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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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gamma ray burst (GRB)

Galilei (1564–1642). See Galilean transformation.

Galilean relativity The variance of physical expressions under the Galilean transformation from one coordinate system to another coordinate system which is moving uniformly with respect to the ﬁrst. Position and velocity are relative or variant under Galilean transformation, but the Laws of Mechanics are invariant under such transformations. Named after Galileo Galilei (1564–1642). See Galilean transformation.

Galilean transformation The transformation of spatial coordinates from one reference system to another reference system, moving uniformly with respect to the ﬁrst, according to the following expression: r = r − vt, where v is the relative velocity between the two reference systems. Under Galilean transformation time is considered an absolute variable; that is, its value is the same for all reference systems. In special relativity, the Galilean transformations are superseded by the Lorentz transformations. Named after Galileo Galilei (1564–1642).

Galilei, Galileo Physicist and astronomer (1564–1642). One of the ﬁrst to use the telescope for astronomical observations, and improved its design. Discovered sunspots, lunar mountains, and valleys, the four largest satellites of Jupiter now known as the Galilean Satellites, and the phases of Venus. In physics he discovered the laws of falling bodies and the law of the pendulum.

Galileo spacecraft A spacecraft launched in 1989 that arrived at Jupiter on December 7, 1995. Galileo’s atmospheric probe plunged into the Jovian atmosphere on the same day, to relay information on its structure and composition, including major cloud decks and lightning. The spacecraft’s orbiter studied the giant planet, its rings and its moons, and the magnetic environment. The primary mission ended in December 1997. A two-year extended mission was then concentrated on Europa and ended with two close ﬂybys of Io in December 1999. A mosaic of images of Europa show several terrain types that provide evidence for the existence of

a liquid ocean under the surface. Galileo is now continuing its mission under another extension, studying Jupiter’s largest moon, Ganymede.

NASA’s Galileo project is managed by JPL. The Galileo probe development and operations are the responsibility of NASA’s Ames Research Center.

Galiliean satellite One of the four Jovian satellites discovered via telescope observations by Galileo: Io, Europa, Ganymede, Callisto, in order of distance from Jupiter.

gallium A silver-white rare metallic element having the symbol Ga, the atomic number 31, atomic weight of 69.72, melting point of 29.78◦C, and a boiling point of 2403◦C, soft enough to cut with a knife. Speciﬁc gravity of solid (29.6◦C), 5.904; speciﬁc gravity of liquid (29.8◦C), 6.095. Gallium compounds, especially gallium arsenide, are used as semiconductors. The metal is used as a substitute for mercury in high temperature thermometers.

gamma A unit of magnetic ﬁeld intensity employed in geophysics, equal to one nanotesla (1.0 × 10−9 tesla) and 1.0 × 10−5 gauss. This unit is used because of its convenient size for expressing ﬂuctuations that occur in the Earth’s ﬁeld, due to solar inﬂuence and geomagnetic storm phenomena. See geomagnetic ﬁeld.

γ -ray Electromagnetic radiation (photons) with energy greater than about 0.1 MeV (wavelength less than about 0.2 Å).

gamma ray burst (GRB) Tremendous ﬂashes of radiation ﬁrst detected in the gammaray region of the spectrum. GRBs were discovered about 30 years ago with instruments intended to monitor nuclear test ban treaties. Presently thought to be the most powerful explosions in nature (after the Big Bang), their sources have only recently been localized by observations of associated afterglows in X-rays, visible light, and radio waves, delayed in that order. In gamma-rays, GRBs last from a millisecond to hours, although optical and radio afterglows may go on for weeks. Several afterglows attributed to GRBs have redshifted spectral lines indicating occurrence at several billion

gamma-ray burst, black hole accretion disks

light-years distant. At this distance, the energy released is similar to the rest-mass energy of the sun. Afterglow features match many of the theoretical predictions of an expanding “ﬁreball,” but the source mechanism itself remains unknown. Other transients, seemingly similar and found with instrumentation designed to study gamma ray bursts, have in recent years been conﬁrmed as distinctly separate phenomena, coming from neutron stars located in this galaxy or its Magellanic cloud satellite galaxies. See soft gamma repeaters, March 5th event.

gamma-ray burst, black hole accretion disks

Under the gamma-ray burst “ﬁreball” paradigm (see gamma-ray bursts, ﬁreball), any gammaray burst engine must produce high energies without ejecting too much baryonic matter. A class of models, all of which produce rapidly accreting tori around black holes, provide natural explanations for the high energies, but low ejecta mass. The gravitational potential energy is converted to energy in a pair/plasma ﬁreball either from the neutrino annihilation or magnetic ﬁeld energy mechanisms, both of which require an asymmetry in the mass accretion. The energy is deposited along the disk rotation axis, producing a beamed jet. This beaming lessens the burst energy requirements (most papers quote gamma-ray burst energies assuming isotropic explosions) and avoids excessive baryon contamination. This class of models includes mergers of double neutron star systems, mergers of black hole and neutron star binaries, mergers of black hole and white dwarf binaries, collapsars, and helium core mergers.

