Gamma-ray astronomy: Difference between revisions

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	'''Gamma-ray astronomy''' is the ] study of the ] with ].

	== Early history ==
	Long before experiments could detect ]s emitted by cosmic sources, scientists had known that the universe should be producing these ]s. Work by ] and H. Primakoff in 1948, ] and I.B. Hutchinson in 1952, and, especially, Morrison in 1958 had led scientists to believe that a number of different processes which were occurring in the universe would result in gamma-ray emission. These processes included ] interactions with ], ] explosions, and interactions of energetic ]s with ]s. However, it was not until the 1960s that our ability to actually detect these emissions came to pass.

	Gamma-rays coming from space are mostly absorbed by the Earth's atmosphere. So gamma-ray astronomy could not develop until it was possible to get our detectors above all or most of the atmosphere, using ]s or spacecraft. The first gamma-ray telescope carried into orbit, on the ] satellite in 1961, picked up fewer than 100 cosmic gamma-ray photons. These appeared to come from all directions in the Universe, implying some sort of uniform "gamma-ray background". Such a background would be expected from the interaction of cosmic rays (very energetic charged particles in space) with gas found between the stars.

	The first true astrophysical gamma-ray sources were solar flares, which revealed the strong 2.223 MeV line predicted by Morrison. This line results from the formation of deuterium via the union of a neutron and proton; in a solar flare the neutrons appear as secondaries from interactions of high-energy ions accelerated in the flare process. These first gamma-ray line observations were from ], ], and the ], the latter spacecraft launched in 1980. The solar observations inspired theoretical work by ] and others.

	Significant gamma-ray emission from our galaxy was first detected in 1967 by the gamma-ray detector aboard the ] satellite. It detected 621 events attributable to cosmic gamma-rays. However, the field of gamma-ray astronomy took great leaps forward with the ] (1972) and the ] (1975-1982) satellites. These two satellites provided an exciting view into the high-energy universe (sometimes called the 'violent' universe, because the kinds of events in space that produce gamma-rays tend to be explosions, high-speed collisions, and such). They confirmed the earlier findings of the gamma-ray background, produced the first detailed map of the sky at gamma-ray wavelengths, and detected a number of point sources. However, the poor resolution of the instruments made it impossible to identify most of these point sources with individual stars or stellar systems.

	== Early discoveries ==
	Perhaps the most spectacular discovery in gamma-ray astronomy came in the late 1960s and early 1970s from a constellation of defense satellites which were put into orbit for a completely different reason. Detectors on board the ] satellite series, designed to detect flashes of gamma-rays from nuclear bomb blasts, began to record bursts of gamma-rays -- not from the vicinity of the Earth, but from deep space. Today, these ]s are seen to last for fractions of a second to minutes, popping off like cosmic flashbulbs from unexpected directions, flickering, and then fading after briefly dominating the gamma-ray sky. Studied for over 25 years now with instruments on board a variety of satellites and space probes, including Soviet ] spacecraft and the ], the sources of these enigmatic high-energy flashes remain a mystery. They appear to come from far away in the Universe, and currently the most likely theory seems to be that at least some of them come from so-called '']'' explosions - supernovas creating ]s rather than ]s.

	== Recent and current observatories ==
	During its ] program in 1977, ] announced plans to build a "great observatory" for gamma-ray astronomy. The ] (CGRO) was designed to take advantage of the major advances in detector technology during the 1980s, and was launched in 1991. The satellite carried four major instruments which have greatly improved the spatial and temporal resolution of gamma-ray observations. The CGRO provided large amounts of data which are being used to improve our understanding of the high-energy processes in our Universe. CGRO was de-orbited in June 2000 as a result of the failure of one of its stabilizing ]s.

	] was launched in 1996 and deorbited in 2003.
	It predominantly studied X-rays, but also observed gamma-ray bursts.
	By identifying the first non-gamma ray counterparts to gamma-ray bursts, it opened the way for their precise position determination and optical observation of their fading remnants in distant galaxies.
	The ] 2 (HETE-2) was launched in October 2000 (on a nominally 2 yr mission) and was still operational in March 2007.
	], a NASA spacecraft, was launched in 2004 and carries the BAT instrument for gamma-ray burst observations.
	Following BeppoSAX and HETE-2, it has observed numerous x-ray and optical counterparts to bursts, leading to distance determinations and detailed optical follow-up.
	These have established that most bursts originate in the explosions of massive stars (]s and ]s) in distant galaxies.

	Currently the main space-based gamma-ray observatories are the INTErnational Gamma-Ray Astrophysics Laboratory, (]), and the ] (GLAST).
	INTEGRAL is an ESA mission with additional contributions from ], Poland, USA and Russia.
	It was launched on ] ].
	NASA launched ] on 11 June 2008.
	In includes LAT, the Large Area Telescope, and GBM, the GLAST Burst Monitor, for studying gamma-ray bursts.

	Very energetic gamma rays, with photon energies over ~30 GeV, can also be detected by ground based experiments.
	The extremely low photon fluxes at such high energies require detector effective areas that are impractically large for current space-based instruments.
	Fortunately such high-energy photons produce extensive showers of secondary particles in the atmosphere that can be observed on the ground, both directly by radiation counters and optically
	via the ] the ultra-relativistic shower particles emit.
	The Imaging Atmospheric Cherenkov Telescope technique currently achieves the highest sensitivity. The ], a steady source of so called TeV gamma-rays, was first detected in 1989 by the Whipple Observatory at Mt. Hopkins, in ] in the USA.
	Modern Cherenkov telescope experiments like ], ], ], and CANGAROO III can detect the Crab Nebula in a few minutes.
	The most energetic photons (up to 16 ]) observed from an extragalactic object originate from the ] ] (Mrk 501).
	These measurements were done by the High-Energy-Gamma-Ray Astronomy (]) air ] telescopes.

	Gamma-ray astronomy observations are still limited by non-gamma ray backgrounds at lower energies, and, at higher energy, by the number of photons that can be detected.
	Larger area detectors and better background suppression are essential for progress in the field.

	== See also ==
	* ] (Alphabetic list)
	* ], GLAST.
	* ]

	== External links ==
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Revision as of 17:50, 25 September 2008