1,007 research outputs found
Appearance of innermost stable circular orbits of accretion discs around rotating neutron stars
The innermost stable cicular orbit (ISCO) of an accretion disc orbiting a
neutron star (NS) is often assumed a unique prediction of general relativity.
However, it has been argued that ISCO also appears around highly elliptic
bodies described by Newtonian theory. In this sense, the behaviour of an ISCO
around a rotating oblate neutron star is formed by the interplay between
relativistic and Newtonian effects. Here we briefly explore the consequences of
this interplay using a straightforward analytic approach as well as numerical
models that involve modern NS equations of state. We examine the ratio K
between the ISCO radius and the radius of the neutron star. We find that, with
growing NS spin, the ratio K first decreases, but then starts to increase. This
non-monotonic behaviour of K can give rise to a neutron star spin interval in
which ISCO appears for two very different ranges of NS mass. This may strongly
affect the distribution of neutron stars that have an ISCO (ISCO-NS). When
(all) neutron stars are distributed around a high mass M0, the ISCO-NS spin
distribution is roughly the same as the spin distribution corresponding to all
neutron stars. In contrast, if M0 is low, the ISCO-NS distribution can only
have a peak around a high value of spin. Finally, an intermediate value of M0
can imply an ISCO-NS distribution divided into two distinct groups of slow and
fast rotators. Our findings have immediate astrophysical applications. They can
be used for example to distinguish between different models of high-frequency
quasiperiodic oscillations observed in low-mass NS X-ray binaries.Comment: 4 pages, 2 figures, accepted by A&A Letter
Static quark-antiquark pair free energy and screening masses: continuum results at the QCD physical point
We study the correlators of Polyakov loops, and the corresponding gauge
invariant free energy of a static quark-antiquark pair in 2+1 flavor QCD at
finite temperature. Our simulations were carried out on = 6, 8, 10, 12,
16 lattices using a Symanzik improved gauge action and a stout improved
staggered action with physical quark masses. The free energies calculated from
the Polyakov loop correlators are extrapolated to the continuum limit. For the
free energies we use a two step renormalization procedure that only uses data
at finite temperature. We also measure correlators with definite Euclidean time
reversal and charge conjugation symmetry to extract two different screening
masses, one in the magnetic, and one in the electric sector, to distinguish two
different correlation lengths in the full Polyakov loop correlator. This
conference contribution is based on the paper: JHEP 1504 (2015) 138Comment: 7 pages, 4 figures. Talk presented at the 33rd International
Symposium on Lattice Field Theory (Lattice 2015), 14-18 July 2015, Kobe
International Conference Center, Kobe, Japa
Black hole spin inferred from 3:2 epicyclic resonance model of high-frequency quasi-periodic oscillations
Estimations of black hole spin in the three Galactic microquasars GRS
1915+105, GRO J1655-40, and XTE J1550-564 have been carried out based on
spectral and timing X-ray measurements and various theoretical concepts. Among
others, a non-linear resonance between axisymmetric epicyclic oscillation modes
of an accretion disc around a Kerr black hole has been considered as a model
for the observed high-frequency quasi-periodic oscillations (HF QPOs).
Estimates of spin predicted by this model have been derived based on the
geodesic approximation of the accreted fluid motion. Here we assume accretion
flow described by the model of a pressure-supported torus and carry out related
corrections to the mass-spin estimates. We find that for dimensionless black
hole spin a<0.9, the resonant eigenfrequencies are very close to those
calculated for the geodesic motion. Their values slightly grow with increasing
torus thickness. These findings agree well with results of a previous study
carried out in the pseudo-Newtonian approximation. The situation becomes
different for a>0.9, in which case the resonant eigenfrequencies rapidly
decrease as the torus thickness increases. We conclude that the assumed
non-geodesic effects shift the lower limit of the spin, implied for the three
microquasars by the epicyclic model and independently measured masses, from
a~0.7 to a~0.6. Their consideration furthermore confirms compatibility of the
model with the rapid spin of GRS 1915+105 and provides highly testable
predictions of the QPO frequencies. Individual sources with a moderate spin
(a<0.9) should exhibit a smaller spread of the measured 3:2 QPO frequencies
than sources with a near-extreme spin (a~1). This should be further examined
using the large amount of high-resolution data expected to become available
with the next generation of X-ray instruments, such as the proposed Large
Observatory for X-ray Timing (LOFT).Comment: 6 pages, 4 figures, accepted by Astronomy & Astrophysic
