Glossary Xx

X-RAYS - High-energy electromagnetic radiation, with short wavelength (~10-0.01 nm) and high frequency (greater than ~1016 Hertz). Although the boundaries are somewhat arbitrary, wavelengths shorter than 0.01 nm are called gamma-rays and those longer than 10 nm extreme ultraviolet (EUV). X-rays would be produced by blackbody radiation at temperatures in excess of 106 K. Sources of astrophysical X-rays include accretion disks, gas impacting on neutron stars, X-ray bursters, and hot gas located in the centers of galaxy clusters.

X-RAY BINARY - Close binary system where a neutron star (or rarely a black hole) accretes matter from what is usually a main sequence star (left). X-ray binaries are some of the most luminous X-ray sources in the sky. X-rays are produced as material from the companion star is drawn to the compact object either through Roche-lobe overflow into an accretion disk (low-mass X-ray binaries) or through direct impact of a stellar wind onto the compact object (high-mass X-ray binaries).

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X-RAY BURSTS - Bursts of X-ray energy that occur in low-mass X-ray binary systems in which a neutron star and low-mass main sequence star are in orbit around one another. Due to their close proximity and the extreme gravity of the neutron star, the companion star overflows its Roche lobe and H is drawn into an accretion disk around the neutron star. H is eventually accreted to the surface of the neutron star, where it is immediately converted into He by the extreme temperatures and pressures that exist there. A thin surface layer of He builds up, and once a critical mass is reached, the He ignites explosively, heating the entire surface of the neutron star and releasing a sudden burst of X-rays. After the burst, the system returns to a quiescent state and the neutron star begins to re-accumulate the He surface layer. The process repeats resulting in recurrent X-ray bursts. The mechanisms that produce X-ray bursts and recurrent novae are similar. Recurrent novae form when a white dwarf accretes a surface layer of H that undergoes explosive burning.

X-ray bursts generally occur at regular intervals separated by several hours or days. They last from a few seconds to a few minutes, with the burst profile showing a rapid rise (0.3-10 s) followed by a slower decline (5-100 seconds). The rapid rise reflects the sudden increase in temperature brought about by explosive He ignition, while the longer decline reflects the slower cooling of the surface of the star.

X-RAY DIFFRACTION - Analytical technique used to determine the structures of crystalline solids. A monochromatic beam of X-rays (usually Cu-Kα) is diffracted off repeating planes of atoms in crystalline samples to produce a diffraction pattern. Through analysis of the diffraction pattern, atomic structures can often be determined.

X-RAY HALO - Halo of high temperatures X-ray emitting gas (106 to 10 x 106 K) surrounding most large elliptical galaxies. Although found predominantly around massive elliptical galaxies, weaker X-ray halos have also been detected around starburst galaxies and more recently around spiral galaxies similar to the Milky Way. The photograph (right) shows the X-ray halo in blue superimposed on a visible light image.

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X-RAY PULSAR - Neutron star with a powerful magnetic field in an X-ray binary. Gas accreted from the companion star is channeled to the magnetic poles of the neutron star and forms X-ray emitting hot spots which move into and out of view as the neutron star spins, giving rise to regular X-ray pulses. The pulsation periods of X-ray pulsars range from 1.6 ms to >10 minutes in length. Long period X-ray pulsars have particularly strong magnetic fields that decrease their rotation rates through torques exerted on its magnetosphere.

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Unlike radio pulsars, which are all spinning down due to energy losses in the form of relativistic particles and magnetic dipole radiation, some X-ray pulsars have been found to be spinning up; whereas, others have relatively stable spin rates or show erratic behavior (alternating periods of spin-up and spin-down). The variations in the spin rate arise because the pulsar can gain, lose or maintain its angular momentum depending on how the accreting material is transferred to the neutron star. They can have persistent mass transfer from Roche-lobe overflow, episodes of mass accretion (possibly due to an eccentric orbit that takes the neutron star close to the companion near periastron), or can be powered by stellar wind accretion. In contrast, a solitary radio pulsar can only lose angular momentum through radiation of energy.

X-WIND - In the X-wind model the magnetic field of a young star interacts with the magnetic field of the circumstellar disk to produce a gap between the star and disk. As gas spirals inward through the disk, it divides at the inner disk edge (the X-region) into two streams. A high angular momentum stream is flung away along the rotating magnetic field lines of the disk (the X-wind); the low angular momentum stream falls onto the star and helps build its mass.

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XENOLITH - Fragment of a rock or meteorite that formed apart from the host material.