590 research outputs found
Alignment Timescale of the Microquasar GRO J1655-40
The microquasar GRO J1655-40 has a black hole with spin angular momentum
apparently misaligned to the orbital plane of its companion star. We
analytically model the system with a steady state disc warped by Lense-Thirring
precession and find the timescale for the alignment of the black hole with the
binary orbit. We make detailed stellar evolution models so as to estimate the
accretion rate and the lifetime of the system in this state. The secondary can
be evolving at the end of the main sequence or across the Hertzsprung gap. The
mass-transfer rate is typically fifty times higher in the latter case but we
find that, in both cases, the lifetime of the mass transfer state is at most a
few times the alignment timescale. The fact that the black hole has not yet
aligned with the orbital plane is therefore consistent with either model. We
conclude that the system may or may not have been counter-aligned after its
supernova kick but that it is most likely to be close to alignment rather than
counteralignment now.Comment: Accepted for publication in MNRA
Aspherical supernova explosions and formation of compact black hole low-mass X-ray binaries
It has been suggested that black-hole low-mass X-ray binaries (BHLMXBs) with
short orbital periods may have evolved from BH binaries with an
intermediate-mass secondary, but the donor star seems to always have higher
effective temperatures than measured in BHLMXBs (Justham, Rappaport &
Podsiadlowski 2006). Here we suggest that the secondary star is originally an
intermediate-mass (\sim 2-5 M_{\sun}) star, which loses a large fraction of
its mass due to the ejecta impact during the aspherical SN explosion that
produced the BH. The resulted secondary star could be of low-mass (\la 1
M_{\sun}). Magnetic braking would shrink the binary orbit, drive mass transfer
between the donor and the BH, producing a compact BHLMXB.Comment: 4 pages, accepted for publication in MNRAS Letter
The Past and Future History of Regulus
We show how the recent discovery of a likely close white dwarf companion to
the well known star Regulus, one of the brightest stars in the sky, leads to
considerable insight into the prior evolutionary history of this star,
including the cause of its current rapid rotation. We infer a relatively narrow
range for the initial masses of the progenitor system: M_{10} = 2.3 +/- 0.2
M_sun and M_{20} = 1.7 +/- 0.2 M_sun, where M_{10} and M_{20} are the initial
masses of the progenitors of the white dwarf and Regulus, respectively. In this
scenario, the age of the Regulus system would exceed 1 Gyr. We also show that
Regulus, with a current orbital period of 40 days, has an interesting future
ahead of it. This includes (i) a common envelope phase, and, quite possibly,
(ii) an sdB phase, followed by (iii) an AM CVn phase with orbital periods < 1
hr. Binary evolution calculations are presented in support of this scenario. We
also discuss alternative possibilities, emphasizing the present uncertainties
in binary evolution theory. Thus, this one particular star system illustrates
many different aspects of binary stellar evolution.Comment: PDFLaTeX, 9 pages with 8 figure
Theory of Nonlinear Dispersive Waves and Selection of the Ground State
A theory of time dependent nonlinear dispersive equations of the Schroedinger
/ Gross-Pitaevskii and Hartree type is developed. The short, intermediate and
large time behavior is found, by deriving nonlinear Master equations (NLME),
governing the evolution of the mode powers, and by a novel multi-time scale
analysis of these equations. The scattering theory is developed and coherent
resonance phenomena and associated lifetimes are derived. Applications include
BEC large time dynamics and nonlinear optical systems. The theory reveals a
nonlinear transition phenomenon, ``selection of the ground state'', and NLME
predicts the decay of excited state, with half its energy transferred to the
ground state and half to radiation modes. Our results predict the recent
experimental observations of Mandelik et. al. in nonlinear optical waveguides
Gap solitons in a model of a hollow optical fiber
We introduce a models for two coupled waves propagating in a hollow-core
fiber: a linear dispersionless core mode, and a dispersive nonlinear
quasi-surface one. The linear coupling between them may open a bandgap, through
the mechanism of the avoidance of crossing between dispersion curves. The
third-order dispersion of the quasi-surface mode is necessary for the existence
of the gap. Numerical investigation reveals that the entire bandgap is filled
with solitons, and they all are stable in direct simulations. The gap-soliton
