# Radiation astronomy/Gamma rays

The Moon is seen by the Compton Gamma Ray Observatory, in gamma rays of greater than 20 MeV. Credit: D. J. Thompson, D. L. Bertsch (NASA/GSFC), D. J. Morris (UNH), R. Mukherjee (NASA/GSFC/USRA).

Gamma-ray astronomy is radiation astronomy applied to the various extraterrestrial gamma-ray sources, especially at night. It is usually conducted above the Earth's atmosphere and at locations away from the Earth as a part of explorational (or exploratory) gamma-ray astronomy.

An introduction to gamma rays may occur at the secondary education level. The study of this type of radiation usually intensifies at the university undergraduate level. The more hazardous aspects of gamma radiation become known when a student embarks on graduate study.

As with general radiation astronomy some cautionary speculation may be introduced unexpectedly to stimulate the imagination and open a small crack in a few doors that may appear closed at present. This advances the learning portion of the resource to being a lecture and part article so some state-of -the-art results from the scholarly literature can be included.

The laboratories of gamma-ray astronomy are limited to the observatories themselves and the computers and other instruments (sometimes off site) used to analyze the results.

"Gamma radiation astronomy" is a term that dates back to 1965: PETERSON, LE. "Experiments in X-ray and gamma-ray astronomy(X-ray and gamma radiation astronomy- OSO MEASUREMENTS)." 1965. 15 P (1965).

## Astronomy

When any effort to acquire a system of laws or knowledge focusing on an astr, aster, or astro, that is, any natural body in the sky especially at night,[1] discovers an entity emitting, reflecting, or fluorescing gamma rays, succeeds even in its smallest measurement, gamma-ray astronomy is the name of the effort and the result. Once an entity, source, or object has been detected as emitting, reflecting, or fluorescing gamma rays, it may be necessary to determine what the mechanism is. Usually this information provides understanding of the same entity, source, or object.

Gamma rays are the most energetic rays of the electromagnetic spectrum.

Gamma rays are emitted during radioactive decay processes such as those occurring in nuclear explosions. Credit: National Nuclear Security Administration, USA.

Def. very high frequency (and therefore very high energy) electromagnetic radiation emitted as a consequence of radioactivity is called a gamma ray.

Def. electromagnetic radiation consisting of gamma rays is called gamma radiation.

Gamma rays typically have frequencies above 10 exahertz (or >1019 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometers (less than the diameter of an atom). However, this is not a hard and fast definition, but rather only a rule-of-thumb description for natural processes. Gamma rays from radioactive decay are defined as gamma rays no matter what their energy, so that there is no lower limit to gamma energy derived from radioactive decay. Gamma decay commonly produces energies of a few hundred keV, and almost always less than 10 MeV. In astronomy, gamma rays are defined by their energy, and no production process need be specified. The energies of gamma rays from astronomical sources range over 10 TeV, at a level far too large to result from radioactive decay. A notable example is extremely powerful bursts of high-energy radiation normally referred to as long duration gamma-ray bursts, which produce gamma rays by a mechanism not compatible with radioactive decay.

"The unusually wide span of the gamma-ray spectral window [covers] at least ten decades of photon energies (~105 - 1015 eV)".[2]

## Actinide minerals

Actinide minerals, or actinides, are those with unusually high concentrations, atomic per cents, or weight per cents, of the actinide elements.

## Autunites

This gamma-ray spectrum contains the typical isotopes of the uranium-radium decay line. Credit: Wusel007.

Elements usually emit a gamma-ray during nuclear decay or fission. The gamma-ray spectrum at right shows typical peaks for 226Ra, 214Pb, and 214Bi. These isotopes are part of the uranium-radium decay line. As 238U is an alpha-ray emitter, it is not shown. The peak at 40 keV is not from the mineral. From the color of the rock shown the yellowish mineral is likely to be autunite.

Autunite occurs as an oxidizing product of uranium minerals in granite pegmatites and hydrothermal deposits.

## Pitchblendes

This is an image of the mineral pitchblende, or uraninite. Credit: Geomartin.

These crystals are uraninite from Trebilcock Pit, Topsham, Maine. Credit: Robert Lavinsky.

Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely uranium dioxide UO2, but also contains uranium trioxide UO3 and oxides of lead, thorium, and rare earth elements. It is most commonly known as pitchblende (from pitch, because of its black color. All uraninite minerals contain a small amount of radium as a radioactive decay product of uranium. Uraninite also always contains small amounts of the lead isotopes 206Pb and 207Pb, the end products of the decay series of the uranium isotopes 238U and 235U respectively. The extremely rare element technetium can be found in uraninite in very small quantities (about 0.2 ng/kg), produced by the spontaneous fission of uranium-238.

The image at left shows well-formed crystals of uraninite. The image at right shows botryoidal uraninite. Because of the uranium decay products, both sources are gamma-ray emitters.

## Thorianites

This specimen of thorianite is from th Ambatofotsy pegmatite in Madagascar. Credit: Robert Lavinsky.

Thorianite is a rare thorium oxide mineral, ThO2.[3] ... [It has a] high percentage of thorium; it also contains the oxides of uranium, lanthanum, cerium, praseodymium and neodymium. ... the mineral is slightly less radioactive than pitchblende, but is harder to shield due to its high energy gamma rays. It is common in the alluvial gem-gravels of Sri Lanka, where it occurs mostly as water worn, small, heavy, black, cubic crystals.

## Torbernites

Torbernitte is a hydrated green copper uranyl phosphate mineral. Credit: Didier Descouens.

Torbernite is a radioactive, hydrated green copper uranyl phosphate mineral, found in granites and other uranium-bearing deposits as a secondary mineral. Torbernite is isostructural with the related uranium mineral, autunite. The chemical formula of torbenite is similar to that of autunite in which a Cu2+ cation replaces a Ca2+. The number of water hydration molecules can vary between 12 and 8, giving rise to the variety of metatorbernite when torbernite spontaneously dehydrates.

## Uranophanes

Uranophane is a calcium uranium silicate hydrate mineral. Credit: United States Geological Survey.

Uranophane Ca(UO2)2(SiO3OH)2·5H2O is a rare calcium uranium [nesosilicate] hydrate mineral that forms from the oxidation of uranium bearing minerals. Uranophane is also known as uranotile. It has a yellow color and is radioactive.

## Continua

In the spectrum above for probably the mineral autunite, there are about a dozen discrete lines superimposed on a smooth continuum.

Because the energy level spectrum of nuclei typically dies out above about 10 MeV, gamma-ray instruments looking to still higher energies generally observe only continuum spectra, so that the moderate spectral resolution of scintillation (often sodium iodide (NaI) or caesium iodide, (CsI) spectrometers), often suffices for such applications.

Like X-rays, the gamma-ray "continuum can arise from bremsstrahlung, black-body radiation, synchrotron radiation, or what is called inverse Compton scattering of lower-energy photons by relativistic electrons, knock-on collisions of fast protons with atomic electrons, and atomic recombination, with or without additional electron transitions.[4]

The acceleration of electrons is revealed by hard X-ray and gamma-ray bremsstrahlung while the acceleration of protons and ions is revealed by gamma-ray lines and continuum.

