Astronomical Glossary

X-Band
A radio band at a wavelength of 3.7 cm (8085 MHz). [H76]
XBL
X-Ray selected BL Lac Object
XBONG
X-Ray Bright Optically Normal galaxy. X-ray satellites have now detected a sizeable number of X-ray sources spectroscopically identified with "normal" galaxies otherwise having no any obvious signs of nuclear activity in their optical spectra. The large X-ray-to-optical flux ratio exceeds the average value for early-type galaxies of similar optical lumisosity, by more than an order of magnitude. The hardness of the X-ray spectrum suggests that highly obscured AGN activity is taking place in their nuclei. However, the lack of optical emission lines could also be explained if the nuclear light is being overwhelmed by either the stellar continuum or a non-thermal component, or if the lines are not efficiently produced.
Xenon

A colorless odorless monatomic element of the rare-gas group. It occurs in trace amounts in air. Xenon is used in thermionic tubes and strobe lighting.
Symbol: Xe; m.p. -111.9°C; b.p. -107.1°C; d. 5.8971 (0°C) kg m^-3; p.n. 54; r.a.m. 131.29. [DC99]
XMM
X-ray Multi-mirror Mission. [LLM96]
X-ogen
An unidentified molecular transition at 3.36 mm (89.19 GHz) discovered in 1970. [H76]
X Particle
Exceedingly massive (hypothetical) particle predicted by grand unified theories to convey a very short-ranged interaction between quarks and leptons. An X particle would be able to change a quark into a lepton or an antiquark. [D89]
X Process
The unknown nucleosynthetic process that Burbidge, Burbidge, Fowler and Hoyle said had formed the light nuclei Deuterium, Lithium, Beryllium, and Boron. [C95]
X-Rays
(a) Photons of wavelengths between about 0.1 Å and 100 Å - more energetic than ultraviolet, but less energetic than gamma -rays. [H76]
(b) A large band of electromagnetic radiation with wavelengths smaller than extreme ultraviolet light. A typical X-ray photon has over one thousand times as much energy as a photon of visible light. [McL97]
X-Ray Astronomy
(a) Detection of stellar and interstellar X-ray emission. Because X-rays are almost entirely filtered out by the Earth's upper atmosphere, the use of balloon- and rocket-borne equipment is essential. [A84]
(b) Astronomy carried out in the waveband roughly 0.1-100 keV. The atmosphere is opaque to radiation at these wavelengths and so observations have to be carried out from above the Earth's atmosphere. It has been found that many classes of object are X-ray emitters, including stars, supernovae and active galaxies. [D89]
X-Ray Pulsars
Pulsars that radiate in the X-ray region of the spectrum. Best verified examples are Her X-1 and Cen X-3. They are thought to be rotating, strongly magnetic neutron stars of about 1 M in a grazing orbit around a more massive star from which they are accreting matter. [H76]
X-Ray Sources
A class of celestial objects whose dominant mechanism of energy dissipation is through X-ray emission. Galactic X-ray sources appear optically as starlike objects, peculiar in their ultraviolet intensity, variability (on time scales ranging from milliseconds to weeks), and spectral features. All known compact X-ray sources are members of close binary systems; a current popular model is mass accretion onto a compact object from a massive companion. (Four X-ray sources - all variable - are known to be associated with globular clusters.) The 21 known extended X-ray sources associated with clusters of galaxies seem to be clouds of hot gas trapped in the cluster's gravitational field. [H76]
XRT
X-Ray Telescope.

X-Band	A radio band at a wavelength of 3.7 cm (8085 MHz). [H76]
XBL	X-Ray selected BL Lac Object
XBONG	X-Ray Bright Optically Normal galaxy. X-ray satellites have now detected a sizeable number of X-ray sources spectroscopically identified with "normal" galaxies otherwise having no any obvious signs of nuclear activity in their optical spectra. The large X-ray-to-optical flux ratio exceeds the average value for early-type galaxies of similar optical lumisosity, by more than an order of magnitude. The hardness of the X-ray spectrum suggests that highly obscured AGN activity is taking place in their nuclei. However, the lack of optical emission lines could also be explained if the nuclear light is being overwhelmed by either the stellar continuum or a non-thermal component, or if the lines are not efficiently produced.
Xenon	A colorless odorless monatomic element of the rare-gas group. It occurs in trace amounts in air. Xenon is used in thermionic tubes and strobe lighting. Symbol: Xe; m.p. -111.9°C; b.p. -107.1°C; d. 5.8971 (0°C) kg m^-3; p.n. 54; r.a.m. 131.29. [DC99]
XMM	X-ray Multi-mirror Mission. [LLM96]
X-ogen	An unidentified molecular transition at 3.36 mm (89.19 GHz) discovered in 1970. [H76]
X Particle	Exceedingly massive (hypothetical) particle predicted by grand unified theories to convey a very short-ranged interaction between quarks and leptons. An X particle would be able to change a quark into a lepton or an antiquark. [D89]
X Process	The unknown nucleosynthetic process that Burbidge, Burbidge, Fowler and Hoyle said had formed the light nuclei Deuterium, Lithium, Beryllium, and Boron. [C95]
X-Rays	(a) Photons of wavelengths between about 0.1 Å and 100 Å - more energetic than ultraviolet, but less energetic than -rays. [H76] (b) A large band of electromagnetic radiation with wavelengths smaller than extreme ultraviolet light. A typical X-ray photon has over one thousand times as much energy as a photon of visible light. [McL97]
X-Ray Astronomy	(a) Detection of stellar and interstellar X-ray emission. Because X-rays are almost entirely filtered out by the Earth's upper atmosphere, the use of balloon- and rocket-borne equipment is essential. [A84] (b) Astronomy carried out in the waveband roughly 0.1-100 keV. The atmosphere is opaque to radiation at these wavelengths and so observations have to be carried out from above the Earth's atmosphere. It has been found that many classes of object are X-ray emitters, including stars, supernovae and active galaxies. [D89]
X-Ray Pulsars	Pulsars that radiate in the X-ray region of the spectrum. Best verified examples are Her X-1 and Cen X-3. They are thought to be rotating, strongly magnetic neutron stars of about 1 M in a grazing orbit around a more massive star from which they are accreting matter. [H76]
X-Ray Sources	A class of celestial objects whose dominant mechanism of energy dissipation is through X-ray emission. Galactic X-ray sources appear optically as starlike objects, peculiar in their ultraviolet intensity, variability (on time scales ranging from milliseconds to weeks), and spectral features. All known compact X-ray sources are members of close binary systems; a current popular model is mass accretion onto a compact object from a massive companion. (Four X-ray sources - all variable - are known to be associated with globular clusters.) The 21 known extended X-ray sources associated with clusters of galaxies seem to be clouds of hot gas trapped in the cluster's gravitational field. [H76]
XRT	X-Ray Telescope.