Soft X-ray Transient (SXT)
gozarâ-ye partow-e X-e narm
An → X-ray binary system that has a long period of → quiescence interrupted by → outbursts of low-energy → soft X-rays. Alternatively known as X-ray novae, the majority (~ 75%) of SXTs contain a → black hole and a low-mass → main sequence → companion star in orbit around one another. It is thought that SXTs arise in a similar manner to → dwarf novae, through instabilities in the → accretion disk around the → compact object (→ disk instability model).
partowhâ-ye iks-e narm
Fr.: rayons X mous
X-ray photons with energies between about 0.1 to 10 keV. → hard X-rays.
Square Kilometer Array (SKA)
An international project to construct a highly sensitive radio interferometer array operating between 0.15 and 20 GHz with an effective collecting area of one square kilometer. The number of individual telescopes will be 2000 to 3000. SKA will have a sensitivity 100 times higher than that of today's best radio telescopes and an angular resolution < 0.1 arcsec at 1.4 GHz. The site will be selected in 2012 and early science with Phase 1 is scheduled for from 2016 on. See also the SKA homepage.
transient X-ray source
xan-e partow-e iks-e gozarâ
Fr.: source de rayons X transitoire
An X-ray source that appears suddenly in the sky, strongly increases its intensity over a few days, and then declines with a lifetime of several months.
ultra-high-energy cosmic ray (UHECR)
partowhâ-ye keyhâni-ye ultar-meh-kâruž
Fr.: rayons cosmiques de très haute énergie
A particle belonging to the most energetic population of → cosmic rays with an energy above ~ 1020 → electron-volts. The UHECRs constitute a real challenge for theoretical models, because their acceleration requires extreme conditions hardly fulfilled by known astrophysical objects. See also → UHECR puzzle, → Greisen-Zatsepin-Kuzmin cutoff.
ultraluminous X-ray source (ULX)
xan-e partow-e iks-e ultar-tâbân
Fr.: source ultralumineuse en rayons X
An X-ray source that is not in the nucleus of a galaxy, and is more luminous than 1039 ergs s-1, brighter than the → Eddington luminosity of a 10 → solar mass → black hole. In general, there is about one ULX per galaxy in galaxies which host ULXs. The Milky Way contains no such objects. ULXs are thought to be powered by → accretion onto a → compact object. Possible explanations include accretion onto → neutron stars with strong → magnetic fields, onto → stellar black holes (of up to 20 → solar masses) at or in excess of the classical Eddington limit, or onto → intermediate-mass black holes (103-105 solar masses). NGC 1313X-1, NGC 5408X-1, and NGC 6946X-1 are three ULXs with X-ray luminosities up to ~ 1040 erg s-1 (Ciro Pinto et al., 2016, Nature 533, N) 7601).
Very Large Array (VLA)
ârast-e besyâr bozorg
Fr.: Very Large Array (VLA)
A radio interferometer consisting of 27 antennas, each 25 m in diameter, in a Y-shaped configuration. It is located about 100 km west of Socorro, New Mexico, and is operated by the United States National Radio Astronomy Observatory. The VLA has the resolution of a single antenna 36 km wide and the sensitivity of a dish 130 m across.
Very Large Baseline Array (VLBA)
ârast bâ pâye-xatt-e besyâr bozorg
Fr.: Very Large Baseline Array (VLBA)
A network of ten 25-m radio telescopes for → very-long-baseline interferometry (VLBI), operated by the U.S. National Radio Astronomy Observatory. Eight of the VLBA telescopes are distributed across the continental United States, while the other two are in Hawaii and the Virgin Islands, giving a maximum baseline of about 8,000 km and a resolution better than a milliarcsecond at its shortest wavelength.
Fr.: Wolf-Rayet WC
A → Wolf-Rayet star whose spectrum
is dominated by emission lines of ionized carbon: C III 5696 Å,
C III / C IV 4650 Å, C IV 5801-12 Å.
This type is divided in sub-types WC4 to WC9.
Fr.: Wolf-Rayet WN
A → Wolf-Rayet star whose spectrum is dominated by emission lines of ionized nitrogen: N II 3995 Å, N III 4634-4661 Å, N III 5314 Å, N IV 3479-3484 Å, N IV 4058 Å, N V 4603 Å, N V 4619 Å, and N V 4933-4944 Å. This type is divided in sub-types WN2 to WN11.
Fr.: Wolf-Rayet WNE
In theoretical models, a → Wolf-Rayet star without hydrogen at its surface (< 10-5 in number) and with surface carbon abundance smaller than nitrogen abundance.
Fr.: Wolf-Rayet WNL
In theoretical models, a → Wolf-Rayet star with hydrogen at its surface (> 10-5 in number). A star enters the Wolf-Rayet phase as a WNL, then may evolve through the sequence WNL → WNE, → WC, → WO. It can end its evolution at any of these stages.