At the same time, there is a continuum containing all the "different kinds" of electromagnetic radiation.

Americium-241 is suitable for calibration of gamma-ray spectrometers in the low-energy range, since its spectrum consists of nearly a single peak and negligible Compton continuum (at least three orders of magnitude lower intensity).[5]

When compared to the more luminous active nuclei (quasars) with strong emission lines, BL Lac objects have spectra dominated by a featureless non-thermal continuum.[6]

## Backgrounds

This graph shows the power density spectrum of the extragalactic or cosmic gamma-ray background (CGB). Credit: pkisscs@konkoly.hu.

In March 2010 it was announced that active galactic nuclei are not responsible for most gamma-ray background radiation.[7] Though active galactic nuclei do produce some of the gamma-ray radiation detected here on Earth, less than 30% originates from these sources. The search now is to locate the sources for the remaining 70% or so of all gamma-rays detected. Possibilities include star forming galaxies, galactic mergers, and yet-to-be explained dark matter interactions.

Sensitivity to celestial sources by Vela 5A and 5B was severely limited by the high intrinsic detector background, equivalent to about 80% of the signal from the Crab Nebula, one of the brightest sources in the sky at these wavelengths.[8]

Kosmos 60 measured the gamma-ray background flux density to be 1.7×104 quanta/(m2·s). As was seen by Ranger 3 and Lunas 10 & 12, the spectrum fell sharply up to 1.5 MeV and was flat for higher energies. Several peaks were observed in the spectra which were attributed to the inelastic interaction of cosmic protons with the materials in the satellite body.

## Planetary sciences

The Gamma Ray and Neutron Detector (GRaND) onboard the Dawn spacecraft is based on similar instruments flown on the Lunar Prospector and Mars Odyssey space missions. It will be used to measure the abundances of the major rock-forming elements (oxygen, magnesium, aluminium, silicon, calcium, titanium, and iron) on Vesta and Ceres, as well as potassium, thorium, uranium, and water (inferred from hydrogen content).[9][10][11][12][13][14]

## Theoretical gamma-ray astronomy

Theoretically a black body emits electromagnetic radiation over the entire spectrum from very low frequency radio waves to x-rays, and gamma-rays creating a continuum of radiation.

"[T]he mass lost during the evolution of very massive stars may be dominated by optically thick, continuum-driven outbursts or explosions, instead of by steady line-driven winds."[15]

"[T]he 19th century outburst of η Carinae, when the star shed 12-20 M or more in less than a decade" is an example of "mass loss during brief eruptions of luminous blue variables (LBVs)."[15]

"[T]he extreme mass loss probably arises from a continuum-driven wind or a hydrodynamic explosion, both of which are insensitive to metallicity."[15]

"If [eruptive mass losses] occur in ... Population III stars, such eruptions ... profoundly affect the chemical yield and types of remnants from early supernovae and hypernovae thought to be the origin of long gamma-ray bursts."[15]

Type Ib and Ic supernovae, like those of Type II, are massive stars that undergo core collapse. However the stars which become Types Ib and Ic supernovae have lost most of their outer (hydrogen) envelopes due to strong stellar winds or else from interaction with a companion.[16] These stars are known as Wolf-Rayet stars, and they occur at moderate to high metallicity where continuum driven winds cause sufficiently high mass loss rates.

## Meteors

This is an XMM Newton image of the Gemini gamma-ray source. Credit: P.A. Caraveo (INAF/IASF), Milan and ESA.

The gamma-ray source Geminga, shown at right in hard X-rays by the satellite XMM Newton, is first observed by the Second Small Astronomy Satellite (SAS-2).

Geminga may be a sort of neutron star: the decaying core of a massive star that exploded as a supernova about 300,000 years ago.[17]

This nearby explosion may be responsible for the low density of the interstellar medium in the immediate vicinity of the Solar System. This low-density area is known as the Local Bubble.[18] Possible evidence for this includes findings by the Arecibo Observatory that local micrometre-sized interstellar meteor particles appear to originate from its direction.[19]

"Geminga is a very weak neutron star and the pulsar next to us, which almost only emits extremely hard gamma-rays, but no radio waves. ... Some thousand years ago our Sun entered this [Local Bubble] several hundred light-years big area, which is nearly dust-free."[20]

## Neutrinos

Solar neutrinos are shown for the proton-proton chain in the Standard Solar Model. Credit: Dorottya Szam.

The following fusion reaction produces neutrinos and accompanying gamma-rays of the energy indicated:

${\displaystyle \mathrm {_{1}^{1}H} +\mathrm {_{1}^{1}H} \rightarrow \mathrm {_{1}^{2}D} +e^{+}+\nu _{e}+\gamma (0.42MeV).}$

Observation of gamma rays of this energy likely indicate this reaction is occurring nearby.

In the Cowan–Reines neutrino experiment, antineutrinos created in a nuclear reactor by beta decay reacted with protons producing neutrons and positrons:

ν
e
+ p+
n0
+ e+

The positron quickly finds an electron, and they annihilate each other. The two resulting gamma rays (γ) 511 keV each are detectable. The neutron can be detected by its capture on an appropriate nucleus, releasing a gamma ray. The coincidence of both events – positron annihilation and neutron capture – gives a unique signature of an antineutrino interaction.

## Superluminals

This is an image of quasar 3C 279 in gamma rays. Credit: NASA EGRET Compton observatory team.

The Rosemary Hill Observatory (RHO) started observing 3C 279 in 1971,[21] and was further observed by the Compton Gamma Ray Observatory in 1991, when it was unexpectedly discovered to be one of the brightest gamma ray objects in the sky.[22] It is also one of the most bright and variable sources in the gamma ray sky monitored by the Fermi Space Telescope. Apparent superluminal motion was detected during observations first made in 1973 in a jet of material departing from the quasar, though it should be understood that this effect is an optical illusion caused by naive estimations of the speed, and no truly superluminal motion is occurring.[23]

Markarian (Mrk) 1501 is the first Seyfert I galaxy to have superluminal motion.[24] Mrk 1501 is an ultraviolet, X-ray, and gamma-ray source.

## Entities

Def. "the ratio of the area of causally connected regions that become active to the observable area of the shell" is called the surface filling factor.[25]

"In the external shock model [for gamma-ray bursts] ... multiple peaks ... [may arise] because various patches (or emitting "entities") on the shell randomly become active. ... [S]o many entities [can] become simultaneously active that the overall envelope appears quite smooth [on the other hand when] fewer entities become active, ... random fluctuations in the number of simultaneously active entities cause the peak structure to be spiky. ... [E]ach observed peak is not necessarily caused by a single entity, but the peak structure is caused by random variations in the number of active entities".[25]

## Sources

Most astronomical gamma-rays are thought to be produced not from radioactive decay, however, but from the same type of accelerations of electrons, and electron-photon interactions, that produce X-rays in astronomy (but occurring at a higher energy in the production of gamma-rays).

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.[26]

## Objects

Over the entire celestial sphere, SIMBAD currently records 3253 gamma-ray objects. Some of these, like 4U 1705-44, are low-mass X-ray binaries (LMXBs). Some, like V779 Centauri, are high-mass X-ray binaries (HXMBs). Others are quasi-stellar objects (PKS 1326-697), supernova remnants (SNRs) like Messier 1 (M 1), and Seyfert galaxies like M 98. Some gamma-ray emitting objects have not been sufficiently resolved to determine what they are.

