24,347 research outputs found
Evolution of AQL X-1 During the Rising Phase of its 1998 Outburst
We present results from 16 snapshots of Aql X-1 with RXTE during the rising
phase of its recent outburst. The observations were carried out at a typical
rate of once or twice per day. The source shows interesting spectral evolution
during this period. Phenomenologically, it bears remarkable similarities to
``atoll'' sources. Shortly after the onset of the outburst, the source is seen
to be in an ``island'' state, but with little X-ray variability. It then
appears to have made a rapid spectral transition (on a time scale less than
half a day) to another ``island'' state, where it evolves slightly and stays
for 4 days. In this state, the observed X-ray flux becomes increasingly
variable as the source brightens. Quasi-period oscillation (QPO) in the X-ray
intensity is detected in the frequency range 670--870 Hz. The QPO frequency
increases with the X-ray flux while its fractional rms decreases. The QPO
becomes undetectable following a transition to a ``banana'' state, where the
source continues its evolution by moving up and down the ``banana'' branch in
the color-color diagram as the flux (presumably, the mass accretion rate)
fluctuates around the peak of the outburst. Throughout the entire period, the
power density spectrum is dominated by very-low frequency noises. Little power
can be seen above ~1 Hz, which is different from typical ``atoll'' sources. In
the ``banana'' state, the overall X-ray variability remains low (with
fractional rms ~3--4%) but roughly constant. The observed X-ray spectrum is
soft with few photons from above 25 keV, implying the thermal origin of
the emission. The evolution of both spectral and temporal X-ray properties is
discussed in the context of disk-instability models.Comment: 13 pages, including one table and five figures. To appear in ApJ
Letters (July 20
Fluctuation-Induced First Order Transition between the Quantum Hall Liquid and Insulator
We study the phase transition between the quantum Hall liquid state and the
insulating state within the framework of the Chern-Simons-Landau-Ginzburg
theory of the quantum Hall effect. For the transition induced by a background
periodic potential in the absence of disorder, the model is described by a
relativistic scalar field coupled to the Chern-Simons gauge field. For this
system, we show that the transition is of the first order, induced by the
fluctuations of the gauge field, rather than second order, with statistical
angle-dependent scaling exponent.Comment: 5 pages, REVTEX 3.0, two PostScript pictures appended, preprint
SU-ITP-94-
Interaction of Individual Skyrmions in Nanostructured Cubic Chiral Magnet
We report the direct evidence of field-dependent character of the interaction
between individual magnetic skyrmions as well as between skyrmions and edges in
B20-type FeGe nanostripes observed by means of high resolution Lorentz
transmission electron microscopy. It is shown that above certain critical
values of external magnetic field the character of such long-range skyrmion
interactions change from attraction to repulsion. Experimentally measured
equilibrium inter-skyrmion and skrymion-edge distances as function of applied
magnetic field shows quantitative agreement with the results of micromagnetic
simulations. Important role of demagnetizing fields and internal symmetry of
three-dimensional magnetic skyrmions are discussed in details.Comment: accepted in PR
Cosmological Models and Latest Observational Data
In this note, we consider the observational constraints on some cosmological
models by using the 307 Union type Ia supernovae (SNIa), the 32 calibrated
Gamma-Ray Bursts (GRBs) at , the updated shift parameter from WMAP
5-year data (WMAP5), and the distance parameter of the measurement of the
baryon acoustic oscillation (BAO) peak in the distribution of SDSS luminous red
galaxies with the updated scalar spectral index from WMAP5. The tighter
constraints obtained here update the ones obtained previously in the
literature.Comment: 10 pages, 5 figures, 1 table, revtex4; v2: discussions added,
accepted by Eur. Phys. J. C; v3: published versio
Evidence for Frame-Dragging Around Spinning Black Holes in X-Ray Binaries
In the context of black hole spin in X-ray binaries, we propose that certain
type of quasi-period oscillations (QPOs) observed in the light curves of black
hole binaries (BHBs) are produced by X-ray modulation at the precession
frequency of accretion disks, due to relativistic dragging of inertial frames
around spinning black holes. These QPOs tend to be relatively stable in their
centroid frequencies. They have been observed in the frequency range of a few
to a few hundred Hz for several black holes with dynamically determined masses.