gamma-ray burst, classical Classical gam- ma-ray bursts make up the bulk of the observed gamma-ray bursts. They do not appear to repeat, and have hard spectra (95% of energy emitted by photons with energy greater than 50 keV). However, beyond these characteristics, classical gamma-ray bursts represent a very heterogeneous set of objects. Burst durations range from 0.01 to 300 s, during which time the burst may be chaotic, exhibiting many luminosity peaks, or it may vary smoothly. They are distributed isotropically and are thought to originate outside of the galaxy (cosmological bursts). This translates to burst energies in the range: 1048

to 1053 ergs, in some cases the most energetic explosions in the universe since the Big Bang.

gamma-ray burst, classiﬁcation Gammaray bursts have been separated into two major classes: soft gamma-ray repeaters and “classical” gamma-ray bursts. Soft gamma-ray repeaters repeat, have average photon energies of 30 to 50 keV, burst durations of 0.1 s, smooth light-curves, and lie in the galactic plane. Classical bursts do not seem to repeat, emit most of their energy 50 keV, have a range of durations (0.01 to 300 s) and light-curve proﬁles. Classical bursts are distributed isotropically and are thought to originate at cosmological distances outside of the galaxy or galactic halo. Soft gamma-ray repeaters are thought to be caused by accretion, magnetic ﬁeld readjustment, or quakes in neutron stars in the galactic disk.

gamma-ray burst, cosmological mechanisms

Spectra of the optical counterpart of GRB970508 reveal many red-shifted (z = 0.835) absorption lines, conﬁrming that at least some gamma-ray bursts are cosmological; that is, they occur outside of the Milky Way. Cosmological models provide a simple explanation for the isotropic spatial distribution of gamma-ray bursts, but require energies in excess of 1051 ergs. Most cosmological models rely upon massive accretion events upon compact objects (e.g., the merger of two neutron stars). Several mechanisms ultimately produce accretion disks around black holes where the gravitational energy released can be more readily converted into relativistic jets. These black-hole accretion disk models provide an ideal geometry for facilitating the potential energy conversion via the neutrino annihilation or magnetic ﬁeld mechanisms into beamed gamma-ray burst jets.

gamma-ray burst, ﬁreball The gamma-ray burst ﬁreball refers to a mechanism by which the energy produced near the black hole or neutron star source is converted into the observed burst. Due to photon-photon scattering and electron scattering opacities at the source of the burst, the burst photons are initially trapped. As the “ﬁreball” expands adiabatically, the optical depth decreases, but so does the temperature. In the current scenario, the ﬁreball does

gamma-ray burst models, collapsar

not become optically thin until much of the internal energy has been converted into kinetic energy through expansion. The observed gammaray burst spectrum is then produced by internal shocks in the ﬁreball or by shocks created as the ﬁreball sweeps up the interstellar medium.

gamma-ray burst, galactic mechanisms

Prior to the launch of the Burst and Transient Source Experiment (BATSE), galactic models were the “favored” mechanisms for gamma-ray bursts. The bulk of these models involve sudden accretion events onto neutron stars or some sort of glitch in the neutron star (e.g., neutron star quakes). However, the data from BATSE revealed that the bursts are isotropically distributed in the sky, limiting galactic models to those that occur in the galactic halo. The advantage of such mechanisms is that the energy requirements for galactic explosions is nearly 8 orders of magnitude lower than their cosmological counterparts ( 1043 ergs). Recent observations of the optical afterglow of gamma-ray bursts, speciﬁcally the detection of redshifted lines in GRB970508, place at least some of the gamma-ray bursts at cosmological redshifts.

gamma-ray burst, GRB 970508 The May 8, 1997 gamma-ray burst is the ﬁrst gammaray burst where a reliable optical counterpart revealed identiﬁable metal absorption lines. The lines are probably caused by some intervening material between our galaxy and the gammaray burst source. A redshift of the absorbing medium of z = 0.835 was inferred from these 8 lines, setting the minimum redshift of the gamma-ray burst (Metzger et al., 1997). Other evidence indicates that the absorbing material was part of the host galaxy of the gamma-ray burst, placing the gamma-ray burst at a redshift z = 0.835. This gamma-ray burst provides indisputable evidence that at least some gammaray bursts are cosmological.

gamma-ray burst, hypernova The hypothetical explosion produced by a collapsar, the collapse of a rotating massive star into a black hole, which would produce a very large γ -ray burst. See gamma-ray burst models, collapsar.

gamma-ray burst, magnetic ﬁelds The generation and stretching of magnetic ﬁeld lines have been proposed as mechanisms to convert the energy of material accreting onto a black hole in active galactic nuclei and gamma-ray bursts alike. The “standard” mechanism is that described by Blandford and Znajek (1977) which uses magnetic ﬁeld interactions in the disk to extract the rotational energy of the black hole. Other mechanisms exist which extract the potential energy of the accreting matter. Gamma-ray bursts require magnetic ﬁeld strengths in excess of 1015 Gauss, which is roughly 10% of the disk equipartion energy.

gamma-ray burst, mechanisms Over 100 proposed distinct mechanisms for classical gamma-ray bursts exist which can be grouped roughly into three categories based on their location: solar neighborhood GRBs, galactic GRBs, and cosmological GRBs. Solar neighborhood GRB mechanisms are the least likely and calculations of the proposed mechanisms do not match the observations. Galactic models have the advantage that they require much less energy than cosmological models. However, the isotropic distribution of bursts require that these models be in the galactic halo. In addition, absorption lines in the spectra of the optical counterparts of GRBs, namely GRB970508, indicate that some bursts must be cosmological. Because of this evidence, cosmological models are the favored class of models, despite their high energy requirements (up to 1053 ergs).