## Sun

The Sun is seen in gamma rays by COMPTEL during a June 15, 1991, solar flare. Credit: COMPTEL team, University of New Hampshire.

The Sun, which has no similar surface of high atomic number [like the Moon] to act as [a] target for cosmic rays, cannot usually be seen at all at these energies [greater than 20 MeV], which are too high to emerge from primary nuclear reactions, such as solar [interior] nuclear fusion (though occasionally the Sun produces gamma rays by cyclotron-type mechanisms, during solar flares).

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.[27] Nuclear gamma rays were observed from the solar flares of August 4 and 7, 1972, and November 22, 1977.[28]

RHESSI was the first satellite to image solar gamma rays from a solar flare[29].

## Mercury

The surface of Mercury over the planet's northern hemisphere has been mapped using gamma-ray counts to determine the distributions of the elements oxygen, silicon, and potassium.[30]

"The lack of a significant variation in the measured Th abundances suggests that there may be considerable variability in the K/Th abundance ratio over the mapped regions."[30]

## Venus

"Concentrations of natural radioactive elements U, Th, and K in the Venusian mountain rocks were obtained by gamma ray spectrometers aboard the Vega 1 and Vega 2 descent modules that landed close to Mermaid Valley and the northeastern slope of Aphrodite Terra, respectively."[31]

"[T]he chemical composition of the Venusian rocks studied is similar to that of basic rocks of the earth's crust, tholeiitic basalts and gabbros."[31]

## Earth

The red dots show some of the ~500 terrestrial gamma-ray flashes daily detected by the Fermi Gamma-ray Space Telescope through 2010. Credit: NASA/Goddard Space Flight Center.

In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles. Mixing matter and antimatter can lead to the annihilation of both, in the same way that mixing antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particle–antiparticle pairs.

The Earth's atmosphere is a relatively bright source of gamma rays produced in interactions of ordinary cosmic ray protons with air atoms.

A number of observations by space-based telescopes have revealed gamma ray emissions, specifically, terrestrial gamma-ray flashes (TGFs). These observations pose a challenge to current theories of lightning, especially with the discovery of the clear signatures of antimatter produced in lightning.[32]

A TGF has been linked to an individual lightning stroke occurring within 1.5 ms of the TGF event,[33] proving for the first time that the TGF was of atmospheric origin and associated with lightning strikes.

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft "has been observing TGFs at a much higher rate, indicating that these occur about 50 times per day globally (still a very small fraction of the total lightning on the planet). The energy levels recorded exceed 20 MeV. [Apparently], the gamma radiation fountains upward from starting points at surprisingly low altitudes in thunderclouds."[34]

"These are higher energy gamma rays than come from the sun. And yet here they are coming from the kind of terrestrial thunderstorm that we see here all the time."[35] In 2009, [the] Fermi Gamma Ray Telescope in Earth orbit observed [an] intense burst of gamma rays corresponding to positron annihilations coming out of a storm formation. Scientists wouldn't have been surprised to see a few positrons accompanying any intense gamma ray burst, but the lightning flash detected by Fermi appeared to have produced about 100 trillion positrons. This has been reported by media in January 2011, it is an effect, never considered to happen before.[36]

Airborne gamma-ray spectrometry is now the accepted leading technique for uranium prospecting with worldwide applications for geological mapping, mineral exploration & environmental monitoring.

## Moon

This image is an elemental map of the Moon using a GRS. Credit: Los Alamos National Laboratory.

The Compton Gamma Ray Observatory has imaged the Moon in gamma rays of energy greater than 20 MeV.[37] These are produced by cosmic ray bombardment of its surface.

Gamma-ray spectrometers have been widely used for the elemental and isotopic analysis of airless bodies in the Solar System, especially the Moon[38] These surfaces are subjected to a continual bombardment of high-energy cosmic rays, which excite nuclei in them to emit characteristic gamma-rays which can be detected from orbit. Thus an orbiting instrument can in principle map the surface distribution of the elements for an entire planet. They are able to measure the abundance and distribution of about 20 primary elements of the periodic table, including silicon, oxygen, iron, magnesium, potassium, aluminum, calcium, sulfur, and carbon. The chemical element thorium is mapped by a GRS, with higher concentrations shown in yellow/orange/red in the left-hand side image shown on the right.

## Mars

"The Gamma-Ray Spectrometer (GRS) on the 2001 Mars Odyssey spacecraft has mapped the surface abundances of [radiogenic heat-producing elements] HPEs across Mars."[39]

## Vesta

"Using Dawn’s Gamma Ray and Neutron Detector, ... Global Fe/O and Fe/Si ratios are consistent with [howardite, eucrite, and diogenite] HED [meteorite] compositions."[40]

## Sagittarius A*

The European Space Agency's gamma-ray observatory INTEGRAL has observed gamma rays interacting with the nearby giant molecular cloud Sagittarius B2, causing x-ray emission from the cloud. This energy was emitted about 350 years before by Sgr A*. The total luminosity from this outburst L ≈ 1,5x1039 erg/s) is an estimated million times stronger than the current output from Sgr A* and is comparable with a typical active galactic nucleus.[41][42] This conclusion has been supported in 2011 by Japanese astronomers [who] observed the Galaxy center with [the] Suzaku satellite.[43]

## Gamma-ray bubbles

This image depicts the two gigantic gamma-ray bubbles at the heart of the Milky Way. Credit: .

In November 2010, two gigantic gamma-ray bubbles were detected at the heart of our galaxy. These bubbles appear as a mirror image[44] of each other. 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.[45] 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.[46][47]

The bubbles stretch up to Grus and to Virgo on the night-sky of the southern hemisphere.

## Gamma-ray bursts

Positions on the sky of all gamma-ray bursts detected during the BATSE mission are shown. Credit: .

Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical burst lasts 20–40 seconds. The initial burst is usually followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).[48]

The distribution [of GRBs as shown in the BATSE mission results figure at right] is isotropic, with no concentration towards the plane of the Milky Way, which runs horizontally through the center of the image.

GRBs are named after the date on which they are discovered: the first two digits being the year, followed by the two-digit month and two-digit day. If two or more GRBs occur on a given day, the letter 'A' is appended to the name for the first burst identified, 'B' for the second, and so on.

On what date was GRB 970228 discovered?

On July 2, 1967, at 14:19 UTC, the Vela 4 and Vela 3 satellites detected a flash of gamma radiation that were unlike any known nuclear weapons signatures.[49]

In 2005, ESO telescopes detected, for the first time, the visible light following a short-duration burst and tracked this light for three weeks. This time, the conclusion was that the short-duration bursts could not be caused by a hypernova. Instead, it is thought that they originate in the violent mergers of neutron stars or black holes.[50] Observations of gamma-ray burst afterglows were also coordinated between the [Very Large Telescope] VLT and the Atacama Pathfinder Experiment (APEX) in order to identify the possible counterpart and its decay at submillimeter wavelengths.[51]

## Geminga pulsar

This all-sky view from GLAST reveals bright gamma-ray emission in the plane of the Milky Way (center), including the bright Geminga pulsar. Credit: NASA/DOE/International LAT Team.

