31 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
Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar
The relativistic plasma flows onto neutron stars that are accreting material
from stellar companions can be used to probe strong-field gravity as well as
the physical conditions in the supranuclear-density interiors of neutron stars.
Plasma inhomogeneities orbiting a few kilometres above the stars are observable
as X-ray brightness fluctuations on the millisecond dynamical timescale of the
flows. Two frequencies in the kilohertz range dominate these fluctuations: the
twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the
origins of these oscillations (based on orbital motions) all predict that they
should be related to the stellar spin frequency, but tests have been difficult
because the spins were not unambiguously known. Here we report the detection of
kHz QPOs from a pulsar whose spin frequency is known. Our measurements
establish a clear link between kHz QPOs and stellar spin, but one not predicted
by any current model. A new approach to understanding kHz QPOs is now required.
We suggest that a resonance between the spin and general relativistic orbital
and epicyclic frequencies could provide the observed relation between QPOs and
spin.Comment: Published in the 2003 July 3 issue of Natur
EXO 0748-676 Rules out Soft Equations of State for Neutron Star Matter
The interiors of neutron stars contain matter at very high densities, in a
state that differs greatly from those found in the early universe or achieved
at terrestrial experiments. Matter in these conditions can only be probed
through astrophysical observations that measure the mass and radius of neutron
stars with sufficient precision. Here I report for the first time a unique
determination of the mass and radius of the neutron star EXO 0748-676, which
appears to rule out all the soft equations of state of neutron star matter. If
this object is typical, then condensates and unconfined quarks do not exist in
the centers of neutron stars.Comment: To appear in Nature, press embargo until publicatio
Activated Magnetospheres of Magnetars
Like the solar corona, the external magnetic field of magnetars is twisted by
surface motions of the star. The twist energy is dissipated over time. We
discuss the theory of this activity and its observational status. (1) Theory
predicts that the magnetosphere tends to untwist in a peculiar way: a bundle of
electric currents (the "j-bundle") is formed with a sharp boundary, which
shrinks toward the magnetic dipole axis. Recent observations of shrinking hot
spots on magnetars are consistent with this behavior. (2) Continual discharge
fills the j-bundle with electron-positron plasma, maintaining a nonthermal
corona around the neutron star. The corona outside a few stellar radii strongly
interacts with the stellar radiation and forms a "radiatively locked" outflow
with a high e+- multiplicity. The locked plasma annihilates near the apexes of
the closed magnetic field lines. (3) New radiative-transfer simulations suggest
a simple mechanism that shapes the observed X-ray spectrum from 0.1 keV to 1
MeV: part of the thermal X-rays emitted by the neutron star are reflected from
the outer corona and then upscattered by the inner relativistic outflow in the
j-bundle, producing a beam of hard X-rays.Comment: 23 pages, 7 figures; review chapter in the proceedings of ICREA
Workshop on the High-Energy Emission from Pulsars and Their Systems, Sant
Cugat, Spain, April 201
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
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
Generalized Flows around Neutron Stars
In this chapter, we present a brief and non-exhaustive review of the
developments of theoretical models for accretion flows around neutron stars. A
somewhat chronological summary of crucial observations and modelling of timing
and spectral properties are given in sections 2 and 3. In section 4, we argue
why and how the Two-Component Advective Flow (TCAF) solution can be applied to
the cases of neutron stars when suitable modifications are made for the NSs. We
showcase some of our findings from Monte Carlo and Smoothed Particle
Hydrodynamic simulations which further strengthens the points raised in section
4. In summary, we remark on the possibility of future works using TCAF for both
weakly magnetic and magnetic Neutron Stars.Comment: 15 pages, 7 figures. arXiv admin note: text overlap with
arXiv:1901.0084
X-ray emission from isolated neutron stars
X-ray emission is a common feature of all varieties of isolated neutron stars
(INS) and, thanks to the advent of sensitive instruments with good
spectroscopic, timing, and imaging capabilities, X-ray observations have become
an essential tool in the study of these objects. Non-thermal X-rays from young,
energetic radio pulsars have been detected since the beginning of X-ray
astronomy, and the long-sought thermal emission from cooling neutron star's
surfaces can now be studied in detail in many pulsars spanning different ages,
magnetic fields, and, possibly, surface compositions. In addition, other
different manifestations of INS have been discovered with X-ray observations.
These new classes of high-energy sources, comprising the nearby X-ray Dim
Isolated Neutron Stars, the Central Compact Objects in supernova remnants, the
Anomalous X-ray Pulsars, and the Soft Gamma-ray Repeaters, now add up to
several tens of confirmed members, plus many candidates, and allow us to study
a variety of phenomena unobservable in "standard'' radio pulsars.Comment: Chapter to be published in the book of proceedings of the 1st Sant
Cugat Forum on Astrophysics, "ICREA Workshop on the high-energy emission from
pulsars and their systems", held in April, 201
Strongly magnetized pulsars: explosive events and evolution
Well before the radio discovery of pulsars offered the first observational
confirmation for their existence (Hewish et al., 1968), it had been suggested
that neutron stars might be endowed with very strong magnetic fields of
-G (Hoyle et al., 1964; Pacini, 1967). It is because of their
magnetic fields that these otherwise small ed inert, cooling dead stars emit
radio pulses and shine in various part of the electromagnetic spectrum. But the
presence of a strong magnetic field has more subtle and sometimes dramatic
consequences: In the last decades of observations indeed, evidence mounted that
it is likely the magnetic field that makes of an isolated neutron star what it
is among the different observational manifestations in which they come. The
contribution of the magnetic field to the energy budget of the neutron star can
be comparable or even exceed the available kinetic energy. The most magnetised
neutron stars in particular, the magnetars, exhibit an amazing assortment of
explosive events, underlining the importance of their magnetic field in their
lives. In this chapter we review the recent observational and theoretical
achievements, which not only confirmed the importance of the magnetic field in
the evolution of neutron stars, but also provide a promising unification scheme
for the different observational manifestations in which they appear. We focus
on the role of their magnetic field as an energy source behind their persistent
emission, but also its critical role in explosive events.Comment: Review commissioned for publication in the White Book of
"NewCompStar" European COST Action MP1304, 43 pages, 8 figure