Gamma-Ray Astrophysics
Astronomy is the study of the universe by the detection of light. However, visible light corresponds to only a small fraction of the electromagnetic spectrum--with an energy range extending from below infrared to above ultraviolet, both of these being invisible to the human eye. Modern astronomers also observe the sky at even lower energies (radio waves) and higher energies (x-rays and gamma rays). Astrophysical gamma rays can have energies higher than those produced by the largest particle accelerators.
Today, a large part of the gamma ray energy range remains unexplored. No telescope has ever been built that can detect gamma rays in this unopened window. Observations in this window may be key to understanding the mechanisms for high energy particle acceleration in some of the most powerful astrophysical sources, including extremely dense objects known as pulsars and extremely bright objects known as active galactic nuclei. Additionally, observations of AGNs, which are at distances comparable to the size of the universe will yield information important to cosmology, the study of the birth and evolution of the universe itself.
To access the unexplored gap in the gamma ray energy range for the first time, a group of physicists and astronomers is building a new experiment called the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE). STACEE is unusual in that much of the experiment has already been built, though for an entirely different purpose: the collection of sunlight for solar energy studies. STACEE is located at the National Solar Thermal Test Facility (NSTTF). a solar energy research center located at Sandia National Laboratories in Albuquerque, NM. This facility contains an array of 212 large mirrors called heliostats, each about 20 feet by 20 feet in area. The heliostats are used during the day to track the sun and focus its light but at night the STACEE group uses them to collect the flashes of blue light that result when gamma rays enter the earth's atmosphere. Using these large mirrors as part of the detector, the STACEE group has built a gamma ray telescope with enormous collecting area for a very limited cost. The large collection area is what makes it possible to explore the new energy range for gamma rays.
Accessing the Unopened Window
Gamma ray astrophysics is a young and dynamic field at the cross-roads of particle physics and astrophysics. Recent discoveries by both space-borne and ground-based instruments have revolutionized the field and led to an explosion of interest around the world. Perhaps the most exciting challenge facing the field is the quest to explore a region of gamma ray energy (or wavelength) which has not been studied by any instrument before. Currently gamma ray energies between 20 and 250 GeV are inaccessable to both spaceborne detectors, such as the EGRET experiment aboard NASA's Compton Gamma Ray Observatory, and ground-based air cherenkov detectors, such as the Whipple Observatory.This unopened window represents an energy regime where many interesting phenomena are expected to occur.
The need for a new detector in the energy range from 20 to 250 GeV is highlighted by comparing the low-energy (EGRET) and high-energy (Whipple and similar detectors) all-sky maps of point sources. While EGRET has seen more than 150 sources at energies up to 20 GeV, ground-based experiments have detected only six convincing gamma ray sources above 300 GeV. Something very interesting is clearly happening in the gamma ray spectrum of these sources in the gap between 20 and 250 GeV. It is believed that observations of sources in the gap will provide important evidence concerning the acceleration mechanisms of the most energetic objects in the Universe, including rotating neutron stars (pulsars), remnants of exploded stars (supernovae), gamma ray bursts, and distant, but intense, active galactic nuclei (quasars). Observations between 20 and 250 GeV may also provide a direct probe of the diffuse intergalactic infrared radiation, which in turn may have profound implications for the cosmological structure of the Universe. The importance of building experiments to explore this energy range is highlighted by the existence of competing devices such as CELESTE as well as new major proposals for both ground-based instruments, such as VERITAS, and space-borne missions, such as GLAST. These major instruments, if built, would greatly expand our knowledge of the high energy Universe, and they could be operational in five to ten years. The STACEE project is pursuing some of the same goals and is expected to be operational by late 1999. For more details on the scientific goals of gamma-ray astrophysics, please consult the list of STACEE scientific papers and related links.
What are the primary science objectives for STACEE?
These are some of the central scientific questions that STACEE has been designed to address:
- What is the maximum energy for photons emitted from AGN jets? What does this tell us about the particles in the beam?
- At what energies are gamma-rays from distant sources attenuated? What can this tell us diffuse intergalactic infra-red and optical emission?
- What are the highest energies seen for pulsed emission from gamma-ray ray pulsars? What does this tell us about the location of particle acceleration in these sources?
- Are supernova remnants sources of gamma-rays at 100 GeV? Does this confirm our expectation that SNR are likely sources of high energy cosmic rays?
- Are there other sources, including over 40 unidentified EGRET sources, that may also be detected in the energy range from 50 to 250 GeV which has thus far remained completely unexplored?
For a complete summary of the current status of the STACEE experimental program, including instrument details, publications, and science motivation, see the main STACEE Collaboration Home Page at UCLA.
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