GLAST Feature Article
June 5, 2008 10:1:52
GLAST - Opening A New Window On The Universe
By Sandra Frederick

 

This is an all sky gamma-ray survey as imaged by EGRET anoard NASA's Compton Gamma-ray Observatory. Photo Credit: NASA

On any given night, a naked eye can look up and see a dark sky peppered with bright stars twinkling, as the moon beams brightly back at Earth. Look closer and it is easy to pick out constellations – the Little Dipper and nearby, The Big Dipper – and if lucky, the international space station taking a sweep across the United States.

 But as much as we can envision the night sky here on Earth, there is another unseen one in deep space – the gamma-ray version where no human eye can scan its contents.

 “The sky wouldn’t be dark, there would be no way of knowing if it was night or day,” said David Thompson, GLAST deputy project scientist at NASA’s Goddard Space Flight Center. “But what you would notice is the Milky Way, brilliant swaths of light across the sky. It is constantly changing, some brighter lights then dimmer lights, then blinding flashes of gamma-ray light and then a gamma-ray burst.”

 But the problem is, Thompson said, up to this point the international scientific community has had limited glimpses of the gamma-ray sky through data sent back to Earth from other observatories, such as EGRET, sent to study gamma-rays. The spacecraft sent back data showing mysterious energy that appeared randomly and unpredictably throughout the sky, begging for more exploration, Charles Meegan, GLAST Burst Monitor principal investigator, said.

 So what makes a gamma-ray sky significantly different than we can view with our own eyes? It is extremely bright because gamma rays produce cosmic particles that blink -- pulsars – and rapidly spinning neutron stars, also rotating as they blink. And there are black holes added to the mix, Thompson explained.

 “It is like looking down the barrel of a gun filled with rapidly changing particles with gamma-ray bursts, extreme versions of exploding stars and black holes forming.”

 

This is GLAST. The key scientific objectives of the GLAST mission are: 

1. Explore the most extreme environments in the Universe, where nature harnesses energies far beyond anything possible on Earth.
2. Search for signs of new laws of physics and what composes the mysterious Dark Matter.
3. Explain how black holes accelerate immense jets of material to nearly light speed.
4. Help crack the mysteries of the stupendously powerful explosions known as gamma-ray bursts.
5. Answer long-standing questions across a broad range of topics, including  solar flares, pulsars, and the origin of cosmic rays.

Photo Credit: Robert Gass

The newest kid on the cosmic block of observatories is GLAST, the Gamma-ray Large Area Space Telescope, being launched June 8 from Canaveral Air Force Station in Cape Canaveral, Florida. NASA, along with partners from five countries (United States, Italy, Sweden, Japan and France) designed and built a $690 million observatory to gather information pertaining to the extreme side of the universe – gamma-rays, the highest energy form of light. Once that information is collected in the LAT instrument on GLAST, scientists from 18 institutions – 270 in all – will decipher it and be able to determine the origins of the energy and crack the mystery of stupendously powerful explosions known as gamma-ray burst. The mission has several other goals -- finding answers to supermassive black-hole systems, pulsars and the origins of cosmic rays. 

 “We are excited about this mission, it’s a great leap in capabilities for viewing the universe,” said Steve Ritz, GLAST project scientist with Goddard Space Flight Center. “We are expecting to find surprises.”

LAUNCH 

GLAST will launch on a Delta II 7920H-10 heavy rocket, which has nine graphite epoxy strap-on solid rocket motors integrated in a 10-foot-diameter composite payload fairing to protect the spacecraft during its ascent through the Earth’s atmosphere. Once launched, GLAST will reside in a low-earth orbit at about 350 miles, an inclination of approximately 24.7 degrees. After liftoff, the spacecraft separation is about 75 minutes. 

The observatory will orbit Earth every 90 minutes and be able to view the entire sky in just two orbits –or about three hours. Once it is in space, the KU-band antenna is deployed and the solar arrays are extended.

“We designed for five years, with a 10-year goal. There are no consumables that are being used up and it is based on solid state electronics so it will degrade gracefully,” Spitz said.

When the lifetime of GLAST is finished, it will reenter the atmosphere and what doesn’t burn up will fall into the Pacific Ocean, he added.