The major constraint of any mechanism is that it must produce sufﬁcient energy ( 1051 ergs for an isotropic cosmological burst) with relatively little contamination from baryons. The low mass in the ejecta is required to achieve the high relativistic velocities ( = 1 + Eburst/Mejecta). Beaming of the burst reduces both the energy requirement (the energies generally quoted in papers assume an isotropic explosion) and limit the baryonic contamination. This beaming is predicted by most

of the viable gamma-ray burst models.
gamma-ray burst models, collapsar	The

cores of stars with masses above 10M overcome electron degeneracy pressure and collapse. For stars with masses 25 to 50M , this

gamma-ray burst models, helium merger

collapse eventually drives a supernova explosion and the formation of a neutron star. However, more massive stars eventually collapse into black holes. If those stars are rotating, they may drive a gamma-ray burst by forming an accretion disk around the collapsed core via the black hole accretion disk paradigm. A hypernova is the observed emission from a collapsar, just as a Type II supernova is the observed emission from a core-collapse supernova.

gamma-ray burst models, helium merger

X-ray binaries are powered by the accretion of material onto a neutron star or black hole from its close-binary companion. In some cases, the star may continue to expand until it engulfs the compact object. The compact object then spirals into the hydrogen envelope of this companion, releasing orbital energy which may eject the envelope. However, if there is insufﬁcient energy to eject the envelope, the neutron star or black hole will merge with the core, accreting rapidly via neutrino emission (see accretion, super-Eddington). The compact object quickly collapses to a black hole (if it is not already) and the angular momentum of the orbit produces an accretion disk around the black hole. This system may power a gamma-ray burst under the black hole accretion disk paradigm.

gamma-ray burst models, merging compact objects Merging compact objects (double neutron star binaries, black hole and neutron star binaries, and black hole and white dwarf binaries) evolve into black hole accretion disk systems which then produce the pair/plasma jet which may ultimately power the burst. The actual merger of two neutron stars ejects too much material and it is not until a black hole forms with a 0.1M accretion disk around it that a viable gamma-ray burst might be powered via the black hole accretion disk paradigm. In the other systems, the compact companion of the black hole is shredded into an accretion disk, again forming a black hole accretion disk system.

gamma-ray burst, soft gamma-ray repeaters

Gamma ray bursts are characterized by the shortduration burst of photons with energies greater than 10 keV. A very small subset of bursts (three

of hundreds) have been observed to repeat. Soft gamma-ray repeaters have softer spectra (photon energies 30 to 50 keV vs. 0.1 to 1 MeV for classical bursts). The three observed soft gamma-ray repeaters also have durations which are on the low end of the gamma-ray burst duration ( 0.1 s vs. the 0.01 to 300 s for the classical bursts). All three soft gamma-ray repeaters lie in the galactic plane and are thought to be the result of accreting neutron stars.

Ganymede Moon of Jupiter, also designated JIII. Discovered by Galileo in 1610, it is one of the four Galilean satellites. Its orbit has an eccentricity of 0.002, an inclination of 0.21◦, a precession of 2.63◦ yr−1, and a semimajor axis

of 1.07×106 km. Its radius is 2631 km, its mass 1.48 × 1023 kg, and its density 1.94 g cm−3. It

has a geometric albedo of 0.42 and orbits Jupiter once every 7.154 Earth days. Ganymede is the largest satellite in the solar system.

garden hose angle Angle between the interplanetary magnetic ﬁeld line and a radius vector from the sun. See Archimedian spiral.

gas constant The quantity = 8.314472 J mol−1 K−1 appearing in the ideal gas law:

R = P V /(nT ) ,

where P is the pressure, V is the volume, T is the temperature, and n is the quantity of the gas, measured in gram-moles.

gaseous shocks Abrupt compression and heating of gas, caused by matter moving at velocity larger than the sound speed of the surrounding medium. Since material is moving supersonically, the surrounding gas has no time to adjust smoothly to the change and a shock front, i.e., a thin region where density and temperature change discontinuously, develops. Shocks can form in any supersonic ﬂow: in astronomy, in the case of supernovae, ﬂare stars, and in stellar winds. Since heating causes emission of radiation, the excitation and chemical composition of the gas can be diagnosed by studying the emitted spectrum.

gas thermometer A thermometer that utilizes the thermal properties of an (almost) ideal

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