31,350 research outputs found
On the Disappearance of Kilohertz Quasi-Periodic Oscillations at a High Mass Accretion Rate in Low-Mass X-ray Binaries
For all sources in which the phenomenon of kilo-Hertz quasi-periodic
oscillation (kHz QPO) is observed, the QPOs disappear abruptly when the
inferred mass accretion rate exceeds a certain threshold. Although the
threshold cannot at present be accurately determined (or even quantified)
observationally, it is clearly higher for bright Z sources than for faint atoll
sources. Here we propose that the observational manifestation of kHz QPOs {\em
requires} direct interaction between the neutron star magnetosphere and the
Keplerian accretion disk and that the cessation of kHz QPOs at high accretion
rate is due to the lack of such an interact when the Keplerian disk terminates
at the last stable orbit and yet the magnetosphere is pushed farther inward.
The threshold is therefore dependent of the magnetic field strength -- the
stronger the magnetic field the higher the threshold. This is certainly in
agreement with the atoll/Z paradigm, but we argue that it is also generally
true, even for individual sources within each (atoll or Z) category. For atoll
sources, the kHz QPOs also seem to vanish at low accretion rate. Perhaps the
``disengagement'' between the magnetosphere and the Keplerian disk also takes
place under such circumstances, because of, for instance, the presence of
quasi-spherical advection-dominated accretion flow (ADAF) close to the neutron
star. Unfortunately, in this case, the estimation of the accretion rate
threshold would require a knowledge of the physical mechanisms that cause the
disengagement. If the ADAF is responsible, the threshold is likely dependent of
the magnetic field of the neutron star.Comment: Minor revisions to match the published versio
Neutron-Capture Elements in the Double-Enhanced Star HE 1305-0007: a New s- and r-Process Paradigm
The star HE 1305-0007 is a metal-poor double-enhanced star with metallicity
[Fe/H] , which is just at the upper limit of the metallicity for the
observed double-enhanced stars. Using a parametric model, we find that almost
all s-elements were made in a single neutron exposure. This star should be a
member of a post-common-envelope binary. After the s-process material has
experienced only one neutron exposure in the nucleosynthesis region and is
dredged-up to its envelope, the AGB evolution is terminated by the onset of
common-envelope evolution. Based on the high radial-velocity of HE 1305-0007,
we speculate that the star could be a runaway star from a binary system, in
which the AIC event has occurred and produced the r-process elements.Comment: 4 pages, 3 figures, paper accepted for publication in Chinese Physics
letter
Polarization anisotropy in the optical properties of silicon ellipsoids
A new real space quantum mechanical approach with local field effects
included is applied to the calculation of the optical properties of silicon
nanocrystals. Silicon ellipsoids are studied and the role of surface
polarization is discussed in details. In particular, surface polarization is
shown to be responsible for a strong optical anisotropy in silicon ellipsoids,
much more pronounced with respect to the case in which only quantum confinement
effects are considered. The static dielectric constant and the absorption
spectra are calculated, showing that the perpendicular and parallel components
have a very different dependence on the ellipsoid aspect ratio. Then, a
comparison with the classical dielectric model is performed, showing that the
model only works for large and regular structures, but it fails for thin
elongated ellipsoids.Comment: 5 pages, 4 figures, International Conference on NANO-Structures
Self-Assemblin
Evidence for "Propeller" Effects In X-ray Pulsars GX 1+4 And GROJ1744-28
We present observational evidence for "propeller" effects in two X-ray
pulsars, GX 1+4 and GROJ1744-28. Both sources were monitored regularly by the
Rossi X-ray Timing Explorer (RXTE) throughout a decaying period in the X-ray
brightness. Quite remarkably, strong X-ray pulsation became unmeasurable when
total X-ray flux had dropped below a certain threshold. Such a phenomenon is a
clear indication of the propeller effects which take place when pulsar
magnetosphere grows beyond the co-rotation radius as a result of the decrease
in mass accretion rate and centrifugal force prevents accreting matter from
