GLOSSARY
Absorption line: manifest as the paucity of light at a certain wavelength due the blocking of light by a specific atom or molecule. See absorption-line spectroscopy.
Absorption-line spectroscopy: The study of gas properties in the line of sight to distant sources by analyzing the selective blocking (“absorption”) of light at certain wavelengths. Like distinct fingerprints, each ion has its own characteristic set of absorption lines that betray their existence. Often absorptionline spectroscopy is used to determine the redshift (and hence distance) of a cosmological source, since the characteristic lines of certain ions shifted redward in a spectrum can be readily identified.
Accretion: the process whereby material falls into/onto a massive body, potentially contributing to the growth of that body.
Accretion disk: a pancake-like region where matter flows toward a central object. The matter is generally fed from another massive body outside the accretion disk and makes its way toward the central object on a spiral-like orbit. In the process of accretion, some of the gravitational potential energy of the matter is liberated in the form of heat, allowing accretion disks to be very hot and luminous.
Afterglow: light detected across the electromagnetic spectrum after the GRB itself has ceased.
Ångström (Å): a unit of length equal to 10−8 cm.
Angular diameter distance: the effective distance such that the apparent size of an object decreases inversely proportional to its distance from an observer.
Arcminute: a measure of angular distance. One arcminute is 1/60th of a degree. The two headlights of a car about twelve miles away would appear to be separated by 1/3 arcminute; the typical human eye would be just incapable of discerning that there are two lights but instead would see the lights merged into one.
Arcsecond: a measure of angular distance, one arcsecond is 1/60th of an arcminute. It is approximately 1/206265th of a radian.
AU: Astronomical Unit; defined as the average distance between the Earth and the Sun and determined to be 1.496 × 108 km.
Background radiation: emission from diffuse and random places in the sky, effectively serving as noise that makes detection of a specific source or event more difficult. Background light is generally considered constant in brightness (for a given wavelength range) but may appear to change in time depending on the location of the satellite and the subsequent shielding of portions of the sky (e.g., by the Earth).
Bandpass: a certain range in wavelengths or energies over which an instrument is sensitive to light.
BAT: the Burst Alert Telescope, on board Swift.
BATSE: the Burst and Transient Source Experiment, on board CGRO.
BeppoSAX: the common name for the Italian-Dutch satellite, “Satellite per Astronomia a Raggi X.”
Black hole (BH): the most dense astrophysical object known, characterized by both mass and spin. The object has no physical surface but has a characteristic radius from which neither matter nor light can escape.
Blackbody: a “perfect” emitter and absorber; it is a thermal radiator that has a spectrum entirely characterized by the temperature of the material.
Brightness distribution: the number of sources or events that are observed to be brighter than some flux level.
c: the speed of light; 2.99 × 1010 cm s−1.
CBM: circumburst medium. The material surrounding the GRB explosion.
Celestial sphere: the apparent location of stars and galaxies on the sky. Since we cannot perceive the distances to far-away light sources (see parallax), all the sources appear to be at the same distance from us; it appears that the sources are projected onto a far-away sphere.
Center-of-momentum frame: in the context of colliding particles, the speed and direction one would need to travel in order to see the total momentum (=mass × velocity for particles with mass) of all the particles sum to zero.
CGRO: the Compton Gamma-ray Observatory.BATSE was one of the experiments on CGRO.
Comoving frame: in the context of an explosion, the point of view of a set of observers who are traveling with the flow, at the same outward expansion rate of the explosion. In a cosmological setting, the point of view of a set of observers within or near to the galaxy where the GRB occurs. In that case, such observers do not see any cosmological time-dilation or redshift effects of the event.
Compact object: a massive astrophysical body, such as a black hole or neutron star, which is much more dense than the Sun and is not supported against collapse by pressure associated with normal nuclear fusion processes (as in the Sun).
Cooling break (vc): a transition in the spectrum of a GRB afterglow between two different powerlaw regimes. At frequencies less than the cooling break frequency (vc), electrons responsible for the emission are said to be “slowly cooling” (radiating much less energy than their kinetic energy). Above vc, fast-cooling electrons are radiating an appreciable amount of their kinetic energy.
Cosmic rays: any charged particle hitting a detector. When the particles are moving slowly, they are usually associated with nuclear decay by-products (e.g., from the material surrounding the detector). Fast-moving cosmic rays, with a high amount of energy per particle, are thought to be generated in astrophysical events (e.g., supernovae).
Cosmological time dilation: an effect of the expanding universe whereby events that occur at high redshift (z) appear to progress more slowly to observers on Earth than to observers nearby the event. Specifically, an event which takes time T to progress as viewed by us takes time T/(1 +z) to occur “in the frame” of the event itself.
Counterpart: the generic term for a transient that is spatially and temporally consistent with the position of another object or transient, usually observed at a different wavelength.