The nature of Geminga was quite unknown for 20 years after its discovery by NASA's Second Small Astronomy Satellite (SAS-2). In March 1991 the ROSAT satellite detected a periodicity of 0.237 seconds in soft x-ray emission. This nearby explosion may be responsible for the low density of the interstellar medium in the immediate vicinity of the Solar System. This low-density area is known as the Local Bubble.[52] Possible evidence for this includes findings by the Arecibo Observatory that local micrometre-sized interstellar meteor particles appear to originate from its direction.[53] Geminga is the first example of a radio-quiet pulsar, and serves as an illustration of the difficulty of associating gamma-ray emission with objects known at other wavelengths: either no credible object is detected in the error region of the gamma-ray source, or a number are present and some characteristic of the gamma-ray source, such as periodicity or variability, must be identified in one of the prospective candidates (or vice-versa as in the case of Geminga).

## BL Lacertae objects

On June 28, 2011, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) detected "a very rapid TeV gamma-ray flare from BL Lacertae"[54]

## Soft gamma repeaters

A soft gamma repeater (SGR) is an astronomical object which emits large bursts of gamma-rays and X-rays at irregular intervals. In 1998,[55][56] astronomer Chryssa Kouveliotou made careful comparisons of the periodicity of soft gamma repeater SGR 1806-20. The period had increased by 0.008 seconds since 1993, and she calculated that this would be explained by a magnetar with a magnetic field strength of 8×1010 teslas (8×1014 gauss). This was enough to convince the international astronomical community that soft gamma repeaters are indeed magnetars.

## Sciences

One of the first catalogs of gamma-ray sources is the catalog of Vela satellite detections. Of the several Vela satellites launched into orbit around the Earth to detect nuclear tests most of them also detected celestial gamma-rays. Of the detections listed in the catalog, each dates from before January 27, 1972. Here are some of these early catalogs:

• "A Preliminary Catalog of Transient Cosmic Gamma-Ray Sources Observed by the VELA Satellites", contains 25 sources of gamma-ray bursts as of 1972, including X-ray sources, detected by the Vela series of satellites, initially designed as part of the monitoring system for nuclear detonations.[57] Sources from this catalog are preceded by a "V" in later catalogs, and dates of detection are included.
• "High-energy gamma-ray results from the second small astronomy satellite, high-energy (> 35 MeV)",[58] contains 20 potential gamma-ray sources, most of which are X-ray sources. The catalog includes dates of observation.
• "Gamma-ray sources observed by COS-B" contains 25 gamma-ray sources at energies > 100 MeV, between August 9, 1975, and April 25, 1982. prefixed with "2CG", and including "CG" catalog sources.[59] The catalog does not contain specific dates of observation. Sources are only in galactic coordinates. The circular error boxes for each source detected have radii ranging between 0.4° and 1.5°.
• "Third EGRET catalogue", contains 271 gamma-ray sources, prefixed with "3EG": 94 blazars, five pulsars, the Large Magellanic Cloud, one solar flare, and 170 sources, with no identification with known astrophysical objects.[60]
• "A General Gamma-Ray Source Catalog", contains 309 objects, from 50 keV to ~ 1 TeV.[61] It includes sources from "1A", "1E", "2CG", "2EG", "2EGS", "3C", "4U", "A", "ESO", "EXO", "EXS", "GEV", "GRO", "GRS", "GS", "GX", "H", "MRK", "NGC", "PKS", "PSR", "QSO", and "SS", among other catalogs. The catalog has dates of observation by a variety of observatories and sounding rockets.
• "The Third IBIS/ISGRI Soft Gamma-Ray Survey Catalog", contains 421 sources, from 18-100 keV.[62] The catalog does not contain dates of observation.

## Balloons

On June 19, 1988, from Birigüi (50° 20' W 21° 20' S) at 10:15 UTC 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.[63] The supernova SN1987A in the Large Magellanic Cloud (LMC) was discovered on February 23, 1987, and its progenitor is a blue supergiant (Sk -69 202) with luminosity of 2-5 x 1038 erg/s.[63] The 847 keV and 1238 keV gamma-ray lines from 56Co decay have been detected.[63]

"Gamma rays at energies of 0.3 to 8 megaelectron volts (MeV) were detected on 15 April 1988 from four nuclear-powered satellites including Cosmos 1900 and Cosmos 1932 as they flew over a double Compton gamma-ray telescope."[64]

## Satellites

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.

"[T]he University of Minnesota Gamma-ray Experiment [aboard OSO 1 ] was designed to provide preliminary measurements of the intensity and directional properties of low-energy gamma-rays in space. The detector operated in the 50 keV - 3 MeV range. For the 50-150 keV range, a NaI(Tl) scintillation crystal monitored radiation through a lead shield. The detector operating in the 0.3-1.0 MeV and 1.0-3.0 MeV energy regions used two scintillators connected as a Compton coincidence telescope. ... The U. Minnesota gamma-ray experiment on OSO 1 produced a measurement of the extraterrestrial gamma-ray flux between 0.5-3.0 MeV, and an indication of its origin on the celestial sphere. Equally important, this experiment began to define the background problems encountered in gamma-ray astronomy."[65] OSO 1 is launched on March 7, 1962.[65]

## Astro‐rivelatore Gamma a Immagini LEggero

AGILE (Astro‐rivelatore Gamma a Immagini LEggero) is an X-ray and Gamma ray astronomical satellite of the Italian Space Agency (ASI). The AGILE mission is to observe Gamma-Ray sources in the universe. AGILE’s instrumentation combines a gamma-ray imager (GRID) (sensitive in the energy range 30 MeV-50 GeV), a hard X-ray imager and monitor: Super-AGILE (sensitive in the range 18-60 KeV), a calorimeter (sensitive in the range 350 KeV-100 MeV) (MCAL), and an anticoincidence system (AC), based on plastic scintillator. AGILE was successfully launched on [April 23,] 2007.

## BeppoSAX

In February 1997 the Italian-Dutch satellite BeppoSAX, launched in April 1996, provided the first accurate positions of gamma-ray bursts, allowing follow-up observations and identification of the sources when the X-ray camera was pointed towards the direction from which the burst GRB 970228 had originated, it detected fading X-ray emission.

## Compton Gamma Ray Observatory

This is a schematic of the various experiments aboard the Gamma-ray Observatory. Credit: NASA/JPL.

Launched in 1991, the Compton Gamma Ray Observatory carried aboard the Burst and Transient Source Explorer (BATSE) instrument, an extremely sensitive gamma-ray detector. The BATSE " the sky for gamma ray bursts (20 to >600 keV) and conducted full sky surveys for long-lived sources. It consisted of eight identical detector modules, one at each of the satellite's corners (left, right; front and back; top and bottom). Each module consisted of both a NaI(Tl) Large Area Detector (LAD) covering the 20 keV to ~2 MeV range, 50.48 cm in dia by 1.27 cm thick, and a 12.7 cm dia by 7.62 cm thick NaI Spectroscopy Detector, which extended the upper energy range to 8 MeV, all surrounded by a plastic scintillator in active anti-coincidence to veto the large background rates due to cosmic rays and trapped radiation. Sudden increases in the LAD rates triggered a high-speed data storage mode, the details of the burst being read out to telemetry later. Bursts were typically detected at rates of roughly one per day over the 9-year CGRO mission. A strong burst could result in the observation of many thousands of gamma rays within a time interval ranging from ~0.1 s up to about 100 s.

