58,689 research outputs found
A model for the Z-track phenomenon in GX 5-1 and observational evidence for the physical origins of the kHz QPO
We present results of a combined investigation of the spectral and kHz QPO
evolution around the Z-track in GX 5-1 based on high-quality RXTE data. The
Extended ADC emission model provides very good fits to the spectra, the results
pointing clearly to a model for the nature of the Z-track, in agreement with
previous results for the similar source GX 340+0. In this model, at the soft
apex of the Z-track, the mass accretion rate Mdot is minimum and the neutron
star has its lowest temperature; but as the source moves along the normal
branch, the luminosity of the Comptonized emission increases, indicating that
Mdot increases and the neutron star gets hotter. The measured flux f of the
neutron star emission increases by a factor of ten becoming super-Eddington,
and we propose that this disrupts the inner disk so forming jets. In flaring,
the luminosity of the dominant Comptonized emission from the ADC is constant,
while the neutron star emission increases, and we propose for the first time
that flaring consists of unstable nuclear burning on the neutron star, and the
measured mass accretion rate per unit area mdot at the onset of flaring agrees
well with the theoretical critical value at which burning becomes unstable.
There is a striking correlation between the frequencies of the kHz QPO and the
ratio of the flux to the Eddington value: f/f_Edd, suggesting an explanation of
the higher frequency QPO and of its variation along the Z-track. It is well
known that a Keplerian orbit in the disk at this frequency corresponds to a
position some distance from the neutron star; we propose that the oscillation
always occurs at the inner disk edge, which moves radially outwards on the
upper normal and horizontal branches as the measured increasing radiation
pressure increasingly disrupts the inner disk.Comment: Astronomy and Astrophysics, in pres
Bright solitary waves in a Bose-Einstein condensate and their interactions
We examine the dynamics of two bright solitary waves with a negative
nonlinear term. The observed repulsion between two solitary waves -- when these
are in an antisymmetric combination -- is attributed to conservation laws.
Slight breaking of parity, in combination with weak relaxation of energy, leads
the two solitary waves to merge. The effective repulsion between solitary waves
requires certain nearly ideal conditions and is thus fragile.Comment: 6 pages, 14 figure
Theoretical pressure distributions over arbitrarily shaped periodic waves in subsonic compressible flow and comparison with experiment
Theoretical solution for pressure distribution over arbitrarily shaped periodic waves using Fourier serie
Very low sound velocities in iron-rich (Mg,Fe)O: Implications for the core-mantle boundary region
The sound velocities of (Mg_(.16)Fe_(.84))O have been measured to 121 GPa at ambient temperature using nuclear resonant inelastic x-ray scattering. The effect of electronic environment of the iron sites on the sound velocities were tracked in situ using synchrotron Mössbauer spectroscopy. We found the sound velocities of (Mg_(.16)Fe_(.84))O to be much lower than those in other presumed mantle phases at similar conditions, most notably at very high pressures. Conservative estimates of the effect of temperature and dilution on aggregate sound velocities show that only a small amount of iron-rich (Mg,Fe)O can greatly reduce the average sound velocity of an assemblage. We propose that iron-rich (Mg,Fe)O be a source of ultra-low velocity zones. Other properties of this phase, such as enhanced density and dynamic stability, strongly support the presence of iron-rich (Mg,Fe)O in localized patches above the core-mantle boundary
Amplitude expansion of the binary phase field crystal model
Amplitude representations of a binary phase field crystal model are developed
for a two dimensional triangular lattice and three dimensional BCC and FCC
crystal structures. The relationship between these amplitude equations and the
standard phase field models for binary alloy solidification with elasticity are
derived, providing an explicit connection between phase field crystal and phase
field models. Sample simulations of solute migration at grain boundaries,
eutectic solidification and quantum dot formation on nano-membranes are also
presented.Comment: 11 pages, 8 figure
Theoretical study of ionization of an alkali atom adsorbed on a metal surface by laser assisted subfemtosecond pulse
The first numerical simulation of the process of ionization of an atom
adsorbed on a metal surface by the subfemtosecond pulse is presented. The
streaking scheme is considered, when a weak sub-femtosecond pulse comes
together with a strong IR pulse with a variable delay between them. The problem
is analyzed with numerical solving the non-stationary Schroedinger equation in
the cylindrical coordinate. The results obtained are compared with ones in the
gas phase. We show that the surface influences the DDCS, but the observation of
this influence, beside the trivial polarization shift of the energy of the
initial state, requires a quite high experimental resolution
Spectral Investigations of the nature of the Sco X-1 like sources
We present results of spectral investigations of the Sco X-1 like Z-track
sources Sco X-1, GX 349+2 and GX 17+2 based on Rossi-XTE observations using an
extended accretion disk corona model. The results are compared with previous
results for the Cyg X-2 like group: Cyg X-2, GX 340+0 and GX 5-1 and a general
model for the Z-track sources proposed. On the normal branch, the Sco-like and
Cyg-like sources are similar, the results indicating an increase of mass
accretion rate Mdot between soft and hard apex, not as in the standard view
that this increases around the Z. In the Cyg-like sources, increasing Mdot
causes the neutron star temperature kT to increase from ~1 to ~2 keV. At the
lower kT, the radiation pressure is small, but at the higher kT, the emitted
flux of the neutron star is several times super-Eddington and the high
radiation pressure disrupts the inner disk launching the relativistic jets
observed on the upper normal and horizontal branches. In the Sco-like sources,
the main physical difference is the high kT of more than 2 keV on all parts of
the Z-track suggesting that jets are always possible, even on the flaring
branch. The flaring branch in the Cyg-like sources is associated with release
of energy on the neutron star consistent with unstable nuclear burning. The
Sco-like sources are very different as flaring appears to be a combination of
unstable burning and an increase of Mdot which makes flaring much stronger.
Analysis of 15 years or RXTE ASM data on all 6 classic Z-track sources shows
the high rate and strength of flaring in the Sco-like sources suggesting that
continual release of energy heats the neutron star causing the high kT. A Sco
X-1 observation with unusually little flaring supports this. GX 17+2 appears to
be transitional between the Cyg and Sco-like types. Our results do not support
the suggestion that Cyg or Sco-like nature is determined by luminosity.Comment: Astronomy and Astrophysics in press; 21 pages, 13 figure
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