(GS) family is extended to include pulses moving relative to the given
reference frame, up to limit values of the corresponding boost ,
beyond which the solitons do not exists. The limit values are nonsymmetric for
and . The extended gap is also entirely filled with the
GSs, all of which are stable in simulations. Recently observed solitons in
hollow-core photonic-crystal fibers may belong to this GS family.Comment: 5 pages, 5 figure
WD + MS systems as the progenitor of SNe Ia
We show the initial and final parameter space for SNe Ia in a () plane and find that the positions of some famous
recurrent novae, as well as a supersoft X-ray source (SSS), RX J0513.9-6951,
are well explained by our model. The model can also explain the space velocity
and mass of Tycho G, which is now suggested to be the companion star of Tycho's
supernova. Our study indicates that the SSS, V Sge, might be the potential
progenitor of supernovae like SN 2002ic if the delayed dynamical-instability
model due to Han & Podsiadlowski (2006) is appropriate. Following the work of
Meng, Chen & Han (2009), we found that the SD model (WD + MS) with an optically
thick wind can explain the birth rate of supernovae like SN 2006X and reproduce
the distribution of the color excess of SNe Ia. The model also predicts that at
least 75% of all SNe Ia may show a polarization signal in their spectra.Comment: 6 pages, 2 figures, accepted for publication in Astrophysics & Space
Science (Proceeding of the 4th Meeting on Hot Subdwarf Stars and Related
Objects, edited by Zhanwen Han, Simon Jeffery & Philipp Podsiadlowski
An Observational Diagnostic for Ultraluminous X-Ray Sources
We consider observational tests for the nature of Ultraluminous X-ray sources
(ULXs). These must distinguish between thermal-timescale mass transfer on to
stellar-mass black holes leading to anisotropic X-ray emission, and accretion
on to intermediate-mass black holes. We suggest that long-term transient
behavior via the thermal-viscous disk instability could discriminate between
these two possibilities for ULXs in regions of young stellar populations.
Thermal-timescale mass transfer generally produces stable disks and persistent
X-ray emission. In contrast, mass transfer from massive stars to black holes
produces unstable disks and thus transient behavior, provided that the black
hole mass exceeds some minimum value. This minimum mass depends primarily on
the donor mass and evolutionary state. We show that it exceeds 50 solar masses
for a large fraction (greater than 90%) of the mass-transfer lifetime for the
most likely donors in young clusters. Thus if long-term monitoring reveals a
large transient fraction among ULXs in a young stellar population, these
systems would be good candidates for intermediate-mass black holes in a
statistical sense; information about the donor star is needed to make this
identification secure in any individual case. A transient ULX population would
imply a much larger population of quiescent systems of the same type.Comment: 4 pages, 2 figure, ApJ Letters, in press (correct figure 2 included
in this version
Cuspons, peakons and regular gap solitons between three dispersion curves
A general wave model with the cubic nonlinearity is introduced to describe a
situation when the linear dispersion relation has three branches, which would
intersect in the absence of linear couplings between the three waves. Actually,
the system contains two waves with a strong linear coupling between them, to
which a third wave is then coupled. This model has two gaps in its linear
spectrum. Realizations of this model can be made in terms of temporal or
spatial evolution of optical fields in, respectively, a planar waveguide or a
bulk-layered medium resembling a photonic-crystal fiber. Another physical
system described by the same model is a set of three internal wave modes in a
density-stratified fluid. A nonlinear analysis is performed for solitons which
have zero velocity in the reference frame in which the group velocity of the
third wave vanishes. Disregarding the self-phase modulation (SPM) term in the
equation for the third wave, we find two coexisting families of solitons:
regular ones, which may be regarded as a smooth deformation of the usual gap
solitons in a two-wave system, and cuspons with a singularity in the first
derivative at their center. Even in the limit when the linear coupling of the
third wave to the first two vanishes, the soliton family remains drastically
different from that in the linearly uncoupled system; in this limit, regular
solitons whose amplitude exceeds a certain critical value are replaced by
peakons. While the regular solitons, cuspons, and peakons are found in an exact
analytical form, their stability is tested numerically, which shows that they
