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

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.

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; → dim; → isolated; → neutron; → star.

A region of → molecular clouds whose chemistry is affected by → X-rays from neighboring sources.

→ X-ray; → dissociation; → region.

An → X-ray source that does not display emission → outbursts, in contrast to → X-ray transients.

→ X-ray; → persistent.

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.

→ X-ray; → pulsar.

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).

→ X-ray; → source.

An instrument for producing an X-ray spectrum and measuring the wavelengths of its components.

→ X-ray; → spectrometer.

A telescope designed to focus X-rays from astronomical objects. X-ray telescopes function from orbital satellites because X-rays are absorbed by the Earth's atmosphere. These telescopes require special techniques since the conventional methods used in optical and radio telescopes are not adequate. → grazing-incidence telescope; → Bragg angle.

→ X-ray; → telescope.

An → X-ray source that enhances its emission well above its usual level for a period typically longer than a week. Recurrences can be → periodic or → aperiodic, but there is no obvious correlation between recurrence time and the luminosity amplitude of the → outburst. X-ray transients seem to be divided into two classes: those associated with → high-mass X-ray binary (HMXB) stars and those associated with → low-mass X-ray binary (LMXB) stars. The HMXBs contain a → neutron star or → black hole paired with a → massive star. Often, the stellar companion is a → Be star, Be star, which sometimes sheds material from its equatorial region. In these systems, the transient event is characterized by having more higher energy X-rays in the spectrum. The LMXBs contain a neutron star or black hole orbiting around a cooler, → low-mass star. These transient events often generate more lower-energy X-rays.

→ X-ray; → transient.