84 research outputs found
R-Modes on Rapidly Rotating, Relativistic Stars: I. Do Type-I Bursts Excite Modes in the Neutron-Star Ocean?
During a Type-I burst, the turbulent deflagation front may excite waves in
the neutron star ocean and upper atmosphere with frequencies,
Hz. These waves may be observed as highly coherent flux oscillations during the
burst. The frequencies of these waves changes as the upper layers of the
neutron star cool which accounts for the small variation in the observed QPO
frequencies. In principle several modes could be excited but the fundamental
buoyant mode exhibits significantly larger variability for a given
excitation than all of the other modes. An analysis of modes in the burning
layers themselves and the underlying ocean shows that it is unlikely these
modes can account for the observed burst oscillations. On the other hand,
photospheric modes which reside in a cooler portion of the neutron star
atmosphere may provide an excellent explanation for the observed oscillations.Comment: 18 pages, 1 figure, substantial changes and additions to reflect
version to appear in Ap
Neutron specific heat in the crust of neutron stars from the nuclear band theory
The inner crust of neutron stars, formed of a crystal lattice of uclear
clusters immersed in a sea of unbound neutrons, may be the nique example of
periodic nuclear systems. We have calculated the neutron specific heat in the
shallow part of the crust using the band theory of solids with Skyrme
nucleon-nucleon interactions. We have also tested the validity of various
approximations. We have found that the neutron specific heat is well described
by that of a Fermi gas, while the motion of the unbound neutrons is strongly
affected by the nuclear lattice. These apparently contradictory results are
explained by the particular properties of the neutron Fermi surface
NGC300 X-1 and IC10 X-1: a new breed of black hole binary?
[ABRIDGED] IC10 X-1 has recently been confirmed as a black hole (BH) +
Wolf-Rayet (WR) X-ray binary, and NGC300 X-1 is thought to be. IC10 X-1 and
NGC300 X-1 have similar X-ray properties, with luminosities ~10^38 erg/s, and
orbital periods ~30 hr. We investigate similarities between these two, as well
as differences between them and the known Galactic BH binary systems. We have
examined XMM-Newton observations of NGC300 X-1 and IC10 X-1. We extracted
lightcurves and spectra; power density spectra (PDS) were constructed from the
lightcurves, and the X-ray emission spectra were modeled. Each source exhibits
PDS that are characteristic of disc-accreting X-ray binaries (XBs) in the high
state. In this state, Galactic XBs with known BH primaries have soft, thermal
emission; however the emission spectra of our targets are predominantly
non-thermal. Furthermore, the Observation 1 spectrum of NGC300 X-1 is
strikingly similar to that of IC10 X-1. The remarkable similarity between the
behaviour of NGC300 X-1 in Observation 1 and that of IC10 X-1 lends strong
evidence for NGC300 X-1 being a (BH+WR) binary. The unusual spectra of NGC300
X-1 and IC10 X-1 may be due to these systems existing in a persistently high
state, whereas all known BH LMXBs are transient. BH XBs in a persistent high
state could retain their corona, and hence exhibit a large non-thermal
component. LMC X-1 is a BH XB that has only been observed in the high state,
and its spectrum is remarkably similar to those of our targets. We therefore
classify NGC300 X-1, IC10 X-1 and perhaps LMC X-1 as a new breed of BH XB,
defined by their persistently high accretion rates and consequent stable disc
configuration and corona. This scenario may also explain the lack of
ultraluminous X-ray sources in the canonical soft state.Comment: Approved for publication in A&A. 8 pages, 5 figure
Oscillations of tori in the pseudo-Newtonian potential
Context. The high-frequency quasi-periodic oscillations (HF QPOs) in neutron
star and stellar-mass black hole X-ray binaries may be the result of a
resonance between the radial and vertical epicyclic oscillations in strong
gravity. Aims. In this paper we investigate the resonant coupling between the
epicyclic modes in a torus in a strong gravitational field. Methods. We perform
numerical simulations of axisymmetric constant angular momentum tori in the
pseudo-Newtonian potential. The epicyclic motion is excited by adding a
constant radial velocity to the torus. Results. We verify that slender tori
perform epicyclic motions at the frequencies of free particles, but the
epicyclic frequencies decrease as the tori grow thicker. More importantly, and
in contrast to previous numerical studies, we do not find a coupling between
the radial and vertical epicyclic motions. The appearance of other modes than
the radial epicyclic motion in our simulations is rather due to small numerical
deviations from exact equilibrium in the initial state of our torus.
