An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A "Large Number of Small Dishes" (LNSD) array designed to be sensitive for → commensal surveys of conventional → radio astronomy projects and → SETI targets at centimeter wavelengths. The ATA will consist of 350 6m-diameter → dishes when completed, which will provide an outstanding survey speed and sensitivity. In addition, the many → antennas and → baseline pairs provide a rich → sampling of the → interferometer  → uv plane, so that a single pointing snapshot of the array of 350 antennas yields an image in a single field with about 15,000 independent → pixels. Other important features of the ATA include continuous frequency coverage over 0.5 GHz to 10 GHz and four simultaneously available 600-MHz bands at the → back-end which can be tuned to different frequencies in the overall band. The ATA is a joint project of the Radio Astronomy Laboratory of the University of California, Berkeley, and the SETI Institute in Mountain View, CA. The ATA is now complete to 42 antennas. Highlights of the system are the frequency agility, the low background and → side lobes of the antennas, the wideband feed and input receiver, the analog fiber optical system, the large spatial dynamic range, the back-end processing systems and the overall low cost (see, e.g., Backer et al., 2009, arXiv:0908.1175.pdf).

Named after Paul G. Allen (1953-2018), an American business magnate, computer programmer, researcher, investor, and philanthropist. A donation of $11.5 million by his foundation in 2004 contributed to the development of the project.

A member of a small class of → X-ray pulsars with long rotation periods (6-12 seconds), short → spin-down times (~ 10³-10⁵ years), and → soft X-ray spectrum. AXPs show no evidence of being → X-ray binary systems. Their magnetic fields, as deduced from their spin-down rate, are the highest known, reaching 10¹³-10¹⁵ → gauss. AXPs are generally believed to be → magnetars.

→ anomalous; → X-ray; → pulsar.

Rays of → sunlight that appear to converge at the → antisolar point. Like → crepuscular rays, they are parallel beams of sunlight from holes in the clouds, and their apparently odd directions are a perspective effect.

→ anti-; → crepuscular rays.

1) A system of telescopes coupled together, using → interferometric techniques, to increase the angular resolution or the sensitivity.
2) A two-dimensional detector comprising a large number of identical, individual detectors that can be used simultaneously, e.g. a → CCD.
3) A series of numbers or symbols arranged in some geometric pattern, as in a matrix.

Array, from M.E. arraien, from Anglo-Norman arraier, from V.L. *arredare.

Ârast "set in order," from ârastan, ârâstan "to set in order," Mid.Pers. ârây-, ârâstan, from â- + Av. râd- "to make ready, prepare;" PIE *ar- "to fit together."

One of the largest ground-based astronomy projects and a major new facility for world astronomy located on the plain of the → Chajnantor Chilean Andes, San Pedro de Atacama, some 5000 m above sea level. ALMA will initially comprise 66 high precision antennas, with the option to expand in the future. There will be an array of fifty 12 m antennas, acting together as an → interferometer to capture → millimeter and → submillimeter wavelengths of 0.3 to 9.6 mm. It will have reconfigurable baselines ranging from 15 m to 18 km. A compact array of 7 m antenna and few 12 m diameter antennas (ACA) will be used to measure the diffuse emission. Resolutions as fine as 0''.005 will be achieved at the highest frequencies. Construction of ALMA started in 2003 and will be completed in 2012. The ALMA project is an international collaboration between Europe, Japan, and North America in cooperation with the Republic of Chile. ALMA is funded in Europe by the → European Southern Observatory (ESO). The first 12 m diameter antenna, built by Mitsubishi Electric Corporation for the National Astronomical Observatory of Japan, was handed over to ESO in 2008. It will shortly be joined by North American and European antennas. ALMA will allow astronomers to study the cool Universe, i.e. the molecular gas and tiny dust grains from which stars, planetary systems, galaxies, and even life are formed.

Atacama the name of a desert, west of the Andes mountains in Chile, covering a 1,000 km strip of land on the Pacific coast of South America; → large; → millimeter; → submillimeter; → array.

A kind of ray generated at the cathode in a vacuum tube, by the electrical discharge.

→ cathode; →ray.

A CCD detector having two dimensions.

→ CCD; → array.

An astronomy satellite launched by NASA in 1999 July, specially designed to detect X-ray emission from very hot regions of the Universe such as exploded stars, clusters of galaxies, and matter around black holes. Chandra carries a high resolution mirror (aperture 1.2 m, focal length 10 m), two imaging detectors (HRC and ACIS), and two sets of transmission grating spectrometer (LETG and HETG). Important Chandra features are: an order of magnitude improvement in spatial resolution, good sensitivity from 0.1 to 10 keV, and the capability for high spectral resolution observations over most of this range. Chandra was initially given an expected lifetime of 5 years, but on 4 September 2001 NASA extended its lifetime to 10 years "based on the observatory's outstanding results." Among the results obtained using Chandra one can mention the spectacular image of the → supernova remnant Cassiopeia A. See also → X-ray astronomy.

