Gamma-ray astronomy

From Wikipedia, the free encyclopedia

Gamma rays are absorbed by the atmosphere and must be studied from a space telescope.
Gamma rays are absorbed by the atmosphere and must be studied from a space telescope.
Compton Gamma Ray Observatory
Compton Gamma Ray Observatory

Gamma-ray astronomy is the astronomical study of the cosmos with gamma rays.

Contents

[edit] Early history

Long before experiments could detect gamma rays emitted by cosmic sources, scientists had known that the universe should be producing these photons. Work by Eugene Feenberg and H. Primakoff in 1948, Sachio Hayakawa 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 cosmic ray interactions with interstellar gas, supernova explosions, and interactions of energetic electrons with magnetic fields. 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 balloons or spacecraft. The first gamma-ray telescope carried into orbit, on the Explorer 11 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 OSO-3, OSO-7, and the Solar Maximum Mission, the latter spacecraft launched in 1980. The solar observations inspired theoretical work by Reuven Ramaty and others.

Significant gamma-ray emission from our galaxy was first detected in 1967 by the gamma-ray detector aboard the OSO-3 satellite. It detected 621 events attributable to cosmic gamma-rays. However, the field of gamma-ray astronomy took great leaps forward with the SAS-2 (1972) and the COS-B (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.

[edit] 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 Vela 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 gamma-ray bursts 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 Venera spacecraft and the Pioneer Venus Orbiter, 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 hypernova explosions - supernovas creating black holes rather than neutron stars.

[edit] Recent and current observatories

During its High Energy Astrophysics Observatory program in 1977, NASA announced plans to build a "great observatory" for gamma-ray astronomy. The Compton Gamma-Ray Observatory (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 gyroscopes.

BeppoSAX was launched in 1996 and deorbited in 2003. Predominantly X-rays but also studied gamma-ray bursts.

The High Energy Transient Explorer 2 (HETE-2) was launched in October 2000 (on a nominally 2 yr mission) and was still operational in March 2007.

Swift a NASA spacecraft was launched in 2004 and carries the BAT instrument.

Currently, the main space-based gamma ray observatory is the INTErnational Gamma-Ray Astrophysics Laboratory, (INTEGRAL). INTEGRAL is an ESA mission with contributions from Czech, Poland, USA and Russia. It was launched on 17 October 2002.

Very energetic gamma-rays can also be detected by ground based experiments. The Imaging Atmospheric Cherenkov Telescope technique currently achieves the highest sensitivity. The Crab Nebula, a steady source of so called TeV gamma-rays was first detected in 1989 by the Whipple Observatory (Az, USA). Modern Cherenkov telescope experiments like H.E.S.S., VERITAS, MAGIC, and CANGAROO III can detect the Crab Nebula in a few minutes. The most energetic photons (up to 16 TeV) observed from an extragalactic object originate from the blazar Markarian 501 (Mrk 501). These measurements were done by the High-Energy-Gamma-Ray Astronomy (HEGRA) air Cherenkov telescopes.

Gamma-ray astronomy is mostly dominated by the number of photons that can be detected. Larger area detectors and better background suppression are essential for progress in the field.

[edit] Future GR observatories

NASA plans to launch GLAST (with LAT and BM) in 2008.

[edit] See also

[edit] External links