reaching the magnetic poles. The entire process should simply reverse as the
accretion rate increases. Indeed, steady X-ray pulsation was reestablished as
the sources emerged from the non-pulsating faint state. These data allow us to
directly derive the surface polar magnetic field strength for both pulsars:
3.1E+13 G for GX 1+4 and 2.4E+11 G for GROJ1744-28. The results are likely to
be accurate to within a factor of 2, with the total uncertainty dominated by
the uncertainty in estimating the distances to the sources. Possible mechanisms
for the persistent emission observed in the faint state are discussed in light
of the extreme magnetic properties of the sources.Comment: 12 pages including 3 ps figures. To appear in ApJ Letters Vol. 48
Two-photon Rabi-Hubbard and Jaynes-Cummings-Hubbard models: photon pair superradiance, Mott insulator and normal phases
We study the ground state phase diagrams of two-photon Dicke, the
one-dimensional Jaynes-Cummings-Hubbard (JCH), and Rabi-Hubbard (RH) models
using mean field, perturbation, quantum Monte Carlo (QMC), and density matrix
renormalization group (DMRG) methods. We first compare mean field predictions
for the phase diagram of the Dicke model with exact QMC results and find
excellent agreement. The phase diagram of the JCH model is then shown to
exhibit a single Mott insulator lobe with two excitons per site, a superfluid
(SF, superradiant) phase and a large region of instability where the
Hamiltonian becomes unbounded. Unlike the one-photon model, there are no higher
Mott lobes. Also unlike the one-photon case, the SF phases above and below the
Mott are surprisingly different: Below the Mott, the SF is that of photon {\it
pairs} as opposed to above the Mott where it is SF of simple photons. The mean
field phase diagram of the RH model predicts a transition from a normal to a
superradiant phase but none is found with QMC.Comment: 14 pages, 14 figure
Rotation in galaxy clusters from MUSIC simulations with the kinetic Sunyaev-Zel'dovich effect
We propose in this work its application for the detection of possible coherent rotational motions in the hot intra-cluster medium. We select a sample of massive, relaxed and rotating galaxy clusters from Marenostrum-mUltidark SImulations of galaxy Clusters (MUSIC), and we produce mock maps of the temperature distortion produced by the kinetic Sunyaev-Zel'dovich effect by exploring six different lines of sight, in the best observational condition. These maps are compared with the expected signal computed from a suitable theoretical model in two cases: (i) focusing only on the contribution from the rotation, and (ii) accounting also for the cluster bulk motion. We find that the parameters of the model assumed for the radial profile of the rotational velocity, averaged over the considered lines of sight, are in agreement within two standard deviations at most with independent estimates from the simulation data, without being significantly affected by the presence of the cluster bulk term. The amplitude of the rotational signal is, on average, of the order of 23 per cent of the total signal accounting also for the cluster bulk motion, and its values are consistent with the literature. The projected bulk velocity of the cluster is also recovered at the different lines of sight, with values in agreement with the simulation dataASB acknowledges funding from Sapienza Università di Roma - Progetti per Avvio alla Ricerca Anno 2017, prot. AR11715C82402BC
Nonlinear dynamics of a cigar-shaped Bose-Einstein condensate coupled with a single cavity mode
We investigate the nonlinear dynamics of a combined system which is composed
of a cigar-shaped Bose-Einstein condensate and an optical cavity. The two sides
couple dispersively. This system is characterized by its nonlinearity: after
integrating out the freedom of the cavity mode, the potential felt by the
condensate depends on the condensate itself. We develop a discrete-mode
approximation for the condensate. Based on this approximation, we map out the
steady configurations of the system. It is found that due to the nonlinearity
of the system, the nonlinear levels of the system can fold up in some parameter
regimes. That will lead to the breakdown of adiabaticity. Analysis of the
dynamical stability of the steady states indicates that the same level
structure also results in optical bistability.Comment: 8 pages, 5 figure
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