anomalous X-ray pulsar (AXP)
pulsâr-e pertwâ-ye iks-e nâsân
Fr.: pulsar X anormal
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.
Chandra X-ray Observatory
nepâhešgâh-e partowhâ-ye X-e Chandra
Fr.: Observatoire des rayons X Chandra
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.
partowhâ-ye X-e saxt (#)
Fr.: rayons X durs
The short wavelength, high energy end of the → electromagnetic spectrum. Hard X-rays are typically those with energies greater than around 10 keV. The dividing line between hard and → soft X-rays is not well defined and can depend on the context.
high-mass X-ray binary (HMXB)
dorin-e partow-e iks-e por-jerm
Fr.: binaire X de forte masse
A member of one of the two main classes of → X-ray binary systems where one of the components is a neutron star or a black hole and the other one a → massive star. HMXBs emit relatively → hard X-rays and usually show regular pulsations, no X-ray bursts, and often X-ray eclipses. Their X-ray luminosity is much larger than their optical luminosity. In our Galaxy HMXBs are found predominantly in the → spiral arms and within the → Galactic disk in young → stellar populations less than 107 years old. One of the most famous HMXB is Cygnus X-1 which was the first stellar-mass black hole discovered. See also: → low-mass X-ray binary.
low-mass X-ray binary (LMXB)
dorin-e partow-e iks-e kam-jerm
Fr.: binaire X de faible masse
A member of one of the two main classes of
→ X-ray binary systems where one of the components is a
→ neutron star or → black hole
and the other component a → low-mass star with a spectral type A
or later. LMXBs mainly emit → soft X-rays.
The ratio of their optical to X-ray luminosities is less than 0.1. They belong
to → old stellar populations
with ages 5-15 × 109 years and are found in
→ globular clusters
and in the → bulge
of our → Milky Way
galaxy; some are also found in the disk.
Hercules X-1 is an example of LMXBs.
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.
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.
→ transient; → X-ray source.
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).
→ ultraluminous; → X-ray source.
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.
→ X-ray; → background.
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.
parâš-e partow-e iks
Fr.: diffraction de rayons X
The diffraction of X-rays by the atoms or ions of a crystal. The wavelength of X-rays are comparable to the size of interatomic spacings in solids. Since the atoms in a crystal are arranged in a set of regular planes, crystals serve as three-dimensional diffraction gratings for X-rays. Planes of repetition within the atomic structure of the mineral diffract the X-rays. The pattern of diffraction thus obtained is therefore used to identify minerals by bombarding them with X-rays.
→ X-ray; → diffraction.
X-ray Dim Isolated Neutron Star (XDINS)
setâre-ye notroni bâ partowhâ-ye X-e nazâr
Fr.: étoile à neutron de faibles rayons X
A member of a class of isolated, radio-silent → pulsars with peculiar properties. They show a purely thermal spectrum at X-ray energies with no evidence for a high-energy, power-law component often detected in other → isolated neutron star classes. The X-ray luminosity is 1031 - 1032 erg s-1, fully consistent with surface blackbody emission with temperatures ~ 40-100 eV and (radiation) radii of a few kilometers, as derived from X-ray spectral fits. With the only exception of RX J1856.5-3754, broad absorption features have been found in all XDINSs. These features have energies ~ 300 - 700 eV, equivalent widths of ~ 50 - 150 eV and, as in the case of RX J0720.4-3125, may be variable.
X-ray Dissociation Region (XDR)
nâhiye-ye vâhazeš-e partowhâ-ye X
Fr.: région de dissociation par rayons X
A region of → molecular clouds whose chemistry is affected by → X-rays from neighboring sources.
→ X-ray; → dissociation; → region.
partow-e X paristandé
Fr.: rayons X persistants
An → X-ray source that does not display emission → outbursts, in contrast to → X-ray transients.
→ X-ray; → persistent.
pulsâr-e partowhâ-ye iks, tapâr-e ~ ~
Fr.: pulsar X
A regularly variable X-ray source in which the pulsation is associated with the rotation of a magnetized neutron star in an → X-ray binary. Periods range from a few seconds to a few minutes. Examples include Hercules X-1, Centaurus X-3, Cygnus X-3.
xan-e partow-e iks
Fr.: source de rayons X
An astronomical object whose dominant mechanism of radiation is through X-ray emission. X-ray sources contain an extremely hot gas at temperatures from 106 to 108 K. They are generated by various physical processes involving high energies, such as accretion on to a compact object, shock waves from supernovae, stellar winds, hot gas in stellar coronae, or hot spaces between galaxies in a cluster. The first celestial X-ray source, after the Sun, to be detected was → Scorpius X-1 by means of rocket flight (Giacconi et al. 1962).