Degeneracy pressure: a stiffness to material that originates from a quantum-mechanical phenomenon. At sufficiently high density, some particles, such as electrons and neutrons, cannot be packed together any closer than some critical distance. At this critical packing, the particles resist attempts to squeeze them together; this acts like a pressure that can strongly resist the crush of gravity.
Doppler shift: the change of the apparent frequency or wavelength of emitted light due to motion of the emitting source with respect to the observer. We are all familiar with the Doppler shift of soundwaves, having listened to the changing pitches of ambulances as they speed toward us and then away from us. Sources moving toward the observer will appear to emit shorterwavelength signals (higher pitch) than if the source was not moving. Sources moving away from the observer will appear to emit longer-wavelength signals (lower pitch), resulting in an apparent redshift.
Dynamics: how a source changes physically in time. In the context of synchrotron blastwaves, it describes the radial evolution of the shock with time.
Ejecta: the material ejected in a supernova explosion or a GRB.
Electromagnetic event: temporal change of an astrophysical source that gives rise to light or photons, the carrier of electromagnetic energy. See also electromagnetic spectrum.
Electromagnetic spectrum: the full range of energy, wavelength, and frequency of light. Radio light occupies the smallest-energy, longest-wavelength, lowest-frequency portion of the electromagnetic spectrum. Gamma-ray light occupies the highest-energy, shortest-wavelength, highest-frequency of the electromagnetic spectrum. Other regions of the spectrum include visible (“optical”), infrared, ultraviolet, and X-ray light.
Electronvolt (eV): the energy required to move one electron across an electric potential of one volt. Numerically, this is 1.602 × 10-12 erg. For reference, the energy required to ionize a hydrogen atom in the ground state is 13.6 eV.
Error box: the location of where a GRB could have occurred on the sky. The uncertainty in this determination often makes the location possible anywhere within some polygon-like projection onto the celestial sphere.
External shock: a relativistic shock that occurs between material ejected by the central engine and material in the circumburst medium. External shocks are thought most likely to give rise to the afterglow emission.
Fermi: a high-energy gamma-ray NASA satellite mission launched in 2008. The on-board instruments can detect GRBs over the photon energy range of 150 keV to 300 GeV. The Large Area Telescope (LAT) has the best imaging resolution, and most LAT-detected GRBs have led to the detection of an afterglow.
Fireball: a hot mixture of (charged) particles, photons, and magnetic fields. The particles move fast with random motions. In the context of the early evolution of a GRB, the fireball carries the energy deposited near the central engine (§2.3) and expands outward radially.
Fluence: the energy collected per unit area. It is the integral over time of the flux from a source. Fluence is related to the total energy output of a source, and flux is related to the instantaneous brightness or luminosity of a source.
Flux: the energy collected per unit area per unit time. See also fluence.
Forward shock: the collisionless shock connecting the outflowing material with the circumburst medium. The forward shock is thought to be the site of the late-time afterglow emission.
γ -ray (gamma ray): wavelength regime of the electromagnetic spectrum shorter than X-rays; photon energies above ~10,000 eV.
Globular cluster: a bound, roughly spherical swarm of millions of old and red stars; globular clusters also contain thousands of neutron stars and white dwarfs. There are a around two hundred globular clusters in and around the Milky Way.
Gpc: Gigaparsec; one billion parsecs, which is 3.085×1022 km.
Gravitational redshift: an apparent Doppler shift of light arising near anything that has mass. The amount of gravitational redshift, predicted by General Relativity, has been verified experimentally.
Gravitational waves (GWs): ripples, produced by accelerating masses, that deform space and time and are thought to travel at the speed of light.
HETE: High-Energy Transient Explorer.
Internal shock: a relativistic shock that occurs between material ejected by the central engine. Internal shocks are the interactions thought most likely to give rise to the prompt emission of GRBs.
Inverse Compton (IC) scattering: an interaction between a photon and (typically) an electron, whereby the kinetic energy of the electron is reduced and the energy of the outgoing photon is higher than the energy of the incoming photon. The direction of the electron’s trajectory (as well as that of the photon) is altered by the interaction.
IPN: Interplanetary Network of Satellites.
IR: infrared; the wavelength regime between visible and the sub-millimeter. Near-IR observations span from about 10,000 Ångström to 26,000 Ångström (1–2.6 µm).
Isotropic/isotropy: the same in all directions. In the context of locations, isotropy implies that the typical distances between different GRBs is the same in all places on the sky. The measurement of the degree of isotropy (or anisotropy) is statistical in nature. In the context a given GRB, isotropy would imply that the energy released in all directions is the same. We do not believe that GRBs emit isotropically but instead are collimated.
Kinetic energy: the energy associated with the motion of a particle or body with mass; at low velocity (v), it is the familiar ½mv 2.
kpc: kiloparsec; one thousand parsecs, which is 3.085 × 1016 km.