Fr.: Wolf-Rayet WO
A → Wolf-Rayet star whose spectrum shows emission lines of carbon and strong emission lines of oxygen O VI 3811-34 Å. In theoretical models, a W-R star whose carbon abundance at surface is larger than nitrogen abundance and has the abundance ratio (C + O) / He > 1 (in number).
Fr.: galaxie Wolf-Rayet
A subset of → starburst galaxies whose integrated spectra show broad emission features attributed to the presence of hundreds to thousands → Wolf-Rayet stars. The most massive stars formed in the burst evolve rapidly into a substantial population of Wolf-Rayet stars in aggregations of ionized gas.
Fr.: étoile Wolf-Rayet
A type of very luminous, very hot (as high as 50,000 K) stars whose spectrum is characterized by broad emission lines (mainly He I and He II), which are presumed to originate from material ejected from the star at very high (~ 2000 km s-1) velocities. The most massive → O stars (M > 25 → solar masses for → solar metallicity) become W-R stars around 2 and 3 million years after their birth, spending only some few hundreds of thousands of years (≤ 106 years) in this phase until they explode as → type Ib and → type Ic supernovae. The minimum stellar mass that an O star needs to reach the W-R phase and its duration is dependent on → metallicity. → WC Wolf-Rayet; → WNE Wolf-Rayet; → WNL Wolf-Rayet; → WO Wolf-Rayet. For a review see: P. A. Crowther, 2007, Annu. Rev. of Astron. Astrophys. 45, 177.
Named after the French astronomers Charles Wolf (1827-1918) and Georges Rayet (1839-1906), of the Paris Observatory. In 1867 they discovered three stars in the constellation Cygnus (now designated HD191765, HD192103, and HD192641), that displayed broad emission bands in their spectra; → star.
partow-e iks (#)
Fr.: rayon X
The → electromagnetic radiation with → wavelengths shorter than that of → ultraviolet radiation and greater than that of → gamma rays. Typical X-rays have a wavelength ranging from 0.1 to 100 Å (0.01 to 10 → nanometers), corresponding to frequencies in the range 3 × 1016 to 3 × 1019 Hz and energies in the range 100 eV to 100 → keV. X-rays are produced artificially when high-speed → electrons collide with a heavy metal target such as tungsten. Astrophysical sources of X-rays include → plasmas with → temperatures in the range 106-108 K, and deceleration process of rapidly moving charges upon interaction with matter (→ bremsstrahlung). X-rays were discovered in 1895 by Wilhelm Röntgen, a German physics professor (→ roentgen). For this discovery he won the first Nobel prize in physics in 1901. See also: → soft X-rays, → hard X-rays.
X stand for "unknown," since Röntgen was not sure what they were; → ray.
axtaršenâsi-ye partowhâ-ye iks (#)
Fr.: astronomie en rayons X
The study of celestial bodies using their X-ray emission. X-ray astronomy deals mainly with Galactic and extragalactic phenomena involving very high-energy photon emissions, covering a band of energies between 0.1 keV and 500 keV. The research field includes: → X-ray binaries, → cataclysmic variables, → pulsars, → black holes, → dark matter, → active galaxies, → galactic clusters → X-ray transients. The Earth's atmosphere absorbs most X-rays coming from outer space. X-ray astronomy therefore requires observations to be done above atmosphere. The first rocket flight which successfully detected a cosmic source of X-ray emission was launched in 1962 by an American research group. A very bright source was detected that they named → Scorpius X-1. Since then several dedicated X-ray astronomy satellites have been launched, among which: Uhuru, INTEGRAL, ROSAT, Rossi X-ray Timing Explorer (RXTE), → Chandra X-ray Observatory, and → XMM-Newton, which have contributed to important advances in astronomy.
pas-zamine-ye iks, zamine-ye ~ (#)
Fr.: fond de rayons X
A diffuse background radiation in X-ray wavelengths which has several origins. At very low energies it is due to hot gas in the → Local Bubble. In the → soft X-ray energy band it comes from active galaxies at moderate redshifts. In → hard X-ray range the background is thought to be due to integrated emission from many → quasars at various redshifts.
dorin-e partow-e iks
Fr.: binaire X
A binary star system where one of the stars has evolved and collapsed into an extremely dense body such as a → white dwarf, a → neutron star, or a → black hole. The enormous gravitational attraction of the massive, dense, but dim component pulls material from the brighter, less massive star in an → accretion disk. The gravitational potential energy of the accreted matter is converted to heat by → viscosity and eventually to high-energy photons in the X-ray range. The brightest X-ray binary is → Scorpius X-1.
belk-e partow-e iks
Fr.: sursaut de rayonnement X
A rapid and intense surge of X-ray emission from some sources. They often last less than one second followed by an exponential decrease of typically a few seconds to a minute. Most X-ray bursts are believed to arise in → X-ray binary systems due to nuclear fusion of material accreted onto a compact companion.