2,716 research outputs found
Light Curves for Rapidly-Rotating Neutron Stars
We present raytracing computations for light emitted from the surface of a
rapidly-rotating neutron star in order to construct light curves for X-ray
pulsars and bursters. These calculations are for realistic models of
rapidly-rotating neutron stars which take into account both the correct
exterior metric and the oblate shape of the star. We find that the most
important effect arising from rotation comes from the oblate shape of the
rotating star. We find that approximating a rotating neutron star as a sphere
introduces serious errors in fitted values of the star's radius and mass if the
rotation rate is very large. However, in most cases acceptable fits to the
ratio M/R can be obtained with the spherical approximation.Comment: Accepted by the Astrophysical Journal. 13 pages & 7 figure
Analyzing X-Ray Pulsar Profiles: Geometry and Beam Pattern of Her X-1
We report on our analysis of a large sample of energy dependent pulse
profiles of the X-ray binary pulsar Hercules X-1. We find that all data are
compatible with the assumption of a slightly distorted magnetic dipole field as
sole cause of the asymmetry of the observed pulse profiles. Further the
analysis provides evidence that the emission from both poles is equal. We
determine an angle of 20 deg between the rotation axis and the local magnetic
axis. One pole has an offset of 5 deg from the antipodal position of the other
pole. The beam pattern shows structures that can be interpreted as pencil- and
fan-beam configurations. Since no assumptions on the polar emission are made,
the results can be compared with various emission models. A comparison of
results obtained from pulse profiles of different phases of the 35-day cycle
indicates different attenuation of the radiation from the poles being
responsible for the change of the pulse shape during the main-on state. These
results also suggest the resolution of an ambiguity within a previous analysis
of pulse profiles of Cen X-3, leading to a unique result for the beam pattern
of this pulsar as well. The analysis of pulse profiles of the short-on state
indicates that a large fraction of the radiation cannot be attributed to the
direct emission from the poles. We give a consistent explanation of both the
evolution of the pulse profile and the spectral changes with the 35-day cycle
in terms of a warped precessing accretion disk.Comment: 24 pages, 12 figures. To appear in ApJ 529 #2, 1 Feb 200
An X-Ray Jet from a White Dwarf - Detection of the Collimated Outflow from CH Cygni with Chandra
Most symbiotic stars consist of a white dwarf accreting material from the
wind of a red giant. An increasing number of these objects have been found to
produce jets. Analysis of archival Chandra data of the symbiotic system CH
Cygni reveals faint extended emission to the south, aligned with the optical
and radio jets seen in earlier HST and VLA observations. CH Cygni thus contains
only the second known white dwarf with an X-ray jet, after R Aquarii. The
X-rays from symbiotic-star jets appear to be produced when jet material is
shock-heated following collision with surrounding gas, as with the outflows
from some protostellar objects and bipolar planetary nebulae.Comment: 4 & a bit pages, 4 figures, accepted by ApJL; uses emulateapj.cls and
revtex4. Minor changes following referees report, & shortened to meet page
limi
Modelling the Galactic Magnetic Field on the Plane in 2D
We present a method for parametric modelling of the physical components of
the Galaxy's magnetised interstellar medium, simulating the observables, and
mapping out the likelihood space using a Markov Chain Monte-Carlo analysis. We
then demonstrate it using total and polarised synchrotron emission data as well
as rotation measures of extragalactic sources. With these three datasets, we
define and study three components of the magnetic field: the large-scale
coherent field, the small-scale isotropic random field, and the ordered field.
In this first paper, we use only data along the Galactic plane and test a
simple 2D logarithmic spiral model for the magnetic field that includes a
compression and a shearing of the random component giving rise to an ordered
component. We demonstrate with simulations that the method can indeed constrain
multiple parameters yielding measures of, for example, the ratios of the
magnetic field components. Though subject to uncertainties in thermal and
cosmic ray electron densities and depending on our particular model
parametrisation, our preliminary analysis shows that the coherent component is
a small fraction of the total magnetic field and that an ordered component
comparable in strength to the isotropic random component is required to explain
the polarisation fraction of synchrotron emission. We outline further work to
extend this type of analysis to study the magnetic spiral arm structure, the
details of the turbulence as well as the 3D structure of the magnetic field.Comment: 18 pages, 11 figures, updated to published MNRAS versio
Limits of control for quantum systems: kinematical bounds on the optimization of observables and the question of dynamical realizability
In this paper we investigate the limits of control for mixed-state quantum
systems. The constraint of unitary evolution for non-dissipative quantum
systems imposes kinematical bounds on the optimization of arbitrary
observables. We summarize our previous results on kinematical bounds and show
that these bounds are dynamically realizable for completely controllable
systems. Moreover, we establish improved bounds for certain partially
controllable systems. Finally, the question of dynamical realizability of the
bounds for arbitary partially controllable systems is shown to depend on the
accessible sets of the associated control system on the unitary group U(N) and
the results of a few control computations are discussed briefly.Comment: 5 pages, orginal June 30, 2000, revised September 28, 200
Orbits of quantum states and geometry of Bloch vectors for -level systems
Physical constraints such as positivity endow the set of quantum states with
a rich geometry if the system dimension is greater than two. To shed some light
on the complicated structure of the set of quantum states, we consider a
stratification with strata given by unitary orbit manifolds, which can be
identified with flag manifolds. The results are applied to study the geometry
of the coherence vector for n-level quantum systems. It is shown that the
unitary orbits can be naturally identified with spheres in R^{n^2-1} only for
n=2. In higher dimensions the coherence vector only defines a non-surjective
embedding into a closed ball. A detailed analysis of the three-level case is
presented. Finally, a refined stratification in terms of symplectic orbits is
considered.Comment: 15 pages LaTeX, 3 figures, reformatted, slightly modified version,
corrected eq.(3), to appear in J. Physics
A Chandra study of particle acceleration in the multiple hotspots of nearby radio galaxies
We present Chandra observations of a small sample of nearby classical double
radio galaxies which have more than one radio hotspot in at least one of their
lobes. The X-ray emission from the hotspots of these comparatively low-power
objects is expected to be synchrotron in origin, and therefore to provide
information about the locations of high-energy particle acceleration. In some
models of the relationship between the jet and hotspot the hotspots that are
not the current jet termination point should be detached from the energy supply
from the active nucleus and therefore not capable of accelerating particles to
high energies. We find that in fact some secondary hotspots are X-ray sources,
and thus probably locations for high-energy particle acceleration after the
initial jet termination shock. In detail, though, we show that the spatial
structures seen in X-ray are not consistent with naive expectations from a
simple shock model: the current locations of the acceleration of the
highest-energy observable particles in powerful radio galaxies need not be
coincident with the peaks of radio or even optical emission.Comment: Accepted for ApJ. 33 pages, 8 figures inc. 2 in colo
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