Light-year: the distance that light will travel in the vacuum of space in one year. This is about 9,436 billion kilometers or 0.306 parsecs.
LIGO: Laser Interferometer Gravitational-Wave Observatory.
LMC: Large Magellanic Cloud, a satellite galaxy of the Milky Way.
Lorentz factor (Γ): a term related to the velocity, v, of source as 
. It is most useful to consider instead of Γ when v is very close to the speed of light c.
Luminosity distance: the effective distance between two sources such that the 1/r2 law for the dimming of light is satisfied.
M
: the mass of the Sun; 1.99 × 1033 gm.
Magnetar: a highly magnetized neutron star thought to be the origin of Soft Gamma-ray Repeaters and possibly some extragalactic short-duration GRBs.
Magnitude: a measurement, in logarithmic scale, of the brightness of a UVOIR event. The “apparent magnitude” is what is measured, and the “absolute magnitude” is the brightness that would be measured of that object if observed from a distance of 10 parsec. The star Vega is defined to have an apparent magnitude = 0 at all wavelengths. Fainter sources have larger magnitudes such that, for every change of 2.5 magnitudes, a source is ten times fainter.
Metallicity: a measurement of the abundance of synthesized (heavy) elements relative to the abundances observed in the Sun. Given in dimensionless logarithmic units, a metallicity of −1 implies an enrichment of metals from the primordial abundances that is ten times less than in the Sun. Galaxies in the Universe two billion years after the Big Bang have typical metallicities in the range −3 to −2.
Mpc: Megparsec; one million parsecs, which is 3.085 × 1019 km.
NH: the column density of hydrogen (H), in units of cm−2. Physically, this is the integral of the number density of hydrogen nH (units of [cm−3]) along a given sightline, ∫ n Hdl.
Neutrinos: very low-mass elementary particles that are copiously produced in fusion and fission processes but interact only very weakly with ordinary matter.
Neutron: an uncharged fundamental particle usually found in the nucleus of atoms heavier than hydrogen. It has about the same mass as a proton.
Neutron star (NS): a dense stellar remnant, much like a white dwarf but supported by degeneracy pressure associated with tightly packed neutrons. Typical sizes of NSs are about 10 km in radius and about one Solar mass. They are thought to form during the collapse of massive stars as those stars explode as supernovae.
NFI: Narrow Field X-ray Instruments, on board the BeppoSAX satellite.
Nucleosynthesis: the process of making new elements (nuclei) from existing elements. Usually nucleosynthesis is associated with the creation of new elements via fusion (rather than fission) processes. Explosive nucleosynthesis is the process of making a large mass of new elements rapidly (on seconds timescales); this happens at the start of a supernova.
Occam’s Razor: the notion that the simplest explanation is likely the correct one; a guiding principle in scientific pursuits.
Optical depth (τ): a dimensionless number related to the stopping power of light. High optical depth (τ 
1) implies that that material is opaque to the transmission of light. Likewise, low optical depth (τ 
1) material is transparent to the passage of light through it.
Pair production: the creation of an electron and positron, usually through the interaction (and annihilation) of two highenergy photons.
Panchromatic: across the electromagnetic spectrum, from radio to gamma-ray wavebands.
Parallax: the apparent wobble of a nearby object relative to more-distant objects on the celestial sphere, where a different vantage point is given to an observer on Earth due to the Earth’s motion around the Sun. Parallax is used to find geometric distances to nearby stars.
Parsec: a measure of distance on astronomical scales equal to 3.085 × 1013 km. Distances to nearby stars are usually given in units of parsec. The word parsec is a conjunction of the words parallax and arcsecond, since it is the distance at which a source has an apparent parallax of one arcsecond.
Periodic: a regular and repeating change in brightness. There is no periodicity seen in GRBs, but SGR light curves are seen to change periodically.
Photometric redshift: the technique of using an afterglow imaged at different wavebands to infer the redshift of a source.
Photon: a parcel (or particle) of light that carries energy and momentum. Light can be considered both a wave and a particle. This is an utterly stupefying concept, but Nature has asked us to wrap our heads around this duality. When considering the detection of light from astronomical sources, it is usual to consider optical, X-ray, and gamma-ray light as individual photons. For long-wavelength light, such as radio light, it is usual to consider the energy carried as waves.
Plane wave: if we think of the propagation of light through space as the motion of the crests of waves (like the crests of ocean waves heading toward a beach), then the crest of a plane wave lies perpendicular to the direction of motion. On a placid lake where a pebble has just dropped, the crests are curved and form concentric rings that move outward. Far from where the pebble has dropped (at later times), the crests will nearly appear as plane waves as they sweep by an observer. Likewise, when far from a light source, there is no apparent curvature in the crests.
Potential energy: the energy associated with a mass far from the local gravitational center.