The Oriented Scintillation Spectrometer Experiment, (OSSE), by the Naval Research Laboratory detected gamma rays entering the field of view of any of four detector modules, which could be pointed individually, and were effective in the 0.05 to 10 MeV range. Each detector had a central scintillation spectrometer crystal of NaI(Tl) 12 in (303 mm) in diameter, by 4 in (102 mm) thick, optically coupled at the rear to a 3 in (76.2 mm) thick CsI(Na) crystal of similar diameter, viewed by seven photomultiplier tubes, operated as a phoswich: i.e., particle and gamma-ray events from the rear produced slow-rise time (~1 μs) pulses, which could be electronically distinguished from pure NaI events from the front, which produced faster (~0.25 μs) pulses. Thus the CsI backing crystal acted as an active anticoncidence shield, vetoing events from the rear. A further barrel-shaped CsI shield, also in electronic anticoincidence, surrounded the central detector on the sides and provided coarse collimation, rejecting gamma rays and charged particles from the sides or most of the forward field-of-view (FOV). A finder level of angular collimation was provided by a tungston slat collimator grid within the outer CsI barrel, which collimated the response to a 3.8° x 11.4° FWHM rectangular FOV. A plastic scintillator across the front of each module vetoed charged particles entering from the front. The four detectors were typically operated in pairs of two. During a gamma-ray source observation, one detector would take observations of the source, while the other would slew slightly off source to measure the background levels. The two detectors would routinely switch roles, allowing for more accurate measurements of both the source and background. The instruments could slew with a speed of approximately 2 degrees per second.

The Imaging Compton Telescope, (COMPTEL) by the Max Planck Institute for Extraterrestrial Physics, the University of New Hampshire, Netherlands Institute for Space Research, and ESA's Astrophysics Division was tuned to the 0.75-30 MeV energy range and determined the angle of arrival of photons to within a degree and the energy to within five percent at higher energies. The instrument had a field of view of one steradian. For cosmic gamma-ray events, the experiment required two nearly simultaneous interactions, in a set of front and rear scintillators. Gamma rays would Compton scatter in a forward detector module, where the interaction energy E1, given to the recoil electron was measured, while the Compton scattered photon would then be caught in one of a second layer of scintillators to the rear, where its total energy, E2, would be measured. From these two energies, E1 and E2, the Compton scattering angle, angle θ, can be determined, along with the total energy, E1 + E2, of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The vector, V, connecting the two interaction points determined a direction to the sky, and the angle θ about this direction, defined a cone about V on which the source of the photon must lie, and a corresponding "event circle" on the sky. Because of the requirement for a near coincidence between the two interactions, with the correct delay of a few nanoseconds, most modes of background production were strongly suppressed. From the collection of many event energies and event circles, a map of the positions of sources, along with their photon fluxes and spectra, could be determined.

Instrument Observing Comparison BATSE 0.02 - 8 MeV OSSE 0.05 - 10 MeV COMPTEL 0.75 - 30 MeV EGRET 20 - 30 GeV

## Cos B

This is an image of the satellite Cos-B superimposed on an Earth-orbit-like background. Credit: NASA.

Cos-B was the first European Space Research Organisation mission to study gamma-ray sources. Scientific results included the 2CG Catalogue listing around 25 gamma ray sources and a map of the Milky Way. The satellite also observed the Cygnus X-3 pulsar.

"Gamma radiation astronomy only developed after specialized satellites had been put into outer space. The results are surveyed which were obtained by satellite COS-B. 25 discrete sources were detected but only 5 cosmic objects of different type were identified."[66]

## Energetic Gamma Ray Experiment Telescope

This is a high-energy gamma radiation image about the Earth, taken from Energetic Gamma Ray Experiment Telescope on the NASA’s Compton Gamma Ray Observatory satellite. Credit: United States Department of Energy.

The Energetic Gamma Ray Experiment Telescope, (EGRET) measured high energy (20 MeV to 30 GeV) gamma ray source positions to a fraction of a degree and photon energy to within 15 percent. EGRET was developed by NASA Goddard Space Flight Center, the Max Planck Institute for Extraterrestrial Physics, and Stanford University. Its detector operated on the principle of electron-positron pair production from high energy photons interacting in the detector. The tracks of the high-energy electron and positron created were measured within the detector volume,and the axis of the V of the two emerging particles projected to the sky. Finally, their total energy was measured in a large calorimeter scintillation detector at the rear of the instrument.

## Explorer 11

Launched on April 27, 1961, Explorer 11 is an American Earth-orbital satellite that carried the first space-borne gamma-ray telescope. This is the earliest beginning of space gamma-ray astronomy. During the spacecraft's seven month lifespan it detected twenty-two events from gamma-rays. The celestial distribution of the thirty-one arrival directions showed no statistically significant correlation with the direction of any potential cosmic source.

The gamma-ray telescope used a combination of a sandwich scintillator detector along with a Cherenkov counter to measure the arrival directions and energies of high-energy gamma rays. Since the telescope could not be aimed, the spacecraft was set in a slow spin to scan the celestial sphere.

## Fermi Gamma-ray Space Telescope

The Fermi Gamma-ray Space Telescope sits on its payload attachment fitting. Credit: NASA/Kim Shiflett.

The Milkyway gamma-ray bubbles have been detected using data of the Fermi Gamma-ray Space Telescope.

The Large Area Telescope (LAT) detects individual gamma rays using technology similar to that used in terrestrial particle accelerators. Photons hit thin metal sheets, converting to electron-positron pairs, via a process known as pair production. These charged particles pass through interleaved layers of silicon microstrip detectors, causing ionization which produce detectable tiny pulses of electric charge. Researchers can combine information from several layers of this tracker to determine the path of the particles. After passing through the tracker, the particles enter the calorimeter, which consists of a stack of caesium iodide scintillator crystals to measure the total energy of the particles. The LAT's field of view is large, about 20% of the sky. The resolution of its images is modest by astronomical standards, a few arc minutes for the highest-energy photons and about 3 degrees at 100 MeV. The LAT is a bigger and better successor to the EGRET instrument on NASA's Compton Gamma Ray Observatory satellite in the 1990s.

The Gamma-ray Burst Monitor (GBM) detects sudden flares of gamma-rays produced by gamma ray bursts and solar flares. Its scintillators are on the sides of the spacecraft to view all of the sky which is not blocked by the earth. The design is optimized for good resolution in time and photon energy. The Gamma-ray Burst Monitor has detected gamma rays from positrons generated in powerful thunderstorms.[67]

## Gamma

Gamma is a Soviet gamma ray telescope launched on 11 July 1990. The Gamma-1 telescope was the main telescope. It consisted of 2 scintillation counters and a gas Cerenkov counter. With an effective area of around 0.2 square metres (2.2 sq ft), it operated in the energy range of 50 MeV to 6 GeV. At 100 MeV it initially had an angular resolution of 1.5 degrees, with a field of view of 5 degrees and an energy resolution of 12%. A Telezvezda star tracker increased the pointing position accuracy of the Gamma-1 telescope to 2 arcminutes by tracking stars up to an apparent magnitude of 5 within its 6 by 6 degree field of view. However, due to the failure of power to a spark chamber, for most of the mission the resolution was around 10 degrees.[68]

## HEAO 1

This image shows the HEAO-1 satellite, a NASA High Energy Astronomy Observatory. Credit: NASA.

