5 research outputs found
Accreting Neutron Stars in Low-Mass X-Ray Binary Systems
Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discovered
that disk-accreting neutron stars with weak magnetic fields produce three
distinct types of high-frequency X-ray oscillations. These oscillations are
powered by release of the binding energy of matter falling into the strong
gravitational field of the star or by the sudden nuclear burning of matter that
has accumulated in the outermost layers of the star. The frequencies of the
oscillations reflect the orbital frequencies of gas deep in the gravitational
field of the star and/or the spin frequency of the star. These oscillations can
therefore be used to explore fundamental physics, such as strong-field gravity
and the properties of matter under extreme conditions, and important
astrophysical questions, such as the formation and evolution of millisecond
pulsars. Observations using RossiXTE have shown that some two dozen neutron
stars in low-mass X-ray binary systems have the spin rates and magnetic fields
required to become millisecond radio-emitting pulsars when accretion ceases,
but that few have spin rates above about 600 Hz. The properties of these stars
show that the paucity of spin rates greater than 600 Hz is due in part to the
magnetic braking component of the accretion torque and to the limited amount of
angular momentum that can be accreted in such systems. Further study will show
whether braking by gravitational radiation is also a factor. Analysis of the
kilohertz oscillations has provided the first evidence for the existence of the
innermost stable circular orbit around dense relativistic stars that is
predicted by strong-field general relativity. It has also greatly narrowed the
possible descriptions of ultradense matter.Comment: 22 pages, 7 figures, updated list of sources and references, to
appear in "Short-period Binary Stars: Observation, Analyses, and Results",
eds. E.F. Milone, D.A. Leahy, and D. Hobill (Dordrecht: Springer,
http://www.springerlink.com
Millisecond Oscillations in X-Ray Binaries
The first millisecond X-ray variability phenomena from accreting compact
objects have recently been discovered with the Rossi X-ray Timing Explorer.
Three new phenomena are observed from low-mass X-ray binaries containing
low-magnetic-field neutron stars: millisecond pulsations, burst oscillations
and kiloHertz quasi-periodic oscillations. Models for these new phenomena
involve the neutron star spin, and orbital motion closely around the neutron
star and rely explicitly on our understanding of strong gravity and dense
matter. I review the observations of these new neutron-star phenomena and
possibly related ones in black-hole candidates, and describe the attempts to
use them to perform measurements of fundamental physical interest in these
systems.Comment: 40 pages, 17 figures, 4 tables - submitted to the Annual Review of
Astronomy and Astrophysics; to appear September 200
High-frequency variability in neutron-star low-mass X-ray binaries
Binary systems with a neutron-star primary accreting from a companion star
display variability in the X-ray band on time scales ranging from years to
milliseconds. With frequencies of up to ~1300 Hz, the kilohertz quasi-periodic
oscillations (kHz QPOs) represent the fastest variability observed from any
astronomical object. The sub-millisecond time scale of this variability implies
that the kHz QPOs are produced in the accretion flow very close to the surface
of the neutron star, providing a unique view of the dynamics of matter under
the influence of some of the strongest gravitational fields in the Universe.
This offers the possibility to probe some of the most extreme predictions of
General Relativity, such as dragging of inertial frames and periastron
precession at rates that are sixteen orders of magnitude faster than those
observed in the solar system and, ultimately, the existence of a minimum
distance at which a stable orbit around a compact object is possible. Here we
review the last twenty years of research on kHz QPOs, and we discuss the
prospects for future developments in this field.Comment: 66 pages, 37 figures, 190 references. Review to appear in T. Belloni,
M. Mendez, C. Zhang, editors, "Timing Neutron Stars: Pulsations, Oscillations
and Explosions", ASSL, Springe