Response to combination therapy with interferon alfa-2a and ribavirin in chronic hepatitis C according to a TNF-alpha promoter polymorphism
Background. Tumor necrosis factor-alpha (TNF-alpha) is involved in the pathogenesis of chronic active hepatitis C. Polymorphisms in the promoter region of the TNF-alpha gene can alter the TNF-alpha expression and modify the host immune response. The present study aimed at the correlation of the G308A TNF-alpha polymorphism with the response to antiviral combination therapy in chronic hepatitis C. Patients and Methods: 62 patients with HCV and 119 healthy unrelated controls were genotyped for the G308A TNF-alpha promoter polymorphism. The patients received 3 x 3 million units of interferon alfa-2a and 1,0001,200 mg ribavirin daily according to their body weight. A response was defined as absence of HCV-RNA and normalization of S-ALT after 6 months of combination therapy. Results:With respect to the allele and genotype frequency, a significant difference was not observed between controls and patients with chronic hepatitis C. Furthermore, such a difference was also not observed if responders and non-responders to antiviral therapy were compared. Conclusions: The promoter polymorphism of the TNF-alpha gene investigated herein is equally distributed in healthy individuals and patients with hepatitis C and does not seem to predict the response to therapy with interferon alfa-2a and ribavirin. Copyright (C) 2003 S. Karger AG, Basel
High Statistics Analysis using Anisotropic Clover Lattices: (IV) Volume Dependence of Light Hadron Masses
The volume dependence of the octet baryon masses and relations among them are
explored with Lattice QCD. Calculations are performed with n_f=2+1 clover
fermion discretization in four lattice volumes, with spatial extent L ~ 2.0,
2.5, 3.0 and 3.9 fm, with an anisotropic lattice spacing of b_s ~ 0.123 fm in
the spatial direction, and b_t = b_s/3.5 in the time direction, and at a pion
mass of m_pi ~ 390 MeV. The typical precision of the ground-state baryon mass
determination is ~0.2%, enabling a precise exploration of the volume dependence
of the masses, the Gell-Mann--Okubo mass relation, and of other mass
combinations. A comparison of the volume dependence with the predictions of
heavy baryon chiral perturbation theory is performed in both the SU(2)_L X
SU(2)_R and SU(3)_L X SU(3)_R expansions. Predictions of the three-flavor
expansion for the hadron masses are found to describe the observed volume
dependences reasonably well. Further, the Delta-N-pi axial coupling constant is
extracted from the volume dependence of the nucleon mass in the two-flavor
expansion, with only small modifications in the three-flavor expansion from the
inclusion of kaons and etas. At a given value of m_pi L, the finite-volume
contributions to the nucleon mass are predicted to be significantly smaller at
m_pi ~ 140 MeV than at m_pi ~ 390 MeV due to a coefficient that scales as ~
m_pi^3. This is relevant for the design of future ensembles of lattice
gauge-field configurations. Finally, the volume dependence of the pion and kaon
masses are analyzed with two-flavor and three-flavor chiral perturbation
theory.Comment: 34 pages, 45 figure
Present Constraints on the H-dibaryon at the Physical Point from Lattice QCD
The current constraints from lattice QCD on the existence of the H-dibaryon
are discussed. With only two significant lattice QCD calculations of the
H-dibaryon binding energy at approximately the same lattice spacing, the forms
of the chiral and continuum extrapolations to the physical point are not
determined. In this brief report, we consider the constraints on the H-dibaryon
imposed by two simple chiral extrapolations. In both instances, the
extrapolation to the physical pion mass allows for a bound H-dibaryon or a
near-threshold scattering state. Further lattice QCD calculations are required
to clarify this situation.Comment: 8 pages, 2 figures, 1 table; revised for the journa
A model for magnetically coupled sympathetic eruptions
Sympathetic eruptions on the Sun have been observed for several decades, but
the mechanisms by which one eruption can trigger another one remain poorly
understood. We present a 3D MHD simulation that suggests two possible magnetic
trigger mechanisms for sympathetic eruptions. We consider a configuration that
contains two coronal flux ropes located within a pseudo-streamer and one rope
located next to it. A sequence of eruptions is initiated by triggering the
eruption of the flux rope next to the streamer. The expansion of the rope leads
to two consecutive reconnection events, each of which triggers the eruption of
a flux rope by removing a sufficient amount of overlying flux. The simulation
qualitatively reproduces important aspects of the global sympathetic event on
2010 August 1 and provides a scenario for so-called twin filament eruptions.
The suggested mechanisms are applicable also for sympathetic eruptions
occurring in other magnetic configurations.Comment: 6 pages, 4 figures, accepted in ApJ Letter
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