Initially called Advanced X-ray Astrophysics Facility (AXAF), the satellite was renamed the Chandra X-ray Observatory in honor of Subrahmanyan Chandrasekhar, the 1983 Nobel Prize in Physics, → Chandrasekhar limit. Moreover, Chandra, or candra- means "moon" or "shining" in Skt., from cand- "to give light, shine;" cf. Gk. kandaros "coal;" L. candela "a light, torch," from candere "to shine;" → X-ray; → Observatory.

Of an optical ray, the parallel ray that passes through the center of the → optical system.

→ conjugate; → ray.

Extremely energetic atomic nuclei which travel through the Universe at practically the speed of light and strike the Earth from all direction. Almost 90% of all the incoming → primary cosmic rays are → protons, about 9% are helium nuclei (→ alpha particles) and about 1% are → electrons (beta minus particles). Some cosmic rays come from the Sun (mainly due to → solar flares), most come from galactic → supernovae, and a few with the highest energy are suspected to originate from outside the → Milky Way. As for their flux, about 1 charged particle per second per cm² impacts the Earth. The typical kinetic energy of these particles is about 10 MeV/nucleon to several GeV/nucleon, although there are some at higher energies. In fact, the cosmic ray with the highest energy has been measured above × 10²⁰ eV. These → ultra-high energy cosmic rays are suspected to be extragalactic, as there is no plausible mechanism of acceleration to these energies by a supernova, for example. Again, compare these energies to those of solar neutrinos that have only 0.26 MeV. Cosmic rays may be divided into → primary cosmic rays and → secondary cosmic rays. Their energy ranges from 10⁹ to 10²⁰  → electron-volts.

→ cosmic; → ray; The term "ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles.

An intense beam of cosmic rays coming from any direction on the sky, which originates outside the solar system.

→ cosmic; → ray; → burst.

Spurious signals in CCD frames caused by ionizing radiation which appear as a set of pixels with intense values sparsely scattered over the CCD frame. High energy particles generate muons, which deposit around 80 electrons per micron in silicon. With a collection depth of 10-20 microns, a cosmic-ray event is seen on a CCD frame as having a signal of up to a few thousand electrons, usually concentrated in one or two pixels. Although attributed to cosmic-ray hits, they may also be due to background terrestrial radiation.

→ cosmic rays; → event.

The ionization of → interstellar medium (ISM) gas by → cosmic rays. Cosmic rays are a primary source of ionization, competing with stellar → ultraviolet photons and → X-rays produced by embedded → young stellar objects. Cosmic rays play a key role in the chemistry and dynamics of the interstellar medium. The ionization fraction in turn drives the chemistry of → molecular clouds and controls the coupling of the gas with the Galactic → magnetic field. Moreover, cosmic rays represent an important source of → heating for → molecular clouds because the energy of primary and secondary electrons produced by the ionization process is in large part converted into heat by → inelastic collisions with ISM atoms and → molecules (see, e.g., Padovanit et al., 2009, arXiv:0904.4149).

→ cosmic; → ray; → ionization.

An extensive (many kilometres wide) → cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a → primary cosmic rays collides with atmospheric nuclei creating many → secondary cosmic rays. Also known as → air shower.

→ cosmic; → ray; → shower.

Rays of sunlight that appear to diverge from a single point in the sky when parallel columns of light, partially blocked by clouds, pour through gaps in clouds. They result from light scattering and an optical effect called perspective.

Crepuscular "of, pertaining to, or resembling twilight," from L. crepuscul(um), "twilight, dusk," from crepus-, from creper "dusky, dark."

Partowhâ "rays," from partow, → ray; nimtâbi "of, pertaining to, or resembling nimtâb" → twilight.

Same as → rainbow ray.

→ Descartes; → ray.

Optics: The → light ray leaving a → medium, in contrast to the → incident ray. If the medium has parallel sides, → angle of incidence and → angle of emergence

→ emergent; → ray.

A → radio interferometer array consisting of 27 25-meter diameter antennas located on the Plains of San Agustin in West-Central New Mexico. EVLA will operate at any frequency between 1.0 and 50 GHz and will have a continuum sensitivity improvement over the → VLA by factors of 5 to 20.The EVLA project is expected to be completed in 2012. See also the EVLA homepage.

→ expand; → very; → large; → array.

When a beam of → unpolarized light is incident on a → doubly refracting crystal, there will be two refracted rays. The ray for which → Snell's law does not hold.

→ extraordinary; → ray.

A space observatory, formerly named GLAST, devoted to the study of → gamma rays emitted from astrophysical objects. Developed by NASA in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States, Fermi was launched on June 11, 2008. The main instrument, the Large Area Telescope (LAT), is an imaging → camera covering the energy range from about 20 → MeV to more than 300 → GeV. Such gamma rays are emitted only in the most extreme conditions, by particles moving very nearly at the → speed of light. The LAT's → field of view covers about 20% of the sky at any time, and it scans continuously, covering the whole sky every three hours. Another instrument, the Gamma-ray Burst Monitor (GBM) has a field of view several times larger than the LAT and provides → spectral coverage of → gamma-ray burst that extends from the lower limit of the LAT down to 10 → keV.

→ Fermi; → gamma ray; → space; → telescope.