* Astronomy

Blobrana · L

GLAST

Blobrana · L

Title: Digging for the Truth: Photon Archaeology with GLAST
Authors: F. W. Stecker (NASA/GSFC)

Stecker, Malkan and Scully, have shown how ongoing deep surveys of galaxy luminosity functions, spectral energy distributions and backwards evolution models of star formation rates can be used to calculate the past history of intergalactic photon densities for energies from 0.03 eV to the Lyman limit at 13.6 eV and for redshifts out to 6 (called here the intergalactic background light or IBL). From these calculations of the IBL at various redshifts, they predict the present and past optical depth of the universe to high energy gamma-rays owing to interactions with photons of the IBL and the 2.7 K CMB. We discuss here how this procedure can be reversed by looking for sharp cut-offs in the spectra of extragalactic gamma-ray sources such as blazars at high redshifts in the multi-GeV energy range with GLAST. By determining the cut-off energies of sources with known redshifts, we can refine our determination of the IBL photon densities in the past, i.e., the "archeo-IBL", and therefore get a better measure of the past history of the total star formation rate. Conversely, observations of sharp high energy cut-offs in the gamma-ray spectra of sources at unknown redshifts can be used instead of spectral lines to give a measure of their redshifts.

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Blobrana · L

Title: The search for Milky Way halo substructure WIMP annihilations using the GLAST LAT
Authors: Lawrence Wai, GLAST LAT Collaboration

The GLAST LAT Collaboration is one among several experimental groups, covering a wide range of approaches, pursuing the search for the nature of dark matter. The GLAST LAT has the unique ability to find new sources of high energy gamma radiation emanating directly from WIMP annihilations in situ in the universe. Using it's wide band spectral and full sky spatial capabilities, the GLAST LAT can form "images" in high energy gamma-rays of dark matter substructures in the gamma-ray sky. We describe a preliminary feasibility study for indirect detection of Milky Way dark matter satellites using the GLAST LAT.

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Blobrana · L

Title: Dark Matter Searches with GLAST
Authors: Lawrence Wai, GLAST LAT Collaboration

Indirect detection of particle dark matter relies upon pair annihilation of Weakly Interaction Massive Particles (WIMPs), which is complementary to the well known techniques of direct detection (WIMP-nucleus scattering) and collider production (WIMP pair production). Pair annihilation of WIMPs results in the production of gamma-rays, neutrinos, and anti-matter. Of the various experiments sensitive to indirect detection of dark matter, the Gamma-ray Large Area Space Telescope (GLAST) may play the most crucial role in the next few years. After launch in late 2007, The GLAST Large Area Telescope (LAT) will survey the gamma-ray sky in the energy range of 20MeV-300GeV. By eliminating charged particle background above 100 MeV, GLAST may be sensitive to as yet to be observed Milky Way dark matter subhalos, as well as WIMP pair annihilation spectral lines from the Milky Way halo. Discovery of gamma-ray signals from dark matter in the Milky Way would not only demonstrate the particle nature of dark matter; it would also open a new observational window on galactic dark matter substructure. Location of new dark matter sources by GLAST would dramatically alter the experimental landscape; ground based gamma ray telescopes could follow up on the new GLAST sources with precision measurements of the WIMP pair annihilation spectrum.

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Blobrana · L

A pioneering US space agency spacecraft is set to launch on a mission to explore the most energetic phenomena in the Universe.
The Gamma-ray Large Area Space Telescope (Glast) has been described as an "extreme physics" laboratory.
The probe is due to launch from Florida's Cape Canaveral base in November on a Boeing Delta II rocket.
The team presented details of the mission at the meeting of the American Astronomical Society in Seattle.

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Blobrana · L

Scientists and engineers have completed assembly of the primary instrument for the Gamma-ray Large Area Space Telescope, or GLAST, a breakthrough orbiting observatory scheduled to launch from NASA's Kennedy Space Centre in autumn 2007.

The main instrument, called the Large Area Telescope, arrived on May 14, 2006, at the U.S. Naval Research Laboratory in Washington for environmental testing.
The mission, led by NASA with the Department of Energy and international partners, brings together the astrophysics and particle physics communities.

"With GLAST, physicists will gain valuable information about the evolution of the universe and physicists will search for signals that may even force revision of some of the basic laws of physics. The completion of the Large Area Telescope assembly and its shipment from the accelerator centre are major milestones in its development" - Peter Michelson, Stanford University, the telescope's principal investigator.

The observatory will detect light billions of times more energetic than what our eyes can see or what optical telescopes such as Hubble can detect. Key targets include powerful particle jets emanating from enormous black holes and possibly the theorized collisions of dark matter particles. The Large Area Telescope will be at least 30 times more sensitive than previous gamma-ray detectors and will have a far greater field of view.