Because GLAST will not have to cruise to a planet, scientists can take their time to calibrate the instruments once the observatory gets into its spot in orbit. This also gives the team time to understand the environment where the observatory is located.

“During the first year of science a sky survey will take place and the information about gamma-ray bursts and super massive black holes will be released to the international science community.”

 

 Diagram of GLAST's Delta II H Booster Photo Credit: ULA

MAKINGS OF GAMMA RAY BURSTS

Gamma-ray bursts (GRBs) were discovered by American surveillance satellites in the late 1960s. These satellites were looking for gamma rays coming from possible clandestine Soviet nuclear tests, but instead found brief but intense flashes of gamma rays coming from random directions in space, according to NASA.

To this day GRBs remain one of the greatest mysteries of modern astronomy. Despite lasting only a few milliseconds to several minutes, they are the brightest gamma-ray phenomena known, outshining all other sources of gamma rays combined.

"An individual GRB can release in a matter of seconds the same amount of energy that our Sun will radiate over its 10-billion-year lifetime," GLAST Deputy Project Scientist Neil Gehrels of NASA's Goddard Space Flight Center, said.

Astronomers have made considerable strides in recent years in understanding GRBs, progress that can be directly atributed to a series of successful NASA missions. The Burst Transient Source Experiment (BATSE) on NASA's Compton Gamma-ray Observatory detected several thousand GRBs and showed that they come from random directions on the sky-which strongly suggested that they are not of galactic origin and must occur at great distances.

On March 19, 2008, Swift discovered a bright Gamma-Ray Burst (GRB 080319B). The image shows the X-ray afterglow as seen by the X-Ray Telescope (left) and the bright optical afterglow as observed by the Ultraviolet/Optical Telescope on board Swift. Credit: NASA/Swift/Stefan Immler

In the late 1990s, the Italian/Dutch BeppoSAX satellite was able to pinpoint the location of several GRBs, which enabled X-ray, optical, and radio telescopes to monitor their afterglows. This was a crucial development, since it enabled astronomers for the first time to measure distances to bursts and observe how they interacted with their surrounding environments.

The now-defunct HETE-2 satellite and the currently operating NASA Swift satellite have significantly extended and improved these capabilities, and have lofted our study of GRBs to new heights.

Thanks to these missions, astronomers now think that most GRBs, those lasting 2 seconds or longer (known as long GRBs), are associated with the explosive deaths of massive stars. As the star's core collapses at the end of its life, it forms a black hole or neutron star.

Computer simulations show that infalling stellar gas can tap the rotational energy of a rapidly spinning core, and magnetic fields can channel that material into two jets traveling at nearly the speed of light. These jets punch their way out of the dying star along its rotation axis. The gamma rays are produced by shock waves created either from material colliding within the jet, or from the jet slamming into surrounding material.

Still there are some GRBs that last less than 2 seconds – known as short GRBs – which may originate in a variety of ways. Most are produced by the merger of two neutron stars, or the merger of a black hole and a neutron star. But others may be triggered by the collapse of the core of a massive star into a black hole, the collapse of a neutron star into a black hole, and powerful flares from magnetars (highly magnetized neutron stars).

“Things are changing there all the time,” Spitz said a few days prior to the launch. “The thing that is most exciting about this mission is the science that is not on anybody’s list yet. We are going to get that with this mission.”

DESIGNING GLAST AND ITS INSTRUMENTS 

GLAST will study the cosmos in the photon energy range of 8,000 electronvolts (8 keV) to greater than 300 billion electronvolts (300 GeV). An electronvolt is a unit of energy close to that of visible light, so GLAST will catch photons with energies thousands to hundreds of billions of times greater than those we see with our eyes (1 keV = 1,000 eV, 1 MeV = 1,000,000 eV, 1 GeV = 1,000,000,000 eV).

GLAST carries two instruments: the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM). The LAT is GLAST’s primary instrument, and the GBM is the complementary instrument.

Following in the footsteps of the Comptom Gamma Ray Observatory (CGRO) and the Swift gamma-ray observatory, a new generation of spacecraft was designed with sunshades added to keep the light out of the field of view from obscuring the star field. It is similar to a visor a driver would flip down to block the sun coming through the windshield on a sunny day, Spitz said.

