2,067 research outputs found

### GR models of the X-ray spectral variability of MCG--6-30-15

We study in detail the GR models of the X-ray spectral variability for
various geometries of the X-ray source and with various relativistic effects
being the dominant cause of spectral variability. The predicted properties are
compared with the Suzaku observational data of the Seyfert 1 galaxy
MCG--6-30-15. The data disfavor models with the X-ray source (1) moving
vertically on the symmetry axis or (2) corotating with the disc and changing
height not far above the disc surface. The most likely explanation for the
observed variability is given by the model involving the X-ray source located
at a very small, varying distance from a rapidly rotating black hole. This
model predicts some enhanced variations in the red wing of the Fe line, which
are not seen in the Suzaku observations. However, the enhanced variability of
the red wing, while ruled out by the Suzaku data, is consistent with an excess
RMS variability, between 5 and 6 keV, reported for some previous ASCA and XMM
observations. We speculate that the presence or lack of such a feature is
related to the change of the ionization state of the innermost part of the
disc, however, investigation of such effects is currently not possible in our
model (where a neutral disc is assumed). If the model, completed by description
of ionization effects, proves to be fully consistent with the observational
data, it will provide a strong indication that the central black hole in
MCG--6-30-15 rotates rapidly, supporting similar conclusions derived from the
Fe line profile.Comment: 15 pages, accepted for publication in A&

### On the light-bending model of X-ray variability of MCG-6-30-15

We apply the light bending model of X-ray variability to Suzaku data of the
Seyfert 1 galaxy MCG-6-30-15. We analyze the energy dependence of the root mean
square (rms) variability, and discuss conditions necessary for the model to
explain the characteristic decrease of the source variability around 5-8 keV. A
model, where the X-ray source moves radially rather than vertically close to
the disk surface, can indeed reproduce the reduced variability near the energy
of the Fe Kalpha line, although the formal fit quality is poor. The model then
predicts the energy spectra, which can be compared to observational data. The
spectra are strongly reflection dominated, and do not provide a good fit to
Suzaku spectral data of the source. The inconsistency of this result with some
previous claims can be traced to our using data in a broader energy band, where
effects of warm absorber in the spectrum cannot be neglected.Comment: 6 pages, PASJ, accepte

### Peierls instability, periodic Bose-Einstein condensates and density waves in quasi-one-dimensional boson-fermion mixtures of atomic gases

We study the quasi-one-dimensional (Q1D) spin-polarized bose-fermi mixture of
atomic gases at zero temperature. Bosonic excitation spectra are calculated in
random phase approximation on the ground state with the uniform BEC, and the
Peierls instabilities are shown to appear in bosonic collective excitation
modes with wave-number $2k_F$ by the coupling between the Bogoliubov-phonon
mode of bosonic atoms and the fermion particle-hole excitations. The
ground-state properties are calculated in the variational method, and,
corresponding to the Peierls instability, the state with a periodic BEC and
fermionic density waves with the period $\pi/k_F$ are shown to have a lower
energy than the uniform one. We also briefly discuss the Q1D system confined in
a harmonic oscillator (HO) potential and derive the Peierls instability
condition for it.Comment: 9 pages, 3figure

### Dynamical properties of dipolar Fermi gases

We investigate dynamical properties of a one-component Fermi gas with
dipole-dipole interaction between particles. Using a variational function based
on the Thomas-Fermi density distribution in phase space representation, the
total energy is described by a function of deformation parameters in both real
and momentum space. Various thermodynamic quantities of a uniform dipolar Fermi
gas are derived, and then instability of this system is discussed. For a
trapped dipolar Fermi gas, the collective oscillation frequencies are derived
with the energy-weighted sum rule method. The frequencies for the monopole and
quadrupole modes are calculated, and softening against collapse is shown as the
dipolar strength approaches the critical value. Finally, we investigate the
effects of the dipolar interaction on the expansion dynamics of the Fermi gas
and show how the dipolar effects manifest in an expanded cloud.Comment: 14 pages, 8 figures, submitted to New J. Phy

### Fluctuations in the formation time of ultracold dimers from fermionic atoms

We investigate the temporal fluctuations characteristic of the formation of
molecular dimers from ultracold fermionic atoms via Raman photoassociation. The
quantum fluctuations inherent to the initial atomic state result in large
fluctuations in the passage time from atoms to molecules. Assuming degeneracy
of kinetic energies of atoms in the strong coupling limit we find that a
heuristic classical stochastic model yields qualitative agreement with the full
quantum treatment in the initial stages of the dynamics. We also show that in
contrast to the association of atoms into dimers, the reverse process of
dissociation from a condensate of bosonic dimers exhibits little passage time
fluctuations. Finally we explore effects due to the non-degeneracy of atomic
kinetic energies.Comment: 7 pages, 6 figure

### Random-phase approximation study of collective excitations in the Bose-Fermi mixed condensate of alkali-metal gases

We perform Random Phase Approximation (RPA) study of collective excitations
in the bose-fermi mixed degenerate gas of Alkali-metal atoms at T=0. The
calculation is done by diagonalization in a model space composed of
particle-hole type excitations from the ground state, the latter being obtained
from the coupled Gross-Pitaevskii and Thomas-Fermi equations. We investigate
strength distributions for different combinations of bose and fermi multipole
($L$) operators with $L=0,1,2,3$. Transition densities and dynamical structure
factors are calculated for collective excitations. Comparison with the sum rule
prediction for the collective frequency is given. Time dependent behavior of
the system after an external impulse is studied.Comment: 28 pages, 13 figures, submitted to Phys. Rev.

- â€¦