gamma-ray astronomy

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The study of gamma rays of cosmic origin. This vast spectral domain extends from an energy of 0.05 MeV (wavelength of 2.5 × 10⁻¹¹ m), the adopted boundary between x-ray and gamma-ray photons, to 10¹¹ MeV (10⁻²³ m), an experimental barrier imposed by the extreme scarcity of ultrahigh-energy photons. See also Gamma rays.

Low-energy (or soft) gamma-ray astronomy (up to a few megaelectronvolts) deals mainly with processes in dense media, such as plasmas confined close to neutron stars and black holes. It also concerns cosmic sites where monoenergetic photons are released either by deexcitation of atomic nuclei (nuclear lines) or by positron annihilation (the 0.511-MeV line). Gamma-ray astronomy at higher energies relates to emissions induced by relativistic particles throughout the whole interstellar medium, as well as in the vicinity of some neutron stars and in the powerful jets beamed by active galactic nuclei. The penetration power of gamma-ray photons enables exploration of regions that are hidden at other wavelengths, such as the galactic center region, as well as of the first stages of the universe, since the cosmos is particularly transparent to gamma rays (with the exception of photons whose energy exceeds 10⁶ MeV).

Because photons in the gamma-ray regime are completely absorbed by the Earth's atmosphere, gamma-ray detectors are placed on board high-altitude balloons or, better still, artificial satellites. Ground-based telescopes, making use of the upper atmospheric layers as a detector, operate successfully in the very high energy gamma-ray band. See also Satellite (astronomy).

Instrumentation

In other regions of the electromagnetic spectrum, sensitivity is increased by the straightforward method of gathering large numbers of photons and concentrating them to form an image, by means of arrangements of reflectors or lenses. Such a method does not apply to gamma-ray telescopes, since gamma-ray photons can be neither reflected nor refracted. However, gamma-ray concentrators based on Laue diffraction have become feasible, and signal-to-noise ratio could be dramatically improved.

Soft gamma-ray telescopes use the coded-aperture technique to image celestial sources. A coded mask is a pattern of tungsten blocks that absorb gamma-ray photons and are arranged so that a given point source at infinity projects on a position-sensitive detector a pattern that is characteristic of the direction of arrival of the photons. The position of the source in the sky is determined by comparing the observed pattern with all possible projection patterns.

High-energy gamma-ray observations are performed with devices derived from particle-physics detectors. High-energy gamma-ray photons interact almost exclusively via electron-positron pair production. After passing undetected through an anticoincidence shield sensitive to charged particles, a photon is converted to an electron-positron pair in one of the conversion foils. Trajectories of the resulting electron and positron are measured by particle tracking detectors. The energy deposited in an underlying calorimeter is used with the trajectory data to determine the arrival direction and total energy of the gamma rays.

As the gamma-ray energy approaches 10⁵ MeV, the intensities of celestial gamma rays become too low for them to be seen with space telescopes. However, at energies above a few thousand megaelectronvolts (wavelengths of less than 10⁻¹⁵ m), a gamma-ray photon induces in the upper atmosphere a shower of secondary relativistic particles whose propagation through the air produces a narrow beam of Cerenkov visible light which can be detected on the ground by a large parabolic mirror. Detailed studies of the Cerenkov light beam enable the determination of the arrival direction of the generating gamma ray to within 0.1° while discriminating gamma-ray-induced events from much more numerous events induced by interactions of very high energy cosmic-ray protons and nuclei. See also Cerenkov radiation; Cosmic rays.

Stellar sources

Other than the Sun, all stellar sources of gamma rays relate to massive stars in their final stages of evolution. Major contributions to the theoretical understanding of explosive nucleosynthesis have come from the data obtained on supernova SN 1987A, which appeared on February 24, 1987, in the Large Magellanic Cloud, a nearby galaxy. See also Nucleosynthesis; Supernova.

Attested by the discovery of radio pulsars in 1967, the capability of neutron stars to accelerate relativistic electrons was confirmed a few years later by the discovery of the gamma-ray emission of the Crab and Vela pulsars. Six or more pulsars were detected by the Compton Gamma-Ray Observatory. The energy spectrum of the Crab pulsar suggests that the gamma radiation results from the synchrotron emission of relativistic electrons in the intense magnetic fields which prevail in the close vicinity of newly formed neutron stars. It is generally agreed that the ultimate source of the radiated energy is the rotational energy of the neutron star. Electrons are accelerated to very high energies by the huge electric fields induced by the pulsar rotation. See also Crab Nebula; Neutron star; Pulsar.

Almost all the known accreting black-hole systems produce strong and variable gamma-ray emission. In systems with low-mass (less than one solar mass) companion stars, huge outbursts called x-ray novae are observed, such as Nova Muscae 1991, which for one week was the brightest source in the soft gamma-ray sky. Several accreting black holes have been observed in the central region of the Milky Way Galaxy. See also Astrophysics, high-energy; Black hole.

Other galactic sources

A large fraction of the cosmic gamma-ray photons originates in interstellar sites. Cosmic-ray-induced interstellar emission results mostly from the interaction of cosmic rays (electrons and protons) with the interstellar gas. The high-energy gamma-ray sky is dominated by radiation from the galactic plane whose spatial distribution and intensity can be reliably modeled from knowledge of the interstellar gas distribution. A large fraction of the pointlike sources observed at medium galactic latitude may be related to the local interstellar medium, and more specifically to the giant cloud complexes of Gould's Belt.

Spectroscopic observations of the whole galactic center region have demonstrated the presence of a large-scale component of 0.511-MeV radiation due to the annihilation of positrons in the interstellar medium. The source of such interstellar positrons is thought to be the β⁺-decay products from radioactive nuclides produced by novae, red giants, Wolf-Rayet stars, and supernovae. See also Giant star; Nova; Positron; Wolf-Rayet star.