The A4, Hard X-ray / Low Energy Gamma-ray experiment, aboard HEAO 1, launched August 12, 1977, used sodium iodide (NaI) scintillation counters to cover the energy range from about 20 keV to 10 MeV.

Each detector was actively shielded by surrounding CsI scintillators, in active-anti-coincidence, so that an extraneous particle or gamma-ray event from the side or rear would be vetoed electronically, and rejected. (It was discovered in early balloon flight by experimenters in the 1960s that passive collimators or shields, made of materials such as lead, actually increase the undesired background rate, due to the intense showers of secondary particles and photons produced by the extremely high energy (GeV) particles characteristic of the space radiation environment.) A plastic anti-coincidence scintillation shield, essentially transparent to gamma-ray photons, protected the detectors from high-energy charged particles entering from the front. For all seven modules, the unwanted background effects of particles or photons entering from the rear was suppressed by a "phoswich" design, in which the active NaI detecting element was optically coupled to a layer of CsI on its rear surface, which was in turn optically coupled to a single photomultiplier tube for each of the seven units.

Because the NaI has a much faster response time (~0.25 μsec) than the CsI (~1 μsec), electronic pulse shape discriminators could distinguish good events××3 in NaI in the NaI from mixed events accompanied by a simultaneous interaction in the CsI.

The largest, or High Energy Detector (HED), occupied the central position and covered the upper range from ~120 keV to 10 MeV, with a field-of-view (FOV) collimated to 37° FWHM. Its NaI detector was 5 in (12.7 cm) in diameter by 3 in (7.62 cm) thick. The extreme penetrating power of photons in this energy range made it necessary to operate the HED in electronic anti-coincidence with the surrounding CsI and also the six other detectors of the hexagon.

Two Low Energy Detectors (LEDs) were located in positions 180° apart on opposite side of the hexagon. They had thin ~3 mm thick NaI detectors, also 5 in (12.7 cm) in diameter, covering the energy range from ~10—200 keV. Their FOV was defined to fan-shaped beams of 1.7° x 20° FWHM by passive, parallel slat-plate collimators. The slats of the two LEDs were inclined to ±30° to the nominal HEAO scanning direction, crossing each other at 60°. Thus, working together, they covered a wide field of view, but could localize celestial sources with a precision determined by their 1.7° narrow fields.

The four Medium Energy Detectors (MEDs), with a nominal energy range of 80 keV — 3 MeV, had 3 in (7.62 cm) dia by 1 in (2.54 cm) thick NaI detector crystals, and occupied the four remaining positions in the hexagon of modules. They had circular FOVs with a 17° FWHM.

Results of the experiment included a catalog of the positions and intensities of hard X-ray (10—200 keV) sources.[69]

## HEAO 3

High Energy Astronomy Observatory number 3 (HEAO 3) undergoes testing prior to launch in its clean room. Credit: NASA.

The HEAO 3, launched on 20 September 1979 carried [an experiment] known as C1, which was a cryogenically cooled germanium (Ge) high-resolution gamma-ray spectrometer.

The C1 instrument was a sky-survey experiment, operating in the hard X-ray and low-energy gamma-ray bands. It was especially designed to search for the 511 keV gamma-ray line produced by the annihilation of positrons in stars, galaxies, and the interstellar medium (ISM), nuclear gamma-ray line emission expected from the interactions of cosmic rays in the ISM, the radioactive products of cosmic nucleosynthesis, and nuclear reactions due to low-energy cosmic rays. In addition, careful study was made of the spectral and time variations of known hard X-ray sources. The experimental package contained four cooled, p-type high-purity Ge gamma-ray detectors with a total volume of about 100 cm${\displaystyle ^{3}}$ , enclosed in a thick (6.6 cm average) caesium iodide (CsI) scintillation shield in active anti-coincidence[70] to suppress extraneous background. The experiment was capable of measuring gamma-ray energies falling within the energy interval from 0.045 to 10 MeV. The Ge detector system had an initial energy resolution better than 2.5 keV at 1.33 MeV and a line sensitivity from 1.E-4 to 1.E-5 photons/sq cm-s, depending on the energy. Key experimental parameters were (1) a geometry factor of 11.1 sq cm-sr, (2) effective area ~75 cm${\displaystyle ^{2}}$  at 100 keV, (3) a field of view of ~30 deg FWHM at 45 keV, and (4) a time resolution of less than 0.1 ms for the germanium detectors and 10 s for the CsI detectors. The gamma-ray spectrometer operated until 1 June 1980, when its cryogen was exhausted.[71] The energy resolution of the Ge detectors was subject to degradation (roughly proportional to energy and time) due to radiation damage.[72]

## High Energy Transient Explorer

This is Explorer 79, the High Energy Transient Explorer 2 (HETE-2). Credit: NASA.

The High Energy Transient Explorer is to carry out the first multiwavelength study of gamma-ray bursts with [a soft X-ray camera or SXC], X-ray, and gamma-ray instruments mounted on a single, compact spacecraft. A unique feature of the HETE mission [is] its capability to localize GRBs with ~10 arc second accuracy in near real time aboard the spacecraft, and to transmit these positions directly to a network of receivers at existing ground-based observatories enabling rapid, sensitive follow-up studies in the radio, IR, and optical bands. HETE-2 was launched on October 9, 2000.

## International Gamma-Ray Astrophysics Laboratory

The European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is an operational Earth satellite, launched in 2002 for detecting some of the most energetic radiation that comes from space. It is the most sensitive gamma ray observatory ever launched.[73] The INTEGRAL imager, IBIS (Imager on-Board the INTEGRAL Satellite) observes from 15 keV (hard X-rays) to 10 MeV (gamma rays). Angular resolution is 12 arcmin, enabling a bright source to be located to better than 1 arcmin. A 95 x 95 mask of rectangular tungsten tiles sits 3.2 meters above the detectors. The detector system contains a forward plane of 128 x 128 Cadmium-Telluride tiles (ISGRI-Integral Soft Gamma-Ray Imager), backed by a 64 x 64 plane of Caesium-Iodide tiles (PICsIT-Pixellated Caesium-Iodide Telescope). ISGRI is sensitive up to 1 MeV, while PICsIT extends to 10 MeV. Both are surrounded by passive shields of tungsten and lead. The primary spectrometer is the SPectrometer for INTEGRAL (SPI). It observes radiation between 20 keV and 8 MeV. SPI consists of a coded mask of hexagonal tungsten tiles, above a detector plane of 19 germanium crystals (also packed hexagonally). The Ge crystals are actively cooled with a mechanical system, and give an energy resolution of 2 keV at 1 MeV.

