Peering into the violent universe

Tests prove new gamma-ray telescope design

Feb. 9, 1999: A new approach to observing the most violent objects in the sky is taking shape and will be reviewed with competing concepts today near Washington. Fiber GLAST - the scintillating fiber detector concept for the next-generation Gamma-ray Large Area Space Telescope - has passed a number of tests that show the basic concept is good and could meet the program's goals in space.

"We're finalizing our design to respond to the Announcement of Opportunity that will lead to a flight project," said Dr. Geoff Pendleton, the principal investigator for FiberGLAST. Pendleton is a gamma-ray astrophysicist with the University of Alabama in Huntsville. He works at NASA's Marshall Space Flight Center.

Right: Pendleton examines optical fiber bundles emerging from a metal stack (left) and feeding into a camera (center) in an arrangement similar to what is planned for FiberGLAST. GLAST would produce images similar to the background image, top, a simulation of gamma-ray sources in the direction opposite the center of our galaxy.

Observations by the Compton Gamma Ray Observatory and other spacecraft over the last few years have provided some of the answers that scientists have sought about the high-energy universe. But they've also raised new questions. A number - including what's at the heart of many gamma-ray sources - have eluded answers.

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In 1998, three science teams, including one led by Pendleton, were awarded initial technology development contracts to pursue promising concepts for GLAST.

"In addition to fundamental instrument performance tests, we've done a lot of system engineering," Pendleton said, "which is a significant part of developing a flight instrument."

That work has been led by the University of New Hampshire, with help from the entire team, including NASA/Marshall, UAH, Louisiana State University, Washington University in St Louis, the University of California at Riverside, and NOVA R&D, Inc.

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Like many ideas for new science instruments, the basic idea is simple. Building FiberGLAST and making it work will be the challenge.

Gamma rays are impossible to focus even with advanced mirrors like the Chandra X-ray Observatory will use. Instead, they are observed indirectly, by measuring light flashes generated as gamma rays hit solid matter. In FiberGLAST, the solid matter will be layers of heavy metal plates. Each interception turns one energetic gamma-ray photon into a series of energetic particles that produce light flashes as they pass through layers of plastic fibers sandwiched between the metal plates. The fibers, arranged at right angles to each other, paint a picture of the particle shower as it expands through the FiberGLAST detector. It's almost like watching a sky rocket blossom into a cascade of sparkles.

Right: Optical fibers sprout from between metal plates in a 1998 test of the FiberGLAST design.

Demonstrating that this would work was one of the first tests for the FiberGLAST concept. In 1998, the FiberGLAST team took a small working model to the Continuous Electron Beam Accelerator Facility at the Thomas Jefferson National Laboratory in Newport News, Va. Because the accelerator produces a precisely controlled stream of electrons, it can also produce a gamma-ray beam of precise energy when the electrons hit the right target. And that lets scientists measure, precisely, the performance of a gamma-ray detector.

"It worked very well," Pendleton said. "The fibers demonstrated high efficiency for detecting the gamma-rays."

Left: Simulations predict that FiberGLAST performs far better than the minimum "baseline" requirement set for the GLAST mission.

But FiberGLAST is more than plastic fibers between metal plates. The light flashes travel down the fibers to multiple anode photomultiplier tubes, the complex, sensitive light detectors.

"We ran a vibration test to simulate launch on the Delta II rocket that will most likely carry GLAST," Pendleton said. "The concern was that it could fall apart. The test proved to the scientific community that our readout system could survive launch."

Once the light flashes reach the detectors arrays, most have to be discarded because they'll really be noise.

GLAST will identify and study nature's high-energy particle accelerators through observations of active galactic nuclei, pulsars, stellar-mass black holes, supernova remnants, gamma-ray bursts, and diffuse galactic and extragalactic high-energy radiation in the energy range from 20 MeV to 100 GeV and higher. GLAST will use these sources to probe important physical parameters of the Galaxy and the Universe that are not readily measured with other observations. The high-energy gamma-rays will be used to search for a variety of fundamentally new phenomena, such as particle dark matter and evaporating black holes. Scientific objectives include:

How do active galactic nuclei (AGNs), such as blazars (as depicted in the artist's concept at right) 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 biggest problem is cosmic rays which go through our system 1,000 to 10,000 times as frequently as gamma rays," Pendleton said. It's a bit like looking for a few drops of rain in the middle of a hail storm.

Most of the cosmic rays are protons zipping along with energies of 4 to 100 billion electron volts (4-100 GeV; a photon of visible light carries about 2 electron-volts of energy).

"If they aren't rejected by the detector system," Pendleton said, "they'll overload the data acquisition and processing system. That system has to be able to make a quick veto of almost all the protons that go through the system."

Simulating everything that happens when both cosmic and gamma rays hit GLAST has been a challenge. Each photon or particle will generate an expanding trail of debris through the entire apparatus. Each encounter has to be observed and quickly recorded or discarded.

"Marshall has a 100 terabyte (100 trillion bytes) data base storage system," Pendleton said. "There aren't many of those around." By way of comparison, it would take almost 667 million floppy disks to store that much data.

"We needed it to develop the hardware readout system," Pendleton said. "In it, we've modeled tens of millions of events. These are large amounts of data that take months to generate in a computer with 17 processors and 1 gigabyte of RAM."

The tests have demonstrated that the team's approach to FiberGLAST will weed out the noise and return just the data they want without swamping the spacecraft's telemetry system.

"Our approach emphasizes particular elements of the science goals," Pendleton said. "At this meeting we will discuss our status and why we favor this design."

Following today's review, the competitors will go home and refine their concepts for the final competition. The draft copy of the Announcement of Opportunity has been released for comments by the scientists. The final AO will be issued in June, with replies due in September. By early 2000, NASA plans to select one concept for development and, ultimately, launch in 2005.

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