5,978 research outputs found
Interpretation of 35 Hz QPO in the Atoll Source 4U 1702-42 as a Low Branch of the Keplerian Oscillations Under the Influence of the Coriolis Force
The recent model of quasi-periodic oscillations in neutron star binaries
(Osherovich and Titarchuk 1999, Titarchuk and Osherovich 1999) has suggested
the existence of two branches of QPOs due to the influence of Coriolis force on
the linear Keplerian oscillator: one branch with frequencies from 400 to 1200
Hz and another branch with frequencies an order of magnitude lower. The
frequencies of the high branch nu_h hold a hybrid frequency relation with the
Keplerian frequency nu_K: nu_h^2=nu_K^2+ [Omega/pi]^2, where Omega is the
rotational frequency of the star's magnetosphere. The frequency of the low
branch is nu_L=(Omega/pi)(nu_K/nu_h) sin(delta), where delta is a small angle
between vector Omega and the vector normal to the plane of Keplerian
oscillations. The observations of the source 4U 1702-42 (Markwardt et al 1999)
have shown that the centroid of the 35 Hz QPO tracks the frequency of the
kilohertz oscillations. We interpret the 35 Hz oscillations as nu_L and find
delta=3.9^o +/- 0.2^o. Our results make 4U 1702-42 the second source (after Sco
X-1) for which the theoretically derived lower branch is identified (within our
model) and delta is calculated. The inferred angle delta stays approximately
the same over the significant range of nu_K (650 - 900 Hz), as expected from
the model. Based on our model we present a classification of QPO frequencies in
the source 4U 1702-42 observed above and below nu_L.Comment: 12 pages, 2 figures, accepted for publication in ApJ Letter
A Template for Implementing Fast Lock-free Trees Using HTM
Algorithms that use hardware transactional memory (HTM) must provide a
software-only fallback path to guarantee progress. The design of the fallback
path can have a profound impact on performance. If the fallback path is allowed
to run concurrently with hardware transactions, then hardware transactions must
be instrumented, adding significant overhead. Otherwise, hardware transactions
must wait for any processes on the fallback path, causing concurrency
bottlenecks, or move to the fallback path. We introduce an approach that
combines the best of both worlds. The key idea is to use three execution paths:
an HTM fast path, an HTM middle path, and a software fallback path, such that
the middle path can run concurrently with each of the other two. The fast path
and fallback path do not run concurrently, so the fast path incurs no
instrumentation overhead. Furthermore, fast path transactions can move to the
middle path instead of waiting or moving to the software path. We demonstrate
our approach by producing an accelerated version of the tree update template of
Brown et al., which can be used to implement fast lock-free data structures
based on down-trees. We used the accelerated template to implement two
lock-free trees: a binary search tree (BST), and an (a,b)-tree (a
generalization of a B-tree). Experiments show that, with 72 concurrent
processes, our accelerated (a,b)-tree performs between 4.0x and 4.2x as many
operations per second as an implementation obtained using the original tree
update template
The Next-to-Leading BFKL Pomeron with Optimal Renormalization
The next-to-leading order (NLO) corrections to the BFKL equation in the BLM
optimal scale setting are briefly discussed. A striking feature of the BLM
approach is rather weak Q^2-dependence of the Pomeron intercept, which might
indicate an approximate conformal symmetry of the equation. An application of
the NLO BFKL resummation for the virtual gamma-gamma total cross section shows
a good agreement with recent L3 data at the CERN LEP2.Comment: 5 pages, 1 eps-figure, to appear in Proceedings of VIIIth Blois
Workshop at IHEP, Protvino, June 28 - July 2, 199
The Next-to-Leading Dynamics of the BFKL Pomeron
The next-to-leading order (NLO) corrections to the BFKL equation in the BLM
optimal scale setting are briefly discussed. A striking feature of the BLM
approach is rather weak Q^2-dependence of the Pomeron intercept, which might
indicate an approximate conformal symmetry of the equation. An application of
the NLO BFKL resummation for the virtual gamma-gamma total cross section shows
a good agreement with recent L3 data at CERN LEP2 energies.Comment: 7 pages, 2 eps-figures, presented at International Symposium on
Multiparticle Dynamics (ISMD99), August 9-13, 1999, Brown University,
Providence, Rhode Islan
Can Any "Invariants" Be Revealed in Quasi-periodic Phenomena Observed From Sco X-1?