It also has a thermal blanket to protect the hardware from the harsh environment of space.

"GLAST is a multipurpose observatory designed to study many phenomena besides gamma-ray bursts, but it promises to greatly extend our knowledge of these incredibly powerful explosions," Peter Michelson of Stanford University, who is the Principal Investigator of GLAST's Large Area Telescope (LAT), said.

Based on knowledge of the gamma-ray sky from previous missions, scientists defined the following requirements for the GLAST instruments:

The LAT detects gamma rays by using Einstein’s famous E = mc2 equation in a technique known as pair production. When a gamma ray, which is pure energy, slams into a layer of tungsten in the detector, it can create a pair of subatomic particles (an electron and its antimatter counterpart, a positron). The direction of the incoming gamma ray is determined by projecting the direction of these particles back to their source using several layers of high-precision silicon tracking detectors. A separate detector, called a calorimeter, absorbs and measures the energy of the particles. Since the energy of the particles created depends on the energy of the original gamma ray, counting up the total energy determines the energy of that gamma ray. Because the LAT in orbit is bombarded by many more particles than gamma rays, it wears a "hat" – a third detector that produces a signal when a particle, but not a gamma ray, goes through it. The combination of no signal in this outer detector ("the dog that did not bark"), plus an electron-positron pair of tracks created inside the LAT, signals a gamma ray. Working one gamma ray at a time, the LAT will make gamma-ray images of astronomical objects, while also determining the energy for each detected gamma ray. Photo Credit: NASA

This is a photo of GLAST's LAT before it's protective Covers were installed. Photo Credit: NASA

 

Large Area Telescope

(1) Because the sky at gamma-ray energies has so many variable sources, the LAT must have a large field of view, over 2 steradians (one-fifth of the entire sky).
(2) To identify and study sources accurately, the LAT must be able to measure the locations of bright sources to within 1 arcminute (about 1/30 of the diameter of the full Moon).
(3) The study of gamma rays covers a broad energy range, so the LAT must catch photons with energies from 30 MeV to greater than 300 GeV. In particular, the LAT will have high sensitivity above 10 GeV, because almost nothing is known about cosmic objects at these energies.
(4) Since gamma-ray bursts can release a torrent of gamma rays within a fraction of a second, the LAT must be able to measure gamma rays over short time intervals.
(5) Because scientists need long observations to understand many types of sources, the LAT should be able to operate for many years without degradation.
(6) Because of the high flux of cosmic rays, which can mask the much smaller flux of gamma rays, the LAT must be able to reject 99.999% of signals generated by cosmic rays.

 

The GLAST Burst Monitor (GBM)

(1) Gamma ray bursts (GRBs) come from random directions of the sky, so the GBM must watch as much of the entire sky as possible at all times.

(2) To gain the most information about GRBs, the GBM should be able to measure photon energies over a wide range, down to 8 keV and up to energies that overlap the LAT energy range.

(3) Since GRBs last from mere microseconds to thousands of seconds, the GBM must be able to detect GRBs over a wide range of timescales.

THE INSTRUMENTS

Large Area Telescope (LAT)

The LAT has four subsystems that work together to detect gamma rays and to reject signals from the intense bombardment of cosmic rays. For every gamma ray that enters the LAT, it will have to filter out 100,000 to one million cosmic rays, charged particles that resemble the particles produced by gamma rays. The four main subsystems are:

    • Tracker
    • Calorimeter
    • Anticoincidence Detector
    • Data Acquisition System

The GLAST team specifically built the other science instrument, the GLAST Burst Monitor (GBM), to address this mystery.

 "The GBM will detect approximately 200 GRBs per year," Meegan said. "It’s amazing that gamma-ray bursts are so powerful that a small detector you could hold in one hand can observe them from distances of billions of light-years."

 The bursts can appear anywhere at anytime so the spacecraft can be repointed for every single burst that shows up, he said.

“We don’t do imaging, we detect bursts not by where they are, but when they are. They only last a few seconds, so the spacecraft can do that itself,” Meegan said.
 Meegan points out that some bursts actually exploded when the universe was less than a billion years old. 

"In fact, we may be seeing back to when the first stars formed," he said. "GRBs may thus turn out to be our best window to the infancy of the universe."  

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