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Searching for Gamma Rays from the Edges of the Galaxy
On December 29 NASA successfully launched the Compton Spectrometer and Imager (COSI) from the McMurdo station in Antarctica. Jointly developed by research teams from Taiwan and the Space Sciences Laboratory at the University of California, Berkeley, COSI was carried by balloon some 40 kilometers into the atmosphere to gather information on gamma rays from distant corners of the universe. The launch represents a milestone in terms of gamma ray detection in astrophysics.
 
The heart of COSI consists of 12 high-purity germanium detectors capable of detecting gamma rays with a photon value of one million volts. Though extremely weak, such gamma rays offer a wealth of information about the galaxy.
 
According to NTHU physics professor Chang Hsiang-kuang, the leader of Taiwan’s research team, the main purpose of this launch was to monitor activity near the center of the Milky Way, where there are strong emissions of annihilation radiation which contains electron-positron pairs. The source of this large amount of positrons has been puzzling astronomers for over half a century. At present the leading theories are that they come from high-density black holes, neutron stars and other black holes, or even low-density dark matter.
 
Professor Chang stated that they hoped during its long flight COSI might even capture a fleeting image of a gamma ray burst and collect data on photon polarization that could help solve yet another mystery in astrophysics.
 
As the new generation of Compton telescopes, COSI is not only smaller and lighter than its predecessors, but also more sensitive. According to Chang, this increased sensitivity is of vital importance to advancing our knowledge of the galaxy. For example, it is generally believed that heavy elements were formed by a supernova explosion, but due to the limited sensitivity of the instrumentation the accompanying gamma-ray radiation has never been clearly observed. Of the several teams working on developing a high-sensitivity Compton telescope, the COSI team has had the fastest progress. Like X-rays, gamma rays are unable to penetrate the Earth's atmosphere. Thus the observation of gamma rays has to be done from outer space, by mounting the instrument on a satellite or spacecraft. Before conducting such a space mission, it is necessary to make a high-altitude test flight to verify that the telescope is working properly. Thus in 2009 COSI was tested in the United States during a 40-hour test flight in preparation for this Antarctica launch.
 
After arriving in Antarctica in October the COSI team set about preparing and testing the equipment. After waiting some time for suitable weather, COSI was finally launched on December 29. Although the team was hoping to keep COSI in flight for one hundred days, the balloon developed a leak and on the second day after being launched COSI landed about 560 kilometers from McMurdo Station. Nonetheless, COSI was still able to gather a considerable amount of information which will prove valuable to prepare for future satellite missions. The development of this highly sensitive gamma-ray detector will also have a positive impact on other fields. For example, if adapted for use in medical imaging technology, such a highly sensitive device will make it possible to reduce the dose of tracer radiation injected into the patient. The other core members of the Taiwan research team were Prof. Chang Yuan-han, the Department of Physics at National Central University, and Lin,Chih-Hsun, Associate Research Scientist of the Institute of Physics at Academia Sinica. Taiwan’s COSI team was initially supported by Taiwan's National Space Organization and is currently funded by the Ministry of Science and Technology.
 
COSI mounted on the gondola of a high-altitude balloon. The semispherical device at the top center of the photo is an iridium satellite antenna.

COSI mounted on the gondola of a high-altitude balloon. The semispherical device at the top center of the photo is an iridium satellite antenna.

Members of Taiwan’s COSI research team at McMurdo Station. From left to right: Tseng, Martin, M.A. student in NTHU’s Institute of Astronomy; Professor Chang Hsiang-kuang; Chiu Hen-lun, who earned his Ph.D. in physics at NTHU and is now a researcher at U.C. Berkeley; and Yang, Chien-Ying, Ph.D. student at the Institute of Astronomy.

Members of Taiwan’s COSI research team at McMurdo Station. From left to right: Tseng, Martin, M.A. student in NTHU’s Institute of Astronomy; Professor Chang Hsiang-kuang; Chiu Hen-lun, who earned his Ph.D. in physics at NTHU and is now a researcher at U.C. Berkeley; and Yang, Chien-Ying, Ph.D. student at the Institute of Astronomy.

Part of the COSI research team at McMurdo Station.

Part of the COSI research team at McMurdo Station.

 

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