Using large number of Rossi X-ray Time Explorer observations of Sco X-1 we
present a detailed investigation of the transition layer (TL) and the
relativistic precession (RP) models. These models predict the existence of the
invariant quantities: an inclination angle delta of the magnetospheric axis
with the normal to the disk for the TLM and a neutron star (NS) mass M_{NS} for
the RPM. Theoretical predictions of both models are tested and their
self-consistency is checked. We establish that: (1) The inferred delta angle is
5.56+/-0.09 degrees. Correlation of the delta-values with the horizontal branch
oscillation (HBO) frequency is rather weak. (2) There is a strong correlation
between an inferred M_{NS} and the HBO frequency in the RPM frameworks. (3) We
infer M_{NS} for different assumptions regarding the relations between the HBO
frequency and the nodal frequency. We find that the inferred M_{NS}=2.7+/-0.1
M_sun cannot be consistent with any EOS of NS matter. We conclude that RPM
fails to describe the data while TLM seems to be compatible.Comment: Accepted for publication in Astrophysical Journal Letters (2002
June/571 issue), 5 pages, 4 figures, uses emulateapj5.st
On the nature of the flux variability during an expansion stage of a type I X-ray burst: Constraints on Neutron Star Parameters for 4U 1820-30
Powerful Type I X-ray burst with strong radial expansion was observed from
the low mass X-ray binary 4U 1820-30 with Rossi X-ray Timing Explorer on May 2,
1997. We investigate closely the flux profile during the burst expansion
stage. Applying a semi-analytical model we are able to uncover the behavior of
a photospheric radius and to simulate the evolution of neutron star
(NS)-accretion disk system. The bottom flux L_{bot} is a few times the
Eddington limit L_{Edd} for outer layers, because the electron cross-section is
a few times less than the Thomson cross-section at such a high temperatures.
The surplus of energy flux with respect to the Eddington, ,
goes into the potential energy of the expanded envelope. As cooling of the
burning zone starts the surplus decreases and thus the envelope shrinks while
the emergent photon flux stays the same . At a certain moment the NS
low-hemisphere, previously screened by the disk, becomes visible to the
observer. Consequently, the flux detected by the observer increases. Indeed, we
observe to the paradoxical situation when the burning zone cools, but the
apparent flux increases because of the NS-accretion disk geometry. We
demonstrate a strong observational evidence of NS-accretion disk occultation in
the behavior of the observed bolometric flux. We estimate the anisotropy due to
geometry and find that the system should have a high inclination angle.
Finally, we apply an analytical model of X-ray spectral formation in the
neutron star atmosphere during burst decay stage to infer the neutron star (NS)
mass-radius relation.Comment: 15 pages, 3 figures, accepted to ApJ
Spatial coherence of forward-scattered light in a turbid medium
We study spatially coherent forward-scattered light propagating in a turbid medium of moderate optical depth (0-9 mean free paths). Coherent detection was achieved by using a tilted heterodyne geometry, which desensitizes coherent detection of the attenuated incident light. We show that the degree of spatial coherence is significantly higher for light scattered only once in comparison with that for multiply scattered light and that it approaches a small constant value for large numbers of scattering events
Quantum Mechanical Realization of a Popescu-Rohrlich Box
We consider quantum ensembles which are determined by pre- and
post-selection. Unlike the case of only pre-selected ensembles, we show that in
this case the probabilities for measurement outcomes at intermediate times
satisfy causality only rarely; such ensembles can in general be used to signal
between causally disconnected regions. We show that under restrictive
conditions, there are certain non-trivial bi-partite ensembles which do satisfy
causality. These ensembles give rise to a violation of the CHSH inequality,
which exceeds the maximal quantum violation given by Tsirelson's bound, , and obtains the Popescu-Rohrlich bound for the maximal
violation, . This may be regarded as an a posteriori
realization of super-correlations, which have recently been termed
Popescu-Rohrlich boxes.Comment: 5 page
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