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A Timeline of Gamma-Ray Astronomy
09.06.07
Early 1960s The first balloon experiments, and NASA's Explorer 11 satellite, detect the first hints of 100 MeV gamma-ray emission from our galaxy, but the results are inconclusive. The lack of an easily detectable signal shows that the predictions of Morrison and others were overly optimistic.
Early 1960s The first generation of ground-based Atmospheric Cherenkov Telescopes (ACTs) become operational in the U.S. and U.S.S.R. ACTs detect blue Cherenkov light created when very-high-energy (hard) gamma rays from space interact with molecules in Earth’s atmosphere. The early ACTs yield inconclusive results, with no definitive detections.
Late 1960s U.S. military Vela satellites serendipitously detect gamma-ray bursts (GRBs) while looking for clandestine Soviet nuclear tests. But the existence of GRBs remains classified until 1973.
1967-1969 NASA’s Orbiting Solar Observatory (OSO-3) detects a grand total of 621 gamma-ray photons from deep space, representing a major breakthrough. It verifies the existence of galactic emission from cosmic-ray interactions, and discovers the diffuse gamma-ray background. OSO-3 results are quickly confirmed by balloon experiments flown by a variety of research groups.
Early 1970s Gamma-ray detectors on the command modules for Apollo 15 and 16, while en route to the Moon, discover a diffuse background of low-energy gamma rays. These same instruments helped map gamma rays emitted from radioactive elements on the lunar surface.
1972 NASA's Small Astronomy Satellite-2 (SAS-2) confirms the diffuse gamma-ray background discovered by OSO-3. SAS-2 shows that the galactic emission is related to the structure of the Milky Way, it studies the Crab and Vela gamma-ray pulsars, and finds an unexpected point source, which later turns out to be the neutron star Geminga.
1975-1981 The European COS-B satellite, which is similar to SAS-2 in size and cost, discovers 25 additional gamma-ray point sources, some of which remain unidentified. Others turn out to be pulsars. Another one of the objects is the first extragalactic gamma-ray source: 3C 273, which is a relatively nearby quasar. It also detects diffuse galactic emission.
1979-1981 NASA'S High-Energy Astrophysics Observatory-3 (HEAO-3) discovers low-energy (soft) gamma rays coming from the galactic center from the annihilation of electrons and positrons. (This is the 511 keV line, which some scientists consider to be hard X rays.) Some still-unknown process must be producing antimatter in the region around the galactic center. These results are confirmed by balloon instruments.
1980-1989 NASA's Solar Maximum Mission detects soft gamma rays from solar flares.
Late 1980s The second generation of ACTs becomes operational, led by the 10-meter Whipple Telescope in Arizona. Whipple indirectly detects hard gamma rays from the direction of the Crab Nebula, but not the pulsar at the center of the nebula.
Late 1980s Balloon experiments detect gamma rays from radioactive elements produced by Supernova 1987A, proving that supernovae produce new elements as predicted by theory.
Image right: A time clock. Credit: NASA
Late 1980s NASA and the U.S.S.R. fly several dedicated missions to study GRBs, increasing the number of bursts and the number of theories about the origins of these mysterious events.
1991-2000 The Compton Gamma-ray Observatory (CGRO), one of NASA's four Great Observatories, revolutionizes gamma-ray astronomy with a series of major discoveries. Designed for two years of operation, CGRO returns data for nine years, and is de-orbited because of a gyro-hardware failure. The BATSE instrument detects more than 2,700 GRBs and shows that they come from all over the sky, strongly suggesting they are explosions occurring in distant galaxies. It also shows that GRBs seem to occur in two types: long (greater than two seconds) and short (less than two seconds). The EGRET instrument finds 271 point sources, including about 70 blazars and six pulsars, but two-thirds of the sources remain unidentified. The discovery of so many blazars was unexpected. The COMPTEL instrument maps the galactic distribution of aluminum-26, showing where stars are forming in the Milky Way. The OSSE instrument maps the annihilation line from the galactic center more accurately, and finds gamma-ray emission from X-ray binaries and Seyfert galaxies.
Early 1990s Ground-based ACTs discover hard gamma rays from several blazars. To the amazement of astronomers, this emission varies on a timescale of just minutes to hours.
1997-2003 The Italian-Dutch BeppoSAX satellite, though mainly used to study X rays, localizes several GRB positions quickly through observations of the afterglows of long GRBs. The positions are precise enough that follow-up ground-based observations, and later observations from the Hubble Space Telescope, prove that the bursts occur at great distances, a major breakthrough.
2000-2007 NASA's High-Energy Transient (HETE-2) satellite helps firm up the connection between long GRBs and supernovae.
2002-present NASA's Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite continues to advance astronomers’ understanding of how particles are accelerated and how energy is released in solar flares. RHESSI serendipitously detects polarization in a GRB, showing that powerful magnetic fields must be involved.
2002-present Among many other studies, the European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite measures aluminum-26 levels throughout our galaxy, demonstrating that the Milky Way produces, on average, about two supernovae per century.
2004-present NASA's Swift satellite is currently detecting about 100 GRBs per year, and localizing many of them for follow-up studies. The mission is showing that GRBs are more diverse than expected, and have a variety of origins. The follow-up of short GRB afterglows lends strong support to the theory that some of these events come from neutron star-neutron star mergers, or black hole-neutron star mergers.
2000s A new generation of advanced ground-based ACTs is providing unprecedented sensitivity and resolution in very-high-energy gamma-ray astronomy. Led by the European High Energy Stereoscopy System (H.E.S.S.), an array of four telescopes in Namibia, these detectors are finding pulsar-wind nebulae, binary systems, supernova remnants, and many unidentified sources. Other important ACTs include CANGAROO (an Australian-Japanese facility based in Australia), MAGIC (located on La Palma in the Canary Islands), and VERITAS (based in Arizona). MILAGRO (based in New Mexico) is using a large swimming pool full of photomultiplier tubes to conduct a survey of the gamma-ray sky. These ground-based experiments complement GLAST by extending detections to the highest gamma-ray energies.
2007 The Italian satellite AGILE launches on April 23. This small spacecraft has a high-energy gamma-ray detector with a sensitivity similar to EGRET's, but with a wider field of view.
2008 Launch of NASA’s Gamma-ray Large Area Space Telescope (GLAST).
By Robert Naeye and David Thompson