## Koronas-Foton

The Koronas-Foton satellite carries an Indian Roentgen Telescope (RT), specifically the RT-2 gamma-ray telescope,[74] for low-energy gamma-ray imaging, and a Konus-RF X-ray and gamma-ray spectrometer.

## Kosmos 60

Kosmos 60 carried a 16-channel NaI(Tl) scintillator 40 x 40 mm in size. It was surrounded in a charged particle rejection scintillator. The spacecraft weighed 1600 kg and the detector was located inside the vehicle. The detector was sensitive to 0.5-2.0 MeV photons.

Kosmos 60 measured the gamma-ray background flux density to be 1.7 × 104 quanta/(m2·s). As was seen by Ranger 3 and Lunas 10 & 12, the spectrum fell sharply up to 1.5 MeV and was flat for higher energies. Several peaks were observed in the spectra which were attributed to the inelastic interaction of cosmic protons with the materials in the satellite body.

## Luna 10

Luna 10 is the Luna orbiter. Credit: USSR Luna orbiter bus (NASA).

Luna 10 (E-6S series) was a Soviet Luna program, robotic spacecraft mission, also called Lunik 10. Scientific instruments included a gamma-ray spectrometer for energies between 0.3—3 MeV (50–500 pJ. Luna 10 conducted extensive research in lunar orbit, gathering important data on the nature of lunar rocks (which were found to be comparable to terrestrial basalt rocks).

## OSO-3

The third orbiting solar observatory (OSO 3) is launched on March 8, 1867. Credit: NASA's HEASARC: Observatories, Goddard Spaceflight Center.

The gamma-ray telescope flown on the Third Solar Observatory (OSO-3) launched in 1967 achieved the first definitive observation of high-energy cosmic gamma rays from both galactic and extragalactic sources.

The gamma-ray instrument onboard registered 621 events attributed to cosmic gamma rays above 50 MeV. A complete sky survey showed that the celestial distribution of gamma-rays is highly anisotropic, being concentrated along the galactic equator. In addition, an extended region around the galactic center showed a higher measured intensity.

## OSO-7

This is OSO 7. Credit: NASA.

Aboard the OSO-7 satellite is the UNH Solar Gamma-Ray Monitor which observed 0.3—10 MeV solar flare gamma rays with a NaI(Tl) scintillation spectrometer in a CsI(Na) active anti-coincidence shield.[75] Specifically, the first observation of solar gamma-ray line emission, due to electron/positron annihilation at 511 keV, from a solar flare in April 1972.

## Ranger 5

This is the Ranger 5 spacecraft diagram. Credit: NASA.

The experimental apparatus included a gamma-ray spectrometer in a 300 mm sphere mounted on a 1.8 m boom. Gamma-ray data were collected for four hours prior to the loss of power.

## Reuven Ramaty High Energy Solar Spectroscopic Imager

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) launched on 5 February 2002 is designed to image solar flares in energetic photons from soft X rays (~3 keV) to gamma rays (up to ~20 MeV) and to provide high resolution spectroscopy up to gamma-ray energies of ~20 MeV. The imaging capability of RHESSI is based on a Fourier-transform technique using a set of 9 Rotational Modulation Collimators (RMCs) as opposed to mirrors and lenses. Each RMC consist of two sets of widely-spaced, fine-scale linear grids. As the spacecraft rotates, these grids block and unblock any X-rays which may be coming from the Sun modulating the photon signal in time. The modulation can be measured with a detector having no spatial resolution placed behind the RMC since the spatial information is now stored in the time domain. The modulation pattern over half a rotation for a single RMC provides the amplitude and phase of many spatial Fourier components over a full range of angular orientations but for a small range of spatial source dimensions. Multiple RMCs, each with different slit widths, provide coverage over a full range of flare source sizes. Images are then reconstructed from the set of measured Fourier components in exact mathematical analogy to multi-baseline radio interferometry. RHESSI provides spatial resolution of 2 arcseconds at X-ray energies from ~4 keV to ~100 keV, 7 arcseconds to ~400 keV, and 36 arcseconds for gamma-ray lines and continuum emission above 1 MeV. RHESSI can also see gamma rays coming from off-solar directions. The more energetic gamma rays pass through the spacecraft structure, and impact the detectors from any angle. This mode is used to observe gamma ray bursts (GRBs).

## Small Astronomy Satellite 2

The SAS 2 is the second small spacecraft designed to detect X-rays, gamma-rays, ultraviolet, visual, and infrared rays. Credit: Mrs. Marjorie R. Townsend Project Manager NASA Goddard Space Flight Center, Dr. Carl E. Fichtel Project Scientist NASA Goddard Space Flight Center, Dr. Nancy G. Roman Program Scientist NASA Headquarters.

"The Small Astronomy Satellite 2 was a NASA gamma ray telescope launched on 15 November 1972 with a primary objective to measure the spatial and energy distribution of primary galactic and extragalactic gamma radiation with energies between 20 and 300 MeV. SAS-2 first detected Geminga, a pulsar believed to be the remnant of a supernova that exploded 300,000 years ago.[17]

## Swift Gamma-Ray Burst Mission

NASA's Swift Gamma-Ray Burst Mission is launched in November 2004. Credit: NASA.

Swift is a multi-wavelength space-based observatory dedicated to the study of gamma-ray bursts (GRBs). Its three instruments work together to observe GRBs and their afterglows in the gamma-ray, X-ray, ultraviolet, and optical wavebands. The Burst Alert Telescope (BAT) detects GRB events and computes their coordinates in the sky. It covers a large fraction of the sky (over one steradian fully coded, three steradians partially coded; by comparison, the full sky solid angle is 4π or about 12.6 steradians). It locates the position of each event with an accuracy of 1 to 4 arc-minutes within 15 seconds. This crude position is immediately relayed to the ground, and some wide-field, rapid-slew ground-based telescopes can catch the GRB with this information. The BAT uses a coded-aperture mask of 52,000 randomly placed 5 mm lead tiles, 1 metre above a detector plane of 32,768 four mm CdZnTe hard X-ray detector tiles; it is purpose-built for Swift. Energy range: 15–150 keV.[76]

## Vela satellites

This photograph shows Vela 5A/B satellites in their cleanroom. Credit: Los Alamos National Laboratory and NASA.

For gamma-ray astronomy, the Vela satellites were the first devices ever to detect cosmic gamma ray bursts.

## WIND spacecraft

This artist's image shows the WIND satellite in space. Credit: NASA.

Launched on 1 November 1994 the experiment consists of two identical gamma ray spectrometers mounted on opposite sites of the spacecraft so all sky is observed.[77]

WIND carries the Transient Gamma-Ray Spectrometer (TGRS) which covers the energy range 15 keV - 10 MeV, with an energy resolution of 2.0 keV @ 1.0 MeV (E/delta E = 500).

"It still provides the highest time, angular, and energy resolution of any of the solar wind monitors.

## Sounding rockets

About a quarter of the sounding rockets were dedicated to stellar and gamma-ray studies.

## Telescopes

"For X-rays, the index of refraction is defined by Rayleigh scattering,"[78] especially in the use of Wolter telescopes.