all may be stable. If the SPM terms are retained, we find that there again
coexist two different families of generic stable soliton solutions, namely,
regular ones and peakons.Comment: a latex file with the text and 10 pdf files with figures. Physical
Review E, in pres
The Cepheid Distance to NGC 1637: A Direct Test of the EPM Distance to SN 1999em
Type II-plateau supernovae (SNe II-P) are the classic variety of
core-collapse events that result from isolated, massive stars with thick
hydrogen envelopes intact at the time of explosion. Their distances are now
routinely estimated through two techniques: the expanding photosphere method
(EPM), a primary distance-determining method, and the recently developed
standard-candle method (SCM), a promising secondary technique. Using Cycle 10
HST observations, we identify 41 Cepheid variable stars in NGC 1637, the host
galaxy of the most thoroughly studied SN II-P to date, SN 1999em. Remarkably,
the Cepheid distance that we derive to NGC 1637, D = 11.7 +/- 1.0 Mpc, is
nearly 50% larger than earlier EPM distance estimates to SN 1999em. This is the
first direct comparison between these two primary distance determining methods
for a galaxy hosting a well-observed, spectroscopically and photometrically
normal, SN II-P. Extensive consistency checks show strong evidence to support
the Cepheid distance scale, so we are led to believe that either SN 1999em is
in some heretofore unsuspected way an unusual SN II-P, or that the SN II-P
distance scale must be revised. Assuming the latter, this one calibration
yields H_0(EPM) = 57 +/- 15 km/s/Mpc and H_0(SCM) = 59 +/- 11 km/s/Mpc;
additional calibrating galaxies are clearly desirable in order to test the
robustness of both determinations of H_0. The HST observations of NGC 1637 also
captured the fading SN 1999em two years after explosion, providing the latest
photometry ever obtained for an SN II-P. Through comparison with photometry of
SN 1987A at similar epochs, we conclude that a slightly greater amount of
radioactive Ni-56, ~0.09 M_sun, was ejected by SN 1999em than was derived for
SN 1987A (0.075 M_sun).Comment: Accepted for publication in The Astrophysical Journal; Version with
full figures available at http://astron.berkeley.edu/~leonard/papers
Autoantibodies against C1q as a diagnostic measure of lupus nephritis:systematic review and meta-analysis
This is a freely-available open access publication. Please cite the published version which is available via the DOI link in this record.Objectives: To evaluate the diagnostic accuracy of C1q autoantibodies in identifying lupus nephritis (LN) in patients with systemic lupus erythematosus (SLE). Data Sources and methods: Citation indexes were searched and 370 articles published from 1977 to 2013 were evaluated. The 31 selected studies included in the meta-analysis were cross-sectional in design. Among the 31 studies, 28 compared anti-C1q antibodies in 2769 SLE patients including those with (n = 1442) and without a history of LN (n = 1327). Nine studies examined anti-C1q in 517 SLE patients with active (n = 249) and inactive LN (n = 268). Hierarchical summary receiver operating characteristic (HSROC) random effects models were fitted to pool estimates of accuracy across the studies. Results: Anti-C1q antibodies discriminated between patients with and without a history of LN, with a median specificity of 73.5%. The HSROC model estimated the corresponding sensitivity to be 70.4%. A hypothetical patient with a 55% prior probability of having a history of LN as opposed to no history (the median prevalence across 28 eligible studies) would have a post-test probability of 76.4% following a positive test result (positive predictive value) or 33.0% following a negative test result (negative predictive value). For differentiating active from inactive LN the median specificity of anti-C1q antibodies was 80%, with a corresponding estimated sensitivity value 75.7% based on the HSROC model. A hypothetical patient with a 56% prior probability of active as opposed to inactive LN (the median prevalence across the 9 eligible studies) would have a post-test probability of 82.8% following a positive test result or 27.9% following a negative test result. Conclusions: Although C1q antibodies are associated with lupus nephritis the post-test probabilities are not sufficiently convincing to provide reasonable certainty of the presence or absence of history of disease/active disease.Arthritis Research UKPeninsula Collaboration for Leadership in Applied
Health Research and Care (CLAHRC)National Health Service
South West, funded by the National Institute for Health Research, U
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