47 research outputs found
Millisecond Time Variations of X-Ray Binaries
The Rossi X-Ray Timing Explorer (RXTE) has found that the neutron stars in
low-mass X-ray binaries exhibit oscillations in the range 300-1200 Hz.
Persistent emission may exhibit one or both of two features. In bursts a nearly
coherent pulsation is seen, which may be the rotation period of the neutron
star. For some the frequency equals the difference between the two higher
frequencies, suggesting a beat frequency model, but in others it is twice the
difference. Similar maximum frequencies suggests that it corresponds to the
Kepler orbit frequency at the minimum stable orbit or the neutron star surface,
either of which would determine the neutron star masses, radii and equation of
state. Theories of accretion onto black holes predict a quasi-periodic
oscillation (QPO) related to the inner accretion disk. The two microquasar
black hole candidates (BHCs) have exhibited candidates for this or related
frequencies.Comment: 4 pages, to be published in the proceedings of IAU Symposium 188: The
Hot Univers
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
Do strange stars exist in the Universe?
Definitely, an affirmative answer to this question would have implications of
fundamental importance for astrophysics (a new class of compact stars), and for
the physics of strong interactions (deconfined phase of quark matter, and
strange matter hypothesis). In the present work, we use observational data for
the newly discovered millisecond X-ray pulsar SAX J1808.4-3658 and for the
atoll source 4U 1728-34 to constrain the radius of the underlying compact
stars. Comparing the mass-radius relation of these two compact stars with
theoretical models for both neutron stars and strange stars, we argue that a
strange star model is more consistent with SAX J1808.4-3658 and 4U 1728-34, and
suggest that they are likely strange star candidates.Comment: In memory of Bhaskar Datta. -- Invited talk at the Pacific Rim
Conference on Stellar Astrophysics (Hong Kong, aug. 1999
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
Timing the Kilohertz Quasi-Periodic Oscillations in Low-mass X-ray Binaries
I describe a new technique that we have been using in the past few years to
get precise measurements of the frequency separation of the kHz QPOs in some
Low-mass X-ray binaries. I show how this technique (that we call
"shift-and-add") works, and I present some of the results we obtained using it.Comment: To appear in the Proceedings of the NATO ASI School "The Neutron Star
- Black Hole Connection" (Kluwer Academic Publishers), C. Kouveliotou, J. van
Paradijs, J. Ventura (eds.). (6 pages; 4 figures
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
Magnetar-like X-ray Bursts from an Anomalous X-ray Pulsar
Anomalous X-ray Pulsars (AXPs) are a class of rare X-ray pulsars whose energy
source has been perplexing for some 20 years. Unlike other, better understood
X-ray pulsars, AXPs cannot be powered by rotation or by accretion from a binary
companion, hence the designation ``anomalous.'' AXP rotational and radiative
properties are strikingly similar to those of another class of exotic objects,
the Soft Gamma Repeaters (SGRs). However, the defining property of SGRs, namely
their low-energy gamma-ray and X-ray bursts, have heretofore not been seen in
AXPs. SGRs are thought to be ``magnetars,'' young neutron stars powered by the
decay of an ultra-high magnetic field. The suggestion that AXPs are magnetars
has been controversial. Here we report the discovery, from the direction of AXP
1E 1048-5937, of two X-ray bursts that have many properties similar to those of
SGR bursts. These events imply a close relationship between AXPs and SGRs, with
both being magnetars.Comment: 14 pages, 2 figures, accepted for publication in Nature. Note: The
content of this paper is embargoed until 1900 hrs London time / 1400 US
Eastern Time on Sept 1
A Close Nuclear Black Hole Pair in the Spiral Galaxy NGC 3393
The current picture of galaxy evolution advocates co-evolution of galaxies
and their nuclear massive black holes (MBHs), through accretion and merging.
Quasar pairs (6,000-300,000 light-years separation) exemplify the first stages
of this gravitational interaction. The final stages, through binary MBHs and
final collapse with gravitational wave emission, are consistent with the
sub-light-year separation MBHs inferred from optical spectra and
light-variability of two quasars. The double active nuclei of few nearby
galaxies with disrupted morphology and intense star formation (e.g., NGC 6240
and Mkn 463; ~2,400 and ~12,000 light-years separation respectively)
demonstrate the importance of major mergers of equal mass spirals in this
evolution, leading to an elliptical galaxy, as in the case of the double radio
nucleus (~15 light-years separation) elliptical 0402+379. Minor mergers of
galaxies with a smaller companion should be a more common occurrence, evolving
into spiral galaxies with active MBH pairs, but have hitherto not been seen.
Here we report the presence of two active MBHs, separated by ~430 light-years,
in the Seyfert galaxy NGC 3393. The regular spiral morphology and predominantly
old circum-nuclear stellar population of this galaxy, and the closeness of the
MBHs embedded in the bulge, suggest the result of minor merger evolution.Comment: Preprint (not final) version of a paper to appear in 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
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