"[T]he strength of the effect drops off as the inverse square of the X-ray energy. This means that at high X-ray energies – and on into low gamma-ray energies – the radiation is not bent enough for a lens to work effectively."[78]

"[T]he index of refraction starts to make a comeback at energies greater than about 700 keV. What is more, while the index of refraction is negative for X-rays, it becomes positive for gamma rays."[78]

"What is new now is that with gamma rays we can really address the extremely high electric field of the nucleus," with Delbrück scattering.[79]

"The measurements indicate that there exists an index of refraction for gamma-ray energies that is substantially larger than people believed before".[80]

"Materials with nuclei that have a large positive charge – such as gold – should be ideal for making gamma-ray lenses".[78]

### Atmospheric Cherenkov telescopes

This diagram depicts an air shower resulting from cosmic rays. Credit: Konrad Bernlöhr.

The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.[81]

### Converted Atmospheric Cherenkov Telescope Using Solar-2

The first astronomical observations started in the fall of 2004. However, the facility had its last observing runs in November 2005 as funds for observational operations from the National Science Foundation were no longer available.

CACTUS is sensitive in the 50-500 GeV energy range.[82]

"The CACTUS atmospheric Cherenkov telescope collaboration recently reported a gamma-ray excess from the Draco dwarf spheroidal galaxy."[83] "[T]he bulk of the signal detected by CACTUS [may come] from direct [weakly interacting massive particles (WIMPs)] WIMP annihilation to two photons"[83].

### High-Energy-Gamma-Ray Astronomy

Two HEGRA reflectors, with the NOT in the background.

HEGRA, which stands for High-Energy-Gamma-Ray Astronomy, was an atmospheric Cherenkov telescope for Gamma-ray astronomy. With its various types of detectors, HEGRA took data between 1987 and 2002, at which point it was dismantled in order to build its successor, MAGIC, at the same site. HEGRA is at 2200 masl.

### High Energy Stereoscopic System

Two HESS telescopes, CT2 and CT3, are part of the HESS array in Namibia. Credit: .

High Energy Stereoscopic System or H.E.S.S. is a next-generation system of Imaging Atmospheric Cherenkov Telescopes (IACT) for the investigation of cosmic gamma rays in the 100 GeV and TeV energy range. ... As of September 2011, there are 62 sources in the HESS catalogue.

### Major Atmospheric Gamma-ray Imaging Cherenkov Telescopes

This is the MAGIC telescope at La Palma, Canary Islands. Credit: Pachango.

MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov Telescopes) is a system of two Imaging Atmospheric Cherenkov telescopes situated at the Roque de los Muchachos Observatory on La Palma, one of the Canary Islands, at about 2200 m above sea level. MAGIC detects particle showers released by gamma rays, using the Cherenkov radiation, i.e., faint light radiated by the charged particles in the showers. With a diameter of 17 meters for the reflecting surface, it is the largest in the world. MAGIC is sensitive to cosmic gamma rays with energies between 50 GeV and 30 TeV due to its large mirror; other ground-based gamma-ray telescopes typically observe gamma energies above 200-300 GeV. Satellite-based detectors detect gamma-rays in the energy range from keV up to several GeV. MAGIC has found pulsed gamma-rays at energies higher than 25 GeV coming from the Crab Pulsar.[84] The presence of such high energies indicates that the gamma-ray source is far out in the pulsar's magnetosphere, in contradiction with many models. A much more controversial observation is an energy dependence in the speed of light of cosmic rays coming from a short burst of the blazar Markarian 501 on July 9, 2005. Photons with energies between 1.2 and 10 TeV arrived 4 minutes after those in a band between .25 and .6 TeV. The average delay was .030±.012 seconds per GeV of energy of the photon. If the relation between the space velocity of a photon and its energy is linear, then this translates into the fractional difference in the speed of light being equal to minus the photon's energy divided by 2 x 1017 GeV.

### Solar Tower Atmospheric Cherenkov Effect Experiment

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE), is a gamma ray detector located near Albuquerque, New Mexico. Observations with STACEE began in October 2001 and concluded in June 2007. Gamma rays were observed from objects such as the Crab Nebula, a supernova remnant, and Markarian 421, a blazar. STACEE uses the heliostats and space on the receiver tower of the National Solar Thermal Test Facility operated by the Sandia National Laboratories on the grounds of the Kirtland Air Force Base. During the night STACEE uses the heliostats to reflect the brief flashes of Čerenkov radiation caused by gamma rays hitting the upper atmosphere to photodetectors mounted in the tower. STACEE is a nonimaging telescope, meaning that it detects the light from a portion of the sky, but does not resolve the light into an image.

## Gamma-ray spectrometers

This is a spectrum of 60Co, with peaks at 1.17 and 1.33 MeV from a spectrometer.

A Gamma-Ray Spectrometer, or (GRS), is an instrument for measuring the distribution (or spectrum—see figure) of the intensity of gamma radiation versus the energy of each photon.

This diagram depicts the generation of gamma rays by cosmic ray exposure. Credit: JPL, NASA.

Using Germanium detectors - a crystal of hyperpure germanium that produces pulses proportional to the captured photon energy; while more sensitive, it has to be cooled to a low temperature, requiring a bulky cryogenic apparatus. When exposed to cosmic rays (charged particles in space that come from the stars, including our sun), chemical elements in soils and rocks emit uniquely identifiable signatures of energy in the form of gamma rays. The gamma ray spectrometer looks at these signatures, or energies, coming from the elements present in the target soil. By measuring gamma rays coming from the target body, it is possible to calculate the abundance of various elements and how they are distributed around the planet's surface. Gamma rays, emitted from the nuclei of atoms, show up as sharp emission lines on the instrument's spectrum output. While the energy represented in these emissions determines which elements are present, the intensity of the spectrum reveals the elements concentrations. Spectrometers are expected to add significantly to the growing understanding of the origin and evolution of planets like Mars and the processes shaping them today and in the past.

## Gamma-ray spectroscopy

The study and analysis of gamma-ray spectra for scientific and technical use is called gamma spectroscopy, and gamma-ray spectrometers are the instruments which observe and collect such data.

Atomic nuclei have an energy-level structure somewhat analogous the energy levels of atoms, so that they may emit (or absorb) photons of particular energies, much as atoms do, but at energies that are thousands to millions of times higher than those typically studied in optical spectroscopy. As with atoms, the particular energy levels of nuclei are characteristic of each species, so that the photon energies of the gamma rays emitted, which correspond to the energy differences of the nuclei, can be used to identify particular elements and isotopes. Distinguishing between gamma-rays of slightly different energy is an important consideration in the analysis of complex spectra, and the ability of a GRS to do so is characterized by the instrument's spectral resolution, or the accuracy with which the energy of each photon is measured. Semi-conductor detectors, based on cooled germanium or silicon detecting elements, have been invaluable for the energy level spectrum of nuclei which typically dies out above about 10 MeV. Gamma-ray instruments looking to still higher energies generally observe only continuum spectra, so that the moderate spectral resolution of scintillation (often sodium iodide (NaI) or caesium iodide, (CsI) spectrometers), often suffices for such applications.

## Hypotheses

1. Positive cloud or above to ground lightning probably emits more gamma rays than X-rays.

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