Quasar: a light source that appears to be as compact as a star in the Milky Way but instead is a massive black hole in a distant galaxy. Quasars are thought to be powered by mass flowing into the BH and are typically tens of thousands of times brighter than the brightest galaxies.
Radian: a measure of angle. There are 2 π radians in a circle of 360 degrees, so one radian is about 57.3 degrees.
Radiation: any form of energy, such as light or gravitational waves, that propagates through space.
Redshift: the apparent shift of a spectrum to longer wavelengths relative to what would be observed in a laboratory. Redshift is often a good-enough proxy for distance in astronomy: at cosmological distances, there is a one-to-one mapping between redshift of a source and its distance. Larger redshifts correspond to larger distances.
Relativistic expansion: outflow at velocities very near the speed of light.
Resolution (imaging): a measure of the blurriness of an image, usually given in angular units ([radians] or [arcsec]) with smaller resolutions being more desirable.
Resolution (spectral): a measure of the how spread out the light is in a spectrum. For analyzing the properties of gas clouds giving rise to afterglow absorption, higher-resolution spectra are generally more desirable.
Restmass energy: the energy equivalent to the mass M of the object, related by E = Mc2.
SFR: star-formation rate, usually given in units of solar mass per year per unit volume.
Shock: a region in space where there is a sharp difference (i.e., a discontinuity) in density, temperature, and/or pressure.
Shockwave: a propagating shock in space and time. A supernova shockwave propagates radially from the center of the explosion.
Sightline: a direction toward or away from a specific astrophysical source through which light travels. Absorbing material in the sightline to that distant source will often leave a characteristic fingerprint in the observed spectrum of that source. See absorptionline spectroscopy.
SMC: Small Magellanic Cloud. See also the LMC.
Soft Gamma-ray Repeaters (SGRs): a class of bursting objects that appear to repeatedly produce gamma-ray events over hours to decades (whereas classical GRBs do not repeat) and show periodic behavior during the gamma-ray event. SGRs are thought to come from magnetars.
Spectral-energy distribution (SED): the spectrum of a source over many decades in frequency or wavelength. A spectrum is the brightness (in flux or luminosity units usually) versus wavelength or frequency.
Spectrum: the brightness of a source as a function of wavelength (or frequency). As a prism splits a light beam into its constituent colors, modern instruments (“spectrographs”) disperse the light of a GRB afterglow to reveal its spectrum.
Supernova (SN): the result of a violent explosion of a star (or two merged stars) whereby a significant amount of mass (> 0.1 M) is expelled at large velocity (> thousand km/s). Supernovae are heated by radioactive elements created at the outset of the explosion. This heating, coupled with the expansion of the ejected material (ejecta), leads to a characteristic rising and falling behavior at optical wavelengths; the time-to-rise of most SNe is from days to weeks following the explosion.
Supernova remnant: the bright, dense, clumpy (and partially radioactive) material ejected during a supernova event. Since the ejecta can move at large velocities, supernova remnants can become very large and likewise appear to have a large angular size. The famous Crab nebula is the remnant of a supernova which was discovered in the year 1054 (see http://wikipedia.org/wiki/Crab_Nebula).
Synchrotron blastwave theory: a theoretical framework that describes the origin of the afterglow emission, connecting the bulk properties of outflowing material (such as velocity and mass), relativistic shocks, and synchrotron radiation.
Synchrotron radiation: light produced by charged particles (usually electrons) moving near the speed of light (i.e., relativistically) in a magnetic field.
Timescale: the characteristic length of time over which there are significant changes in some observed property (e.g., brightness/flux).
UVOIR: a span over the optical, infrared, and ultraviolet portions of the electromagnetic spectrum, from several hundreds of Ångström to a few ten thousands of Ångström.
UVOT: the Ultraviolet-Optical Telescope, on board Swift.
Vela: a series of U.S. satellites operating from the 1960s and 1970s.
VLA: the Very Large Array; a radio observatory located in New Mexico.
Waveband: a certain span in wavelength, frequency, or energy of the electromagnetic spectrum.
WFC: Wide-Field Camera, on board the BeppoSAX satellite.
White dwarf (WD): the remnant core of a former star that is no longer undergoing fusion processes. This dense and compact object is supported against gravitational collapse by degeneracy pressure associated with the tight packing of electrons. The brightness of a WD is determined by the rate at which residual heat leaks out from the surface of the object. The typical size of a WD is comparable to the size of the Earth but has a mass similar to that of the Sun.
H(z): the average neutral fractional of hydrogen at a given redshift (Hz). The value H(z) is the ratio of the number density of neutral hydrogen atoms divided by the number density of neutral hydrogen atoms plus protons.
X-ray: wavelength regime of the electromagnetic spectrum between ultraviolet and gamma ray; photon energies from 0.1 keV to ~10 keV.