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Heavy metal hit parade could point scientists to source of gamma-ray bursts

Sept. 2, 1998: Blazars, pulsars, and bursts: a GLAST for blasts from the past. No, it's not the promotional billing for yet another summer hard rock reunion tour. Rather, it's all part of NASA's next wave of gamma-ray astronomy.

Right: A computer model depicts the large stack - comprising optical fibers and metal layers - sitting atop a satellite bus.

To witness the most energetic explosions in the universe, Dr. Geoffrey N. Pendleton of the University of Alabama in Huntsville (UAH) has proposed an instrument that will capture images of the universe in the shortest of short wavelength radiation - gamma rays. By definition, higher energy explosions produce shorter wavelength radiation, and gamma rays are energy waves that are hundreds of billions times shorter than visible light.

One of the major accomplishments of gamma-ray astronomy has been the detection of high-energy gamma rays from "blazars," a class of active galaxies. The gamma rays are thought to be produced in jets containing plasma moving close to the speed of light.

Pendleton, a gamma-ray astronomer working at NASA's Marshall Space Flight Center, is the principal investigator on a competitive proposal to provide technology for the next-generation gamma-ray telescope.

Left: An artist's concept of a two jets of matter erupting from a blazar .

NASA has chosen Pendleton's proposal - along with proposals from researchers at Stanford University and the University of California at Riverside - to develop technology for the Gamma-ray Large Area Space Telescope (GLAST). Pendleton's GLAST team comprises NASA/Marshall, UAH, and Washington University in St. Louis.

Special star appearances

Pendleton has prepared a preliminary design for an instrument that will help pin down the position and energy of blazars, bursters, and of other, steadier gamma-ray sources. A blazar is an active galactic nucleus with a black hole - more than a billion times as massive as our sun - at its center. As matter swirls into the hole, it erupts from the core into deep space and - by a mechanism that is not yet understood - generates intense blasts of gamma radiation.

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Gamma-ray bursters are a different cosmic character, brief, intense flashes of high-energy radiation that appear on average about once a day at an unpredictable time and place in the sky. BATSE's discovery of the random nature in which the bursts appear on the sky was the first indication to scientists that the bursts originate beyond our galaxy. Originally scientists believed that the bursts were limited to our galaxy, and expected to find most in the plane of the Milky Way. But the BATSE data and eventually redshift measurements enabled by observations from other gamma-ray telescopes and optical telescopes on the ground show that the bursts originate from the deepest parts of space, literally billions of light years away.

"This instrument is designed for measuring very high energy gamma-ray signals from throughout the universe," said Pendleton. The instrument, called a scintillating fiber detector system, will act as both a tracker and a calorimeter, observing the direction of the bursts while also measuring their energy level.

Current gamma-ray telescopes such as the Burst and Transient Source Experiment (BATSE) aboard NASA's Compton Gamma Ray Observatory launched in 1991 have led to a new understanding of the origin and distribution of matter in the universe. Gamma-ray bursts reveal information about objects and events occurring in the farthest parts of our universe.

The new instrument offers a wide angle look at the sky with an 160-degree field of view. That's 320 times the apparent diameter of the moon, or 41 percent of the total sky - about as wide as the human eye's field of view. Within this wide area of view, GLAST will be able to pinpoint energy source locations to within 0.08 to 0.008 degrees - 1/6 to 1/60 the apparent diameter of the moon.

Heavy metal smash hits

High-energy gamma rays are too energetic to be focused and detected the same as visible light or even X-rays. When they collide with dense matter, they produce intense showers of charged particles that flash - scintillate - as they pass through glass or plastic. Pendleton proposes building a 125-layer spaghetti sandwich - layers of optical fibers between thin sheets of lead or tantalum.

The upper part of the detector (right) is the tracker with 90 layers of fibers and metal.

The metal sheets intercept the incoming gamma rays, resulting in a showers of charged particles that are recorded as they light up the scintillating fibers. When the photons collide with the layers of metal, they produce negatively and positively charged particles that show up on the scintillating fibers. The fibers run at right angles to each other, like X and Y axes on a grid, and the points where the photons and secondary particles interact with the fibers can be mapped on the grid like pencil marks on a piece of graph paper.

Fundamental questions for GLAST

  • How do active galactic nuclei (AGNs) form and evolve?
  • What powers the jets emanating from AGNs and galactic black holes and how are the particles in the jets accelerated? How are these structures connected with similar structures seen at smaller scales?
  • At what energies are the breaks in the gamma-ray spectra of AGNs? Are high energy spectral cutoffs due to source-intrinsic absorption effects or to absorption by extragalactic background light? What is the redshift dependence of these effects? Is there a class of AGNs that can be used as high-energy "standard candles"?
  • What is the origin of the isotropic "diffuse" gamma-ray background?
  • What are the sites and mechanisms of cosmic-ray acceleration? How do rotation-powered pulsars generate high-energy gamma-rays and what is the relation of this radiation to emission in lower energy bands?
  • What is the rate of supernovae in the Galaxy and where are the unobserved supernovae of the past several hundred years?
  • What are gamma-ray bursts and how do they generate high-energy radiation?
  • What are the unidentified high-energy gamma-ray sources in our galaxy?
  • The showers of particles then hit the next sheet of metal and spread out even more.