Conclusions. We find that there is no pressure coupling between the two
axisymmetric epicyclic modes as long as the torus is symmetric with respect to
the equatorial plane. However we also find that there are other modes in the
disc that may be more attractive for explaining the HF QPOs.Comment: 8 pages, 9 figure
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
Recent X-ray measurements of the accretion-powered pulsar 4U 1907+09
X-ray observations of the accreting X-ray pulsar 4U~1907+09, obtained during
February 1996 with the Proportional Counter Array on the Rossi X-ray Timing
Experiment (RXTE), have enabled the first measurement of the intrinsic pulse
period Ppulse since 1984: Ppulse=440.341[+0.012,-0.017] s. 4U 1907+09 is in a
binary system with a blue supergiant. The orbital parameters were solved and
this enabled the correction for orbital delay effects of a measurement of
Ppulse obtained in 1990 with Ginga. Thus, three spin down rates could be
extracted from four pulse periods obtained in 1983, 1984, 1990, and 1996. These
are within 8% equal to a value of dPpulse/dt=+0.225 s/yr. This suggest that the
pulsar is perhaps in a monotonous spin down mode since its discovery in 1983.
Furthermore, the RXTE observations show transient ~18 s oscillations during a
flare that lasted about 1 hour. The oscillations may be interpreted as
Keplerian motion of an accretion disk near the magnetospheric radius. This, and
the notion that the co-rotation radius is much larger than any conceivable
value for the magnetospheric radius (because of the long spin period), renders
it unlikely that this pulsar spins near equilibrium like is suspected for other
slowing accreting X-ray pulsars. We suggest as an alternative that perhaps the
frequent occurrence of a retrograde transient accretion disk may be
consistently slowing the pulsar down. Further observations of flares can
provide more evidence of this.Comment: 26 pages, 11 figures, to be published in Astrophysical Journal part I
on March 20, 199
Neutron degeneracy and plasma physics effects on radiative neutron captures in neutron star crust
We consider the astrophysical reaction rates for radiative neutron capture
reactions () in the crust of a neutron star. The presence of
degenerate neutrons at high densities (mainly in the inner crust) can
drastically affect the reaction rates. Standard rates assuming a
Maxwell-Boltzmann distribution for neutrons can underestimate the rates by
several orders of magnitude. We derive simple analytical expressions for
reaction rates at a variety of conditions with account for neutron degeneracy.
We also discuss the plasma effects on the outgoing radiative transition channel
in neutron radiative capture reactions and show that these effects can also
increase the reaction rates by a few orders of magnitude. In addition, using
detailed balance, we analyze the effects of neutron degeneracy and plasma
physics on reverse () photodisintegration. We discuss the dependence
of the reaction rates on temperature and neutron chemical potential and outline
the efficiency of these reactions in the neutron star crust.Comment: 11 pages, 6 figure
Nonradial Oscillations of Neutron Stars with a Solid Crust -- Analysis in the Relativistic Cowling Approximation--
Nonradial oscillations of relativistic neutron stars with a solid crust are
computed in the relativistic Cowling approximation, in which all metric
perturbations are ignored. For the modal analysis, we employ three-component
relativistic neutron star models with a solid crust, a fluid core, and a fluid
ocean. As a measure for the relativistic effects on the oscillation modes, we
calculate the relative frequency difference defined as
, where
and are, respectively, the relativistic and the
Newtonian oscillation frequencies. The relative difference
takes various values for different oscillation modes of
the neutron star model, and the value of for a given mode
depends on the physical properties of the models. We find that
is less than for most of the oscillation
modes we calculate, although there are a few exceptions such as the fundamental
(nodeless) toroidal torsional modes in the crust, the surface gravity modes
confined in the surface ocean, and the core gravity modes trapped in the fluid
core. We also find that the modal properties, represented by the
eigenfunctions, are not strongly affected by introducing general relativity. It
is however shown that the mode characters of the two interfacial modes,
associated with the core/crust and crust/ocean interfaces, have been
interchanged between the two through an avoided crossing when we move from
Newtonian dynamics to general relativistic dynamics.Comment: 14 pages, 9 figures, To appear in Astronomy and Astrophysic
Physics of Neutron Star Crusts
The physics of neutron star crusts is vast, involving many different research
fields, from nuclear and condensed matter physics to general relativity. This
review summarizes the progress, which has been achieved over the last few
years, in modeling neutron star crusts, both at the microscopic and macroscopic
levels. The confrontation of these theoretical models with observations is also
briefly discussed.Comment: 182 pages, published version available at
<http://www.livingreviews.org/lrr-2008-10
Accreting Millisecond X-Ray Pulsars
Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories
without parallel in the study of extreme physics. In this chapter we review the
past fifteen years of discoveries in the field. We summarize the observations
of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength
observations that have been carried out since the discovery of the first AMXP
in 1998. We review accretion torque theory, the pulse formation process, and
how AMXP observations have changed our view on the interaction of plasma and
magnetic fields in strong gravity. We also explain how the AMXPs have deepened
our understanding of the thermonuclear burst process, in particular the
phenomenon of burst oscillations. We conclude with a discussion of the open
problems that remain to be addressed in the future.Comment: Review to appear in "Timing neutron stars: pulsations, oscillations
and explosions", T. Belloni, M. Mendez, C.M. Zhang Eds., ASSL, Springer;
[revision with literature updated, several typos removed, 1 new AMXP added
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