Extragalactic sources

With the exception of the Large Magellanic Cloud, all localized extragalactic sources of gamma rays are active galactic nuclei, the most energetic and distant objects in the universe. These include radio sources such as Seyfert galaxies and quasars, all with different properties depending on the observing wavelength. The ultimate source of active galactic nuclei activity is believed to be massive (10⁶–10⁹-solar-mass) black holes, accreting 10–100 solar masses per year to account for their overall luminosity. An accretion disk with a collimated perpendicular jet is the favored model for explaining the luminous, broadband radiation emitted from the central engines of active galactic nuclei. See also Galaxy, external; Quasar.

Gamma-ray bursts

Gamma-ray bursts were first detected by means of the Vela spacecraft in 1967, and their discovery was announced in 1973. More than 2700 bursts were detected by the Compton Gamma-Ray Observatory. Its observations give strong evidence for an isotropic distribution of bursts; that is, the burst directions appear to be random and show no preference for the galactic plane, in particular. The nearest reasonable source location is probably in an extended halo surrounding the Milky Way Galaxy; a great many astrophysicists now favor greater (cosmological) distances.

Knowledge about gamma-ray bursts has increased rapidly in recent years due to the launch of spacecraft such as BeppoSAX, and to the development of a very rapid alert network (the Gamma-Ray Burst Coordinate Network) to allow follow-up observations by ground observers. Optical, x-ray, and radio afterglows have been detected for a number of gamma-ray bursts, and in a few cases redshifts have been reported for optical sources in the gamma-ray burst error boxes.

The arguments for cosmological distances of most gamma-ray bursts include the highly isotropic distribution of sources and the redshifts reported for a small number of gamma-ray bursts, at least for optical sources within the error boxes for the bursts. To account for the enormous amounts of energy required for such cosmological distances (greater than 10⁵⁴ ergs or 10⁴⁷ J) models have been developed based on “hypernovae” (much brighter than supernovae), collisions between two neutron stars, narrow searchlight beams of emission aimed at the Earths, and even gravitational lensing to increase the intensity.

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

Compton Gamma Ray Observatory

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

Contents 1 Early history 2 Early discoveries 3 Balloon flights 4 Solar flares 5 Recent and current observatories 6 See also 7 References 8 External links

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, Philip 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.

Most gamma rays coming from space are absorbed by the Earth's atmosphere, so gamma-ray astronomy could not develop until it was possible to get detectors above all or most of the atmosphere using balloons and 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. They 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 interstellar gas.

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 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 high-speed collisions and similar processes). 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 resolution of the instruments was insufficient to identify most of these point sources with specific visible 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 military defense satellites. 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 from deep space rather than the vicinity of the Earth. Later detectors determined that these gamma-ray bursts are seen to last for fractions of a second to minutes, appearing suddenly from unexpected directions, flickering, and then fading after briefly dominating the gamma-ray sky. Studied since the mid-1980s 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.

Balloon flights

On Jun 19 1988 from Birigüi (50° 20' W 21° 20' S) at 10:15 UT a balloon launch occurred which carried two NaI(Tl) detectors (600 cm² total area) to an air pressure altitude of 5.5 mb for a total observation time of 6 hr.^[1] The supernova SN1987A in the Large Magellanic Cloud (LMC) was discovered on Feb 23 1987 and its progenitor is a blue supergiant (Sk -69 202) with luminosity of 2-5 x 10³⁸ erg/s.^[1] The 847 keV and 1238 keV gamma ray lines from ⁵⁶Co decay have been detected.^[1]

Solar flares

Nuclear gamma rays were observed from the solar flares of Aug 4 and 7 1972 and Nov 22 1977.^[2]

Recent and current observatories

During its High Energy Astronomy 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. 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 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 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 (supernovas and hypernovas) in distant galaxies.

Currently the main space-based gamma-ray observatories are the INTErnational Gamma-Ray Astrophysics Laboratory, (INTEGRAL), and the Gamma-ray Large Area Space Telescope (GLAST). INTEGRAL is an ESA mission with additional contributions from Czech, Poland, USA and Russia. It was launched on 17 October 2002. NASA launched GLAST 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 Cherenkov light the ultra-relativistic shower particles emit. 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 at Mt. Hopkins, in Arizona in the 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 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

• Gamma-ray telescopes (Alphabetic list)
• Gamma-ray Large Area Space Telescope, GLAST.
• X-ray astronomy
• Cosmic-ray observatory

References

1. ^ ^{a b c} Figueiredo N, Villela T, Jayanthi UB, Wuensche CA, Neri JACF, Cesta RC (1990). "Gamma-ray observations of SN1987A". Rev Mex Astron Astrofis. 21: 459-62. http://articles.adsabs.harvard.edu//full/1990RMxAA..21..459F/0000459.000.html. 
2. ^ Ramaty R, Kozlovsky B, Lingenfelter RE (Jul 1979). "Nuclear gamma-rays from energetic particle interactions". Ap J Suppl Ser. 40: 487-526. http://adsabs.harvard.edu/full/1979ApJS...40..487R. 

External links

• A History of Gamma-Ray Astronomy Including Related Discoveries
• The HEGRA Atmospheric Cherenkov Telescope System
• The HESS Ground Based Gamma-Ray Experiment
• The MAGIC Telescope Project
• The VERITAS Ground Based Gamma-Ray Experiment
• The space-borne INTEGRAL observatory
• NASA's Swift gamma-ray burst mission
• The CACTUS Ground Based Air Cherenkov Telescope
• NASA HETE-2 satellite

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