"The relative range of light energies that the instrument can detect is thousands of times wider than that of an optical telescope, which captures only a thin slice of the electromagnetic spectrum. The observatory provides a huge leap in capabilities in this important energy band, and it opens a wide window for exploration and discovery" - Steven Ritz , Project Scientist at NASA's Goddard Space Flight Centre, Greenbelt, Md.

Unlike visible light, gamma rays are too energetic to be focused by traditional telescope mirrors onto a detector. The Large Area Telescope will employ detectors that convert incoming gamma rays into electrons and their antimatter partners, called positrons. This technique, a change of light into matter as described by Einstein's equation E=mc^2, is called pair conversion. It will enable scientists to track the direction of gamma rays and measure their energy.
The telescope will now undergo three gruelling months of ‘shake and bake’ testing to ensure it will survive the intense vibration and noise during launch and operate properly in space. Electromagnetic interference tests also will be performed to ensure Large Area Telescope operations do not interfere with the spacecraft. When testing is finished at the Naval Research Laboratory, the instrument will be shipped to Arizona, where engineers at General Dynamics C4 Systems will integrate the Large Area Telescope and a second instrument, the Burst Monitor, onto the spacecraft.

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Blobrana · L

After more than a decade of work, a team led by physicists at the University of California, Santa Cruz, has completed a major detector subsystem for NASA's Gamma-ray Large Area Space Telescope (GLAST). Completion of the tracking detector is a significant milestone for the telescope project, scheduled for launch in 2007.

GLAST will give astronomers a powerful new tool for studying the sources of high-energy gamma rays in the universe. It will help astronomers investigate poorly understood phenomena such as active galactic nuclei, quasars, pulsars, and gamma-ray bursts. Scientists also hope to learn about the fundamental properties of the universe itself by studying how gamma rays propagate across the universe.

"With GLAST, we can look at gamma rays emitted from the farthest reaches of the universe and study the sources of those gamma rays. We expect that GLAST will see thousands of quasars and related objects" - Robert Johnson, a professor of physics at UCSC and manager of the tracker subsystem for the telescope.

The tracking detector will give GLAST unprecedented sensitivity for detecting high-energy gamma rays and pinpointing their sources in the sky. The design, engineering, and fabrication of the detector was overseen by Johnson and other researchers at the Santa Cruz Institute for Particle Physics (SCIPP) at UCSC. The project was a collaborative effort involving SCIPP, the Stanford Linear Accelerator Centre (SLAC), and institutions in Italy and Japan.

GLAST will be much more powerful than its predecessor, the EGRET experiment that operated on the Compton Gamma Ray Observatory in the 1990s, Johnson said.

"Currently there is no other telescope that can detect gamma rays within the energy range that GLAST will cover, and it will be 30 to 100 times more sensitive than any previous gamma-ray telescope" - Robert Johnson.

The tracking detector has nearly 900,000 detector channels, each connected to its own amplifier. Packed into 16 compact modules are nearly 80 square meters of solid-state silicon detectors and 576 electronics boards, all supported on a carbon-composite structure. The whole system operates on just 160 watts of power, and it had to be designed to withstand severe vibrations during the rocket launch that will carry the telescope into orbit.

"It is really an unprecedented design for a detector system that will be put into space" - Robert Johnson.

The initial design work for the GLAST detector was begun in 1992 by William Atwood, then at SLAC and now an adjunct professor of physics at UCSC. Johnson began collaborating with Atwood on the detector project in 1994, and in 2000, NASA chose their detector from among several competing designs for the GLAST mission. Three weeks ago, SCIPP scientists and their international team delivered the last of the detector modules to the integration and test team at SLAC, where the modules were installed into the overall instrument structure.

Over the years, many UCSC undergraduates and graduate students have contributed to SCIPP's work on the GLAST project, Johnson said. In addition to Atwood and Johnson, other UCSC faculty involved in the project are Hartmut Sadrozinski and Terry Schalk, both adjunct professors of physics.

High-energy astrophysicists at UCSC and elsewhere are eagerly awaiting the day when GLAST will begin collecting data. GLAST's main instrument, the Large Area Telescope for which the tracking detector was built, will provide unprecedented sensitivity to gamma rays in the energy range of about 20 MeV to about 300 GeV. Other space-based gamma-ray instruments, such as NASA's Swift mission, cover lower energy ranges, while new ground-based instruments will detect higher-energy gamma rays.

"The new generation of ground-based gamma-ray observatories cover complementary energy ranges and will work closely with GLAST. We are looking forward to quite an exciting time in terms of being able to reach into new energy ranges and hopefully find the answers to fundamental questions about the universe" - Robert Johnson.

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