伽馬射線天文學

伽馬射線會被地球大氣層完全吸收，必須使用太空望遠鏡觀測

伽瑪射線天文學是指以伽瑪射線研究宇宙的天文學分支。伽瑪射線是可穿透整個宇宙的電磁波中最高能量的波段，也是電磁波譜中波長最短的部分。

伽瑪射線可由太空中的超新星、正電子湮滅、黑洞形成、甚至是放射衰變產生。例如超新星 SN 1987A 就發射了來自超新星爆炸的放射性產物鈷56釋放的伽瑪射線。大多數天體釋放的伽瑪射線一般認為並非來自放射衰變，而是和X射线天文学一樣來自加速的電子、電子和正電子作用（但因為能量較高而產生伽瑪射線）。

康普顿伽玛射线天文台

早期歷史

早在開發出可以偵測到宇宙中伽瑪射線的儀器之前，天文學家就已經知道在宇宙中應該有天體可產生如此高能的光子。1948年時的尤金·芬伯格和亨利·普里馬科夫；1952年的早川幸男和 I·B·哈欽松、特別是1958年時菲利浦·莫里森的研究讓科學家相信在宇宙中有多種不同的物理機制可產生伽瑪射線輻射。這些機制包含宇宙線和星际物质的交互作用、超新星爆炸、加速電子和磁場交互作用。但直到1960年代人類才有能力偵測到宇宙中的伽瑪射線。

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.

在太空中發現伽馬射線源

A 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.

兩個位在銀河系中心的巨大伽瑪射線泡

2010年11月，费米伽玛射线空间望远镜發現了兩個位於銀河系中心的巨大伽瑪射線泡。這兩個伽瑪射線泡外觀是互相鏡像對稱 [1]。 These bubbles of high-energy radiation are suspected as erupting from a massive black hole or evidence of a burst of star formations from millions of years ago. These bubbles have been measured and span 25,000 light-years across. They were discovered after scientists filtered out the "fog of background gamma-rays suffusing the sky". This discovery confirmed previous clues that a large unknown "structure" was in the center of the Milky Way.

探空氣球

1988年6月19日 10:15 UTC，一個探空氣球在巴西比裡吉（50° 20' W 21° 20' S）被釋放，該氣球搭載了兩個總面積 600 cm² 的碘化鈉偵測器，並上升到氣壓高度 5.5 mb 處進行6小時觀測。大麥哲倫星系中的超新星SN 1987A於1987年2月23日被發現，其前身星是光度 2-5 x 10³⁸ erg/s 的藍超巨星 Sanduleak -69° 202a。該次觀測發現了來自鈷56放射衰變產生的 847 keV 和 1238 keV 伽瑪射線譜線。

耀斑

A solar flare is an explosion in a solar atmosphere and was originally detected visually in our own sun. Solar flares create massive amounts of radiation across the full electromagnetic spectrum from the longest wavelength, radio waves, to high energy gamma rays. The correlations of the high energy electrons energized during the flare and the gamma rays are mostly caused by nuclear combinations of high energy protons and other heavier ions. These gamma-rays can be observed and allow scientists to determine the major results of the energy released, which is not provided by the emissions from other wavelengths. Nuclear gamma rays were observed from the solar flares of August 4 and 7, 1972, and November 22, 1977.

最近與未來的伽瑪射線望遠鏡

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 Fermi. INTEGRAL is an ESA mission with additional contributions from Czech, Poland, USA and Russia. It was launched on 17 October 2002. NASA launched Fermi on 11 June 2008. It 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.

參見

• 伽馬射線望遠鏡列表
• 费米伽玛射线空间望远镜（原名 Gamma-ray Large Area Space Telescope, GLAST，大面積伽瑪射線太空望遠鏡）
• X射线天文学
• 宇宙線天文學

外部連結

• 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
• NASA HETE-2 satellite
• TeVCat, a TeV gamma-ray sources catalog.
• GammaLib, a versatile toolbox for high-level analysis of astronomical gamma-ray data.