satellite, have captured information from the early stages of a gamma-ray burst - the most violent and luminous explosions occurring in the universe since the Big Bang.

Swift is able to both locate and point at gamma-ray bursts (GRBs) far quicker than any other telescope. By using its Ultra-Violet/Optical Telescope (UVOT) the astronomers were able to obtain an ultraviolet spectrum of a GRB just 251 seconds after its onset - the earliest ever captured. Further use of the instrument in this way will allow them to calculate the distance and brightness of GRBs within a few hundred seconds of their initial outburst and gather new information about the causes of bursts and the galaxies they originate from.

It is currently thought that immense explosions following the collapse of the core of a rapidly rotating, high-mass star into a black hole cause some GRBs, but many mysteries still surround these events.

"The UVOT's wavelength range, coupled with the fact that Swift is a space observatory with a speedy response rate, unconstrained by time of day or weather, has allowed us to collect this early ultraviolet spectrum," said Martin Still from the Mullard Space Science Laboratory (MSSL) at University College London (UCL).

"By looking at these earlier moments of gamma-ray bursts, we not only will be able to better calculate things such as the luminosity and distance of a burst, but also to find out more about the galaxies that play host to them and the impact these explosions have on their environments," Paul Kuin said, also from MSSL, who works on the calibration of the UVOT instrument. "Once this new technique is applied to much brighter bursts, we'll have a wealth of new data."

Massimiliano De Pasquale, a GRB scientist of the UVOT team from MSSL, said, "The UVOT instrument is particularly suited to study bursts with an average to high redshift - a part of the ultraviolet spectrum that is difficult for even the very big ground-based telescopes to study. Using UVOT with Swift, we can now find redshifts for bursts that were difficult to capture in the past and find out more about their distant host galaxies, about ten billion light years away."

Since its launch in 2004, the Swift satellite has provided the most comprehensive study so far of GRBs and their afterglows. Using the UVOT to obtain ultraviolet spectrums, the Swift team will be able to build on this study and even determine more about the host galaxies' chemistry.

"The new spectrum has not only allowed us to determine the distance of the gamma-ray burst's host galaxy, but also has revealed the density of its hydrogen clouds," said Paul Kuin. "Learning more about these far-away galaxies helps us understand how they formed during the early universe. The gamma-ray burst observed on this occasion originated in a galaxy 8 billion light years from Earth."

Fireworks From a Flaring Gamma-ray Star

February 10, 2009

Astronomers using NASA's Swift satellite and Fermi Gamma-ray Space Telescope are seeing frequent blasts from a stellar remnant 30,000 light-years away. The high-energy fireworks arise from a rare type of neutron star known as a soft-gamma-ray repeater. Such objects unpredictably send out a series of X-ray and gamma-ray flares.

"At times, this remarkable object has erupted with more than a hundred flares in as little as 20 minutes," said Loredana Vetere, who is coordinating the Swift observations at Pennsylvania State University. "The most intense flares emitted more total energy than the Sun does in 20 years."

The object, which has long been known as an X-ray source, lies in the southern constellation Norma. During the past two years, astronomers have identified pulsing radio and X-ray signals from it. The object began a series of modest eruptions October 3, 2008, and then settled down. It roared back to life January 22, 2009, with an intense episode.

Because of the recent outbursts, astronomers will classify the object as a soft-gamma-ray repeater — only the sixth known. In 2004, a giant flare from another soft-gamma-ray repeater was so intense it measurably affected Earth's upper atmosphere from 50,000 light-years away.

Scientists think the source is a spinning neutron star, which is the super dense, city-sized remains of an exploded star. Although only about 12 miles (19 kilometers) across, a neutron star contains more mass than the Sun. The object has been cataloged as SGR J1550-5418.

While neutron stars typically possess intense magnetic fields, a subgroup displays fields 1,000 times stronger. These so-called magnetars have the strongest magnetic fields of any known object in the universe. SGR J1550-5418, which rotates once every 2.07 seconds, holds the record for the fastest-spinning magnetar. Astronomers think magnetars power their flares by tapping into the tremendous energy of their magnetic fields.

"The ability of Fermi's gamma-ray burst monitor to resolve the fine structure within these events will help us better understand how magnetars unleash their energy," said Chryssa Kouveliotou, an astrophysicist at NASA's Marshall Space Flight Center in Huntsville, Alabama. The object has triggered the instrument more than 95 times since January 22, 2009.

Using data from Swift's X-ray telescope, Jules Halpern at Columbia University, New York City, captured the first "light echoes" ever seen from a soft-gamma-ray repeater. Images acquired when the latest flaring episode began show what appear to be expanding halos around the source. Multiple rings form as Xrays interact with dust clouds at different distances, with closer clouds producing larger rings. Both the rings and their apparent expansion are an illusion caused by the finite speed of light and the longer path the scattered light must travel.

"Xrays from the brightest bursts scatter off of dust clouds between us and the star," Halpern said. "As a result, we don't really know the distance to this object as well as we would like. These images will help us make a more precise measurement and also determine the distance to the dust clouds."