By comparing the computed disk precession frequency with that of the observed
QPO, we can derive the black hole angular momentum, given its mass. When
applying this model to GRO J1655-40, GRS 1915+105, Cyg X-1, and GS 1124-68, we
found that the black holes in GRO J1655-40 and GRS 1915+105, the only known
BHBs that occasionally produce superluminal radio jets, spin at a rate close to
the maximum limit, while Cyg X-1 and GS 1124-68, typical (persistent and
transient) BHBs, contain only moderately rotating ones. Extending the model to
the general population of black hole candidates, the fact that only
low-frequency QPOs have been detected is consistent with the presence of only
slowly spinning black holes in these systems. Our results are in good agreement
with those derived from spectral data, thus strongly support the classification
scheme that we proposed previously for BHBs.Comment: new title, minor revisions; change title to conform to ApJL rules;
replaced with the updated version to avoid confusio
Hydraulic transient modeling and analysis of the district heating network
The hydraulic transients are obvious during the operation and adjustment of large-scale district heating (DH) network. Hydraulic transient modeling and simulation methods can provide basis for efficient operation of the DH network. In this paper, two hydraulic transient models, the distributed parameter model (DPM) and lumped parameter model (LPM), were established for DH network. Efficient numerical algorithms were presented for the two models. The proposed DPM and LPM were both applied to a DH network to simulate and analyze the network hydraulic transients caused by the valve and pump operations. Results indicate that with the increase of the distance from the adjustment location, the transient time for the flow rate and pressure gets longer. As the adjustment duration decreases, the transient time increases. Comparison of the simulation performances between the two models indicates that the DPM can capture the high-frequency hydraulic transients subtly. With the decrease of adjustment duration, more severe high-frequency hydraulic transients will occur. However, only the average hydraulic fluctuations can be described by LPM. The computation time of LPM is shorter than DPM because of the vectorizable calculation procedure and the less data to record, since the LPM neglects the pressure waves and the fluid compressibility
A Single Scalar Field Model of Dark Energy with Equation of State Crossing -1
In this paper we study the possibility of building models of dark energy with
equation of state across -1 and propose explicitly a model with a single scalar
field which gives rise to an equation of state larger than -1 in the past and
less than -1 at the present time, consistent with the current observations.Comment: 4 pages, 1 figure, the version accepted by JCAP, presentation
improved and references adde
Cytogenetic and Embryologic Analyses of the Mechanism of Parthenogenesis Induced by 1BL/1RS Male Sterile Lines in Wheat
In this study, we employed electron microscopy to investigate the cytogenetic and embryologic mechanisms of parthenogenesis induced in the 1BL/1RS male sterile lines of wheat. Analysis of the root tips and acid polyacrylamide gel electrophoresis indicated that all of the male sterile lines and their maintainer lines were 1BL/1RS translocation lines, whereas the restorer lines were non-1BL/1RS translocation lines. Furthermore, the chromosomes of 1BL/1RS wheat lines with T. aestivum cytoplasm and Aegilops cytoplasm (include Ae. kotschyi, Ae. ventricosa, Ae. variabilis) paired abnormally at different rates during meiotic metaphase I (MMI). The translocated segment size of the 1RS chromosome and the specific nuclear–alloplasm interaction impaired the pairing of homologous chromosome in the background of the specific Aegilops cytoplasm at MMI. In addition, the frequency of abnormal chromosomal pairing was directly affected by the frequency of haploid production induced by parthenogenesis. The results of this study provide significant insights into the mechanism of parthenogenesis, which is probably due to the abnormal fertilization of synergid cells in alloplasmic 1BL/1RS wheat
- …