    The particles are tracked as they cascade deeper through metal sheets and grid-like scintillating fibers. Heavy metal, like lead or tantalum, is used to increase the chances of intercepting the gamma rays.

    Then, like forensic detectives determining the trajectory of a bullet by looking at the trail of destruction it left behind, scientists use computer programs to reconstruct the direction and energy of the gamma ray at the moment of impact.

    Right: Cross section diagram shows the layers of metal and fibers that will be used in GLAST. (links to 701x619-pixel, 187KB JPG; a larger 1403x1239-pixel, 688KB JPG is available).

    In addition to providing improved gamma-ray tracking, Pendleton's system is capable of measuring gamma rays of even higher energy levels than current instruments, like the ones aboard the Compton Gamma Ray Observatory.

    "Whereas BATSE can detect gamma rays ranging from 20 thousand to 10 million electron volts, this proposed system can detect up to 300 billion electron volts," said Pendleton. "That means the photons it sees are 300 billion times as energetic as visible light."

    The lower section of the system is the calorimeter, the section that measures the energy. This section is made up of 35 additional planes of metal and fiber and is designed to measure extremely high-energy gamma rays.

    "The calorimeter employs essentially the same technology as the tracker, but uses thicker, more closely packed converter sheets to contain the shower and provide an energy measurement for higher energy photons," said Pendleton.

    The data acquisition system includes photomultiplier tubes to detect the flashes and computers to translate the flashes into understandable images.

    The diagram at left (links to 600x808-pixel, 50KB GIF) depicts a typical event: A 300 GeV (300 billion electron volts) gamma ray penetrates almost halfway through GLAST before hitting a metal nucleus and triggering a shower of charged particles that flash (red dots) as they pass through optical fibers and when they exit through the anticoincidence counters (green). This links to a larger image that also shows the lesser events when a 10 GeV gamma ray passes hits. The bottom two images (in the larger image) depict how 10 GeV protons will usually pass through and trigger the anticoincidence detectors. One in 10 will cause a cascade of particles that have a different shower easily distinguished from a gamma-ray hit. (also available: A 1913x2575-pixel, 164KB JPG).

    The entire assembly, about the size of a large restaurant refrigerator, is enclosed by shields. These shields are made of plastic scintillator boards without the heavy metals so that they can detect noise from lower energy charged particles. This lets the scientists avoid confusing cosmic rays with high energy gamma rays.

    NASA/Marshall and its partners have extensive experience with this approach. A prototype, the Scintillating Fiber Telescope for Energetic Radiation (SIFTER), has been developed for a balloon flight and has just completed tests in the Continuous Electron Beam Accelerator Facility at the Jefferson National Laboratory in Hampton, Va. The Scintillating Optical Fiber Calorimeter (SOFCAL) experiment, flown on a balloon in 1997, used similar technologies although cosmic rays were the quarry.

    GLAST model

     
    A model of the GLAST detector - where the layers of optical fibers and metal are clearly visible (above, left) - was tested recently in the Continuous Electron Beam Accelerator Facility which produces a steady beam of electrons at precisely set energy level. When the electrons collide with the right target material, they produce gamma rays in a narrow energy range. This lets scientists measure the exact performance of the GLAST detectors, and thus refine their design. Above, right, a GLAST scientist leans into the equipment to make final adjustments before a run.

    A GLAST for the blasts

    Among the principal objectives of GLAST are to study the physical processes driving blazar- causing gamma-ray bursts, to identify new gamma-ray sources, and to examine gamma-ray emission from blazars. Blazars are super active galaxies surrounding black holes that produce collimated blasts of radiation. Blazars are thought to be powered by black holes a billion times as massive as the Sun.

    "When we talk about high energy gamma rays, we're talking about the biggest explosions in the universe," said Pendleton. Indeed, the future of gamma-ray astronomy reflected in his work is no small talk.

    Web Links

    GLAST project web site at NASA's Goddard Space Flight Center. GLAST is also discussed as part of a larger NASA/Goddard web site giving a larger overview of gamma-ray astronomy.

    Scintillating Optical Fiber Calorimeter (SOFCAL) designed by scientists at Marshall and University of Alabama in Huntsville to detect cosmic rays from a high altitude balloon. It uses technologies similar

    Cosmic gamma-ray bursts news and information, including BATSE updates.

    Blazars are described in a broader discussion of where gamma-ray astronomy is now, and what directions it should take.

    GLAST partners

    NASA/Marshall SIFTER home page includes views of the spacecraft and instrument plan (SIFTER is an earlier name for the program).

    The University of Alabama in Huntsville has been teamed with NASA/Marshall for years.

    Washington University has several images of the instrument package tested in the particle accelerator.


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    Author: Tom Kelleher
    Curator: Bryan Walls